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COURSE TITLE: Fundamentals and Control of D.C. Generators and Motors
DUTY TITLLE: Operation of 2 & 3 Wire Control
POS #: 1900
TASK : Operation of Two & Three Wire Control with Overloads
PURPOSE: To Understand the Connection and Troubleshooting Techniques of Two (2) Wire Control Circuits with Overloads.
TASKS:
|1901 |Identify symbols and terms used in electro-mechanical motor control circuits. |
|1902 |Identify relays, contactors and motor starters. |
|1903 |Read schematic wiring diagrams of motors and their controls. |
|1904 |Wire a simple two- and three-wire motor control circuit. |
|1905 |Wire a reversing starter. |
|1906 |Wire multiple push button/jogging control circuits. |
|1907 |Wire sequential control circuits. |
|1908 |Wire and test electrical control circuits. |
|1909 |Perform preventive maintenance and troubleshooting on motor controls. |
|1910 |Identify and describe classes of wire insulation. |
|1911 |Describe conductor ampacity. |
|1912 |Describe how to select “wire size” and “wire type” for a specific wiring application. |
|1913 |Demonstrate procedures for the correct labeling of wires. |
|1914 |Interpret electrical diagrams. |
NOTE: This task is not on the current Program of Study Task Listing; however this is an important task the students must learn for the Electrical trade. The P.O.S. numbers shown are from a previous task listing.
REVISION: 2019
|ENGLISH LANGUAGE ARTS |
|CC.1.2.11-12.J Acquire and use accurately general academic and domain-specific words and phrases, sufficient for reading, writing, speaking, and |
|listening at the college and career readiness level; demonstrate independence in gathering vocabulary knowledge when considering a word or phrase |
|important to comprehension or expression |
|CC.1.3.11-12.I Determine or clarify the meaning of unknown and multiple-meaning words and phrases based on grade level reading and content, |
|choosing flexibly from a range of strategies and tools. |
|MATH |
|CC.2.1.HS.F.4 Use units as a way to understand problems and to guide the solution of multi-step problems. |
|CC.2.1.HS.F.6 Extend the knowledge of arithmetic operations and apply to complex numbers. |
|READING IN SCIENCE & TECHNOLOGY |
|CC.3.5.11-12.B. Determine the central ideas or conclusions of a text; summarize complex concepts, processes, or information presented in a text by |
|paraphrasing them in simpler but still accurate terms. |
|CC.3.5.11-12.C. Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; |
|analyze the specific results based on explanations in the text. |
|WRITING IN SCIENCE & TECHNOLOGY |
|CC.3.6.11-12.E. Use technology, including the Internet, to produce, publish, and update individual or shared writing products in response to |
|ongoing feedback, including new arguments or information. |
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*ACADEMIC STANDARDS *
|READING, WRITING, SPEAKING & LISTENING |
|1.1.11.A Locate various texts, assigned for independent projects before reading. |
|1.1.11.D Identify strategies that were most effective in learning |
|1.1.11.E Establish a reading vocabulary by using new words |
|1.1.11.F Understanding the meaning of, and apply key vocabulary across the various subject areas |
|1.4.11.D Maintain a written record of activities |
|1.6.11.A Listen to others, ask questions, and take notes |
|MATH |
|2.2.11.A Develop and use computation concepts |
|2.2.11.B Use estimation for problems that don’t need exact answers |
|2.2.11.C Constructing and applying mathematical models |
|2.2.11.D Describe and explain errors that may occur in estimates |
|2.2.11.E Recognize that the degree of precision need in calculating |
|2.3.11.A Selecting and using the right units and tools to measure precise measurements |
|2.5.11.A Using appropriate mathematical concepts for multi-step problems |
|2.5.11.B Use symbols, terminology, mathematical rules, Etc. |
|2.5.11.C Presenting mathematical procedures and results |
|SCIENCE |
|3.1.12.A Apply concepts of systems, subsystems feedback and control to solve complex technological problems |
|3.1.12.B Apply concepts of models as a method predict and understand science and technology |
|3.1.12.C Assess and apply patterns in science and technology |
|3.1.12D Analyze scale as a way of relating concepts and ideas to one another by some measure |
|3.1.12.E Evaluate change in nature, physical systems and man-made systems |
|3.2.12.A Evaluate the nature of scientific and technological knowledge |
|3.2.12.B Evaluate experimental information for appropriateness |
|3.2.12.C Apply elements of scientific inquiry to solve multi – step problems |
|3.2.12.D Analyze the technological design process to solve problems |
|3.4.12.A Apply concepts about the structure and properties of matter |
|3.4.12.B Apply energy sources and conversions and their relationship to heat and temperature |
|3.4.12.C Apply the principles of motion and force |
|3.8.12.A Synthesize the interactions and constraints of science |
|3.8.12.B Use of ingenuity and technological resources to solve specific societal needs and improve the quality of life |
|3.8.12.C Evaluate the consequences and impacts of scientific and technological solutions |
|ECOLOGY STANDARDS |
|4.2.10.A Explain that renewable and non-renewable resources supply energy and material. |
|4.2.10.B Evaluate factors affecting availability of natural resources. |
|4.2.10.C Analyze the use of renewable and non-renewable resources. |
|4.2.12.B Analyze factors affecting the availability of renewable and non-renewable resources. |
|4.3.10.A Describe environmental health issues. |
|4.3.10.B Explain how multiple variables determine the effects of pollution on environmental health, natural processes and human practices. |
|4.3.12.C Analyze the need for a healthy environment. |
|4.8.12.A Explain how technology has influenced the sustainability of natural resources over time. |
|CAREER & EDUCATION |
|13.1.11.A Relate careers to individual interest, abilities, and aptitudes |
|13.2.11.E Demonstrate in the career acquisition process the essential knowledge needed |
|13.3.11.A Evaluate personal attitudes that support career advancement |
|ASSESSMENT ANCHORS |
|M11.A.3.1.1 Simplify expressions using the order of operations |
|M11.A.2.1.3 Use proportional relationships in problem solving settings |
|M11.A.1.2 Apply any number theory concepts to show relationships between real numbers in problem solving |
STUDENT
The student will be able to terminate and troubleshoot various two (2) and three (3) wire control circuits with overloads.
TERMINAL PERFORMANCE OBJECTIVE
Given all the electrical tools and materials required, the student will be able to terminate and troubleshoot various two (2) and three (3) wire control circuits with overloads.
SAFETY
• Always wear safety glasses when working in the shop.
• Always check with the instructor before turning power on.
• Always use tools in the correct manner.
• Keep work area clean and free of debris.
• Never wire a project without the correct wiring diagram.
• Always use the correct size N.E.M.A. enclosure for starters.
RELATED INFORMATION
1. Attend lecture by instructor.
2. Obtain handout.
3. Review chapters in textbook.
4. Define vocabulary words.
5. Complete all questions in this packet.
6. Complete all projects in this packet.
7. Complete K-W-L Literacy Assignment by Picking an Article From the
“Electrical Contractor” Magazine Located in the Theory Room. You can pick any article you feel is important to the electrical trade.
EQUIPMENT & SUPPLIES
1. Safety glasses 11. THHN wire
2. Hammer 12. Single phase power supply
3. Screw driver 13. Three phase power supply
4. Awl 14. Three way switches
5. Wire strippers 15. Push button stations
6. Side cutters 16. NEMA enclosure
7. Cable rippers 17. Single pole switch
8. Lineman pliers 18. Control relays
9. Needle nose pliers 19. Starter with overloads
10. Multimeter 20. Assortment of overloads
|VOCABULARY |
|CC.1.3.11-12.I Determine or clarify the meaning of unknown and multiple-meaning words and phrases based on grade level reading and content, |
|choosing flexibly from a range of strategies and tool |
|CC.3.5.11-12.D. Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific |
|or technical context relevant to grades 11–12 texts and topics. |
• Current Relay:
• Double Acting Pushbutton:
• Float Switch:
• Motor Starter:
• Pressure Switch:
• Flow switch:
• Mechanical Starter (Three Phase):
• Overload Heater:
• Mechanical Lockout Push Buttons:
• Overload Relay:
• Manual Starter (Single Phase):
• Control Transformer:
• N.E.M.A:
• Across The Line:
• Automatic Starter:
• Auxiliary Contacts:
• Bridge Rectifier:
• Contactor:
• Drum Controller:
• Drum Switch:
• Electrical Interlock:
• Holding Contacts:
• Interlock:
• Magnetic Contactor:
• Motor Controller:
• Multispeed Starter:
• Overload Protection:
• Overload Relay:
PROCEDURE
CC.2.1.HS.F.4 Use units as a way to understand problems and to guide the solution of multi-step problems.
CC.3.5.11-12.C. Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; analyze the specific results based on explanations in the text.
1.6.11A Listen to others, ask questions, and take notes
3.4.12.B Apply energy sources and conversions and their relationship to heat and temperature
GENERAL PRINCIPLES OF ELECTRIC MOTOR CONTROL
OBJECTIVES
• State the purpose and general principles of electric motor control
• State the difference between manual and remote control
• List the conditions of starting and stopping, speed control, and protection of electric motors
• Explain the difference between compensating and definite time delay action
1. A controller is a device or group of devices that governs the delivery of a predetermined amount of electric power to apparatus connected to it. The controller’s function is to start, stop, reverse, accelerate, decelerate, regulate, or protect devices connected to it.
2. A remote control initiates or causes a change in the operation of an electrical device or apparatus and is placed at a different location than the device being controlled. An example of a remote control is the push button which energizes a magnetic motor starter which starts a motor.
3. Current limiting or compensating time acceleration refers to the amount of current or voltage required to open or close magnetic switches. This type of control action is generally used for starting time periods on direct-current motor control.
• Float switch: raises or lowers liquids.
• Pressure switch: maintains liquid, gas, or air pressures within a desired range.
• Time clock: provides a definite “on and off” action.
• Thermostat: maintains temperature ranges.
• Limit switch: used as an over travel stop on machines.
• Interlock: assures that all systems are correct before the motor is started
Switches
Introduction There are a wide variety of switches used in the electrical industry. In this section only those switches commonly used in water and wastewater pumping installations will be discussed.
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On – Off
• ON/OFF switches are also called supplementary contact switches. This switch appears as a vertical lever on the control panel. When the switch is operated it remains in position until it is physically moved. This type of switch is called a single pole, single throw switch, (SPST).
• DPST In some instances a switch contains two sets of contacts. They are both located at the back of the same switch and are operated by the same lever.
• This type of switch is called a double pole, single throw switch (DPST).
• In the symbol for the DPST switch, notice there is a dotted line running from the top switch set to the bottom.
• This type of dotted line indicates the two switches are physically connected together. The moving of one switch will move the second switch.
ON OFF
H-O-A
• The hand-off-auto switch (H-O-A) is actually a SPDT (single pole, double throw) switch. This switch has three positions, hand, off, and auto. In some instances the labels may be changed to Local-off-Remote. The switch looks almost identical to the ON/OFF switch.
• The switch stays in whatever position it is set until it is physically changed.
• There are two common H-O-A switches used in pumping installations: single-gang and the multi-gang switch.
• This indicates the two switches are physically in one housing.
• Moving the switch lever changes the position of both switches. With the H-O-A switch, the left position is normally the Hand, the center position is Off and the right position is Auto.
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Momentary Contact
• A switch commonly used with the “push-to-start” circuit is the momentary contact switch. This switch only operates when the button is depressed.
• Operation there are two basic styles of this switch: normally open (N.O.) and normally closed (N.C.). With the normally open momentary contact switch, electrical contact is only made when the push button is depressed.
• In a control circuit, such as the “push-to-start” circuit, the push button is labeled the START button.
• The normally closed switch is opened when the button is depressed. In a control circuit this button is usually called the STOP button.
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Maintained Contact
• Rather than using the momentary contact on-off switch in a circuit, the designer may choose the maintained contact push button switch. This is a single push button that looks like the momentary contact switch.
• When the button is depressed, the circuit is closed and current flows. To turn the circuit off or open the circuit, the button is pressed a second time and then released. This opens the contacts and opens the circuit.
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Floats
• There are three types of float switches used in pumping systems. Floats are used to turn pumps on, and off, as emergency alarms, or shut downs.
• A float used to turn a pump on or off in a water reservoir is the mechanical lever operated float switch3.
• In wastewater lift stations and some drinking water reservoirs the mercury float is often used.
• A simple lever operated float switch can be installed in the water jacket of a low pressure boiler or hydro pneumatic tank to shut down the system when there is a loss of water.
• Float switches are available with either normally open or normally closed contacts.
• The same symbol is used to identify all three of these float switches:
• Mechanical, the mechanical float uses a copper or plastic float attached to a rod. The rod passes through a holding bracket. The vertical movement of the rod is restricted by collars above and below the holder. These same collars are used to operate a lever that is attached to a set of contacts. As the float moves up and down, the lever is moved, opening or closing the contacts.
• Lever The lever float switch is composed of a small metal float on the end of a shaft. The shaft slides through an opening in a lever. The other end of the lever is connected to a set of contacts. As the float is raised or lowered by water level, the switch is opened or closed by the collars attached to the float shaft. The distance between the collars determines the water level differential.
• Mercury The heart of the mercury float switch is a vial, usually made of glass, with two contacts and a puddle of mercury.
• The vial is encased with a very dense foam material which is covered with a waterproof plastic case. A weight is usually placed at the top of the float. With a normally open float switch the electrodes are at the top of the glass vial. When the water level rises, the float tips sideways and the mercury runs across the electrodes, making connection. With a normally closed float, the electrodes are in the bottom of the vial.
• Tipping the float causes the mercury to run away from the electrodes, opening the circuit.
• Special Considerations Mercury float switches are one of the most common sewage lift station controls. In a normal two pump lift station there are four floats. The bottom float is the pump off float, the second float is the lead pump on float, the third is the lag pump on, and the fourth float is the high level alarm float.
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Pressure Switch
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• In many instances, it is desirable to turn pumps on or off based on system pressure. It may also be desirable to prevent a pump from coming on if the suction pressure drops below a certain level or if the discharge pressure goes above a predetermined level. Pressure switches are used for this purpose. There are three common pressure switches: the diaphragm, the metal bellows, and the piston operated hydraulic switch. The diaphragm pressure switches are used for low pressure, below 250 psi; the metal bellows in pressure conditions up to 2,000 psi; and the piston operated hydraulic switches in pressure conditions up to 15,000 psi. Because low pressure diaphragm switches are the common pressure switch used in pumping installations, only their operation will be discussed.
• The diaphragm pressure switch uses a flexible, rubber like diaphragm to sense the water pressure, a spring to hold the contacts in their normal position, and one or more sets of contacts.
• Pressure switch contacts can be N.O. or N.C.; the following explanation uses the N.C. switch shown above. When the pressure below the diaphragm increases the upward force sufficiently to overcome the spring tension, the diaphragm is raised up causing the lever to rotate to the left, opening the contacts. When the pressure on the diaphragm drops sufficiently for the spring tension to overcome its upward force the contacts are closed. This type of pressure switch could be used to turn on a pump when the system pressure drops below a set point and turn the pump off when the pressure exceeds a set point.
• A common installation for this type of pressure switch is a small groundwater system that uses a hydro pneumatic tank. This pressure switch is installed between the pump and the tank and is used to turn the well pump on and off. The switch shown on the bottom of page 202 contains two controls, low pressure and differential pressure. It would turn the pump on when the pressure dropped below a set pressure and turn it off when the pressure rises above a set value. This upper value is controlled by the differential pressure adjustment.
Mercroid Switch
• A second method used to turn pumps on and off based on pressure is the Mercroid switch. This switch utilizes a bourdon tube and a glass vial with two electrodes and a puddle of mercury. These switches are used to turn pumps on and off and perform fail safe functions such as sensing low or high pressure.
Elapsed Time Meters
• Elapsed time meters and hour meters are used to determine the number of hours a pump has operated. This data is used to identify preventive maintenance requirements and to determine if the system is operating properly.
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Bubbler System
• A second, very common method used to control pumps in a sewage lift station is the bubbler system.
• This system forces air pressure through a small tube into the wet well of the lift station. The back pressure caused by water above the end of the tube is sensed by a group of pressure switches. These pressure switches are used to turn pumps on, off, and send a high water alarm.
• The most common bubbler systems use a small compressor, air filter, rotometer, air control valve, a manifold, four pressure switches, and a plastic or stainless steel tube in the wet well. Air supplied by the compressor is filtered to prevent damage to the pressure switches. The rotometer is used to indicate the rate of air flow. In some installations air is bubbled through a water-filled glass tube to indicate rate of air flow. The air is piped into a manifold in the electrical control panel. Four pressure switches are connected to the manifold. The pressure switches may be the single pressure diaphragm type or mercroid switches.
• The pressure in the manifold is directly proportional to the height of water above the top of the bubbler tube in the wet well. This pressure is sensed by the pressure switches. The first switch is set at a low pressure and is used to turn the pumps off. The second switch is set at a slightly higher pressure and is used to turn on the lead pump. The third switch turns on the lag pump and the fourth switch turns on the high water alarm.
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Electrode
• Another method used to turn pumps on and off is the electrode. This device is used in storage tanks and chemical tanks. Two to four electrodes of different lengths are placed in the tank. Water makes an electrical connection between the electrodes starting or stopping pumps and/or opening and closing valves.
• B/W Controls There are two popular electrode systems used to control water levels. One is the B/W Controls Inc. system and the other is the Warrick level control. The B/W
• Controls system uses an “A”-shaped laminated core coil, two sets of contacts, and two electrodes. One electrode is the pump on and the second is the pump off. This system is designed to be used with a metal tank. The ground is connected to the tank. When a nonmetallic tank is used, a third reference electrode that extends nearly to the bottom of the tank is used.
• B/W Controls AC power is applied to the primary windings of the relay. When power is applied, current flowing through the primary coil produces magnetic lines of flux. These lines of flux take the path of least resistance, through the bottom portion of the “A”. This induces a current in the secondary coil. However, since the secondary coil is attached across the two electrodes, which are not connected, an open circuit exists and there is no current flow in the secondary coil.
• When the water level rises so that there is a connection between the two electrodes, (pump start and pump stop electrodes) a current flows in the secondary coil. This current creates magnetic lines of flux in the lower part of the “A” that break the lines of flux created by the primary coil.
• The result is the lines of flux in the laminated core are diverted into the legs of the “A” causing the armature of the coil to be lifted to the core. This closes the contacts at 8, 9, 6, and 7.
• The closing of the contacts between 8 & 9 connects the circuit between the electrodes and ground. This is called a holding circuit and prevents the pump from shutting off until the water level is lowered below the off electrode.
• The Warrick controls use two to five electrodes. The most common in wastewater lift stations are the five electrode set-ups. These electrodes are the reference, pumps off, lead pump, lag pump, and high level alarm.
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Flow Switch
• In many pumping installations it is desirable to prevent a device from turning on if there is no flow in a line or to turn the pump off if flow in the line suddenly stops. To do this a flow switch is used. One of the common uses of a flow switch is in a small ground water system with a hypochlorite and/or a fluoride saturator feed system. In this instance, it is not desirable for the chemical feed pumps to operate if there is no flow from the well pump.
• There are two common types of flow switches used in pumping system; RTD and paddle.
• With the paddle type, a metal paddle is placed directly into the line. The upper end of the paddle is connected to a lever that opens or closes a set of contacts. When there is sufficient flow in the line, the paddle is moved forward, closing the contacts in the switch.
• With the RTD (Resistance Temperature Device) two electrodes are heated. Flow past the heated electrodes reduces their temp. This drop in temperature and reduces the electrical potential between them and the unheated electrode indicating a flow.
• Feed Pumps When the flow switch is installed with a chemical feed pump, it is wired to control power through the auxiliary contacts on the motor starter which are used to control power to the duplex plug used to supply power to the chemical feed pump. When the flow switch verifies flow power is applied to the duplex plug thus staring the chemical feed pump.
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Transducer
• Transducers are electro-mechanical devices which convert physical quantities, such as pressure, to a proportional electrical value, such as voltage or current.
• Pressure transducers are very effective in detecting water level and are used to control pumps in wet wells.
• There is a wide variety of pressure transducers used to detect changes in pressure. The most common types are the inductance, capacitance, and resistance.
• The resistance type is the most common in the water and wastewater industry. The resistance transducer uses a strain gauge. The strain gauge is wired into an electrical circuit and placed against a flexible diaphragm.
• The strain gauge is made by placing a special wire onto a ceramic wafer. When the wafer is distorted by pressure, the length of the wire is increased and its cross-sectional area is reduced. The result is an increase in its resistance. The wire is placed into a special circuit called a wheatstone bridge. In the bridge all four resistors are the same exact value causing the voltage difference between points “A” and “B” to be zero. When the resistance of the strain gauge is changed, the voltage difference between points “A” and “B” is changed causing a current to flow between these two points. The current flow is directly proportional to the pressure applied to the gauge. The current flow is very small and requires an electronic amplifier to be used in order to provide a signal that is usable. Standard transducers produce a 4 to 20 mA signal.
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Proximity Sensor
• Proximity sensors have no external arms or levers.
• When an object comes close (proximity) to the sensor, it opens or closes a set of contacts. The sensor may use inductance, capacitance, or a magnet in order to operate. Inductive and capacitance proximity sensors are solid state devices with no moving parts. They are used to detect the position of a valve positioner on a pump control valve or the position of the arm on a swing check valve. In this latter use, the sensor may be wired to shut down a pump if the check valve fails to open.
• Magnetically operated proximity sensors use the limit switch symbol.
• The inductive proximity sensors used in the water and wastewater industry are solid state devices housed in a metal tube.
• The operation of solid state devices is beyond the scope of this text. Therefore, a magnetically-operated, mechanical proximity sensor will be used to explain its operation. The magnetic sensor is composed of two magnets, (shown as “L” and “R” below), and a movable armature (A) all housed in a plastic case. The magnets are different lengths and the armature holds the contacts.
• Under normal conditions the “R” magnet holds the contacts just above it in a closed position. When a ferrous device is placed close to the “R” magnet, the lines of flux travel through the ferrous device, reducing their strength on the armature and allowing magnet “L” to switch the position of the contacts. Moving the ferrous device away from the sensor allows the contacts to return to their original position.
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Limit Switch
• One of the common uses of limit switches is with wide body globe valves used to control pressure surge from a pump. The limit switch is placed on top of the globe valve. The valve stem is used to turn the switch on or off and thus control when the pump shuts down.
• The mechanical limit switch uses an arm with a small wheel attached to the end. This wheel runs against a rod attached to the control valve diaphragm. A movable stop is attached to the rod. The switch arm is attached to a second arm. This second arm can be made to contact a contact jumper that is held away from the contacts by a spring.
• When the control valve closes, the rod on the valve diaphragm drops down and the movable stop causes the arm of the limit switch to move. As this arm moves, it moves through the pre-travel area. If the valve continues to close, the arm is moved further, causing the second arm to push the jumper across the contacts.
• This sends a signal to stop the pump. The arm may continue to move beyond the initial contact position.
• This is because it takes time for the valve to completely close. This last distance is called “over travel.
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Temperature Switches
• Temperature switches can be used to turn circulation pumps on and off to control temperature in a digester or water circulation loop as found in the arctic and subarctic. They can also be used to regulate the temperature in low pressure boilers or they could be used to merely provide an electronic signal that is used to indicate the temperature of the water, air, etc. In a low pressure boiler system these devices are called aquastats.
• The temperature switch is composed of a sealed bulb filled with a temperature sensitive fluid. The bulb is attached to the switch through a capillary tube that may be a few inches or a few feet in length. At the top of the capillary tube is a diaphragm much like the diaphragm used in a pressure switch. The fluid that is in the bulb also fills the capillary tube and lower area under the diaphragm. The diaphragm is attached mechanically to a normally open or normally closed set of contacts.
• The temperature sensitive fluid in the switch expands with an increase in temperature. The fluid was formulated so that the rate of expansion is linear with an increase in temperature. As the temperature rises, the diaphragm is flexed upward, moving the lever and opening or closing the switch contacts. When the temperature decreases, the diaphragm lowers, lowering the rod and returning the contacts to their original position.
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Lights
Indicator (pilot) Lights
• Lights of various colors are placed on control panels to indicate the status of a pump (on or off) and signal various types of alarms (low water, high water, high motor heat, water in the motor casing, water in the oil, etc.) These lights are usually 120 volt devices that utilize a small wattage bulb. The bulb is covered by a colored lens.
• The letter in the center of the symbol indicates color.
• The common letters and colors are:
1. A = amber
2. B = blue
3. C = clear
4. G = green
5. O = orange
6. R = red
7. W = white
• One of the major operational problems with indicator lights is burning out. The only way they can be tested is to remove the colored lens and bulb. As a result, some designs place “push to test” indicator lights in critical alarm positions.
• Transformer Type A second option is to use a transformer type indicator light. This type extends the life of the bulb by using a low voltage (6V) bulb. However, it costs more than the 120V type.
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Push to Test Indicator (pilot) Lights
• A push to test indicator light provides all the functions described above with regular indicator lights plus the opportunity to determine if the bulb is working without removing the bulb.
• The push to test light contains the standard light circuit and an additional switch connection. When the lens of the light is pushed in, it causes a momentary contact switch to be disconnected from the standard circuit and pressed against a second lead that is connected directly to the control power hot lead. If the bulb is good, it will be illuminated.
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Electro-Mechanical Devices
Solenoids
• Solenoids were discussed in great detail in the lesson on electromagnetism; therefore this portion will be a general review. Solenoids are used to open and close valves and move mechanical devices. They are an electro- mechanical device.
• Below is an example of a solenoid as it would be seen in an electrical diagram.
• The solenoid is composed of an electromagnet and a movable metal rod called the armature. In the solenoid below the armature is connected to the movable disc in a globe valve. A spring is placed between the valve bonnet and the disc.
• There are two types of solenoids, the normally open (NO) and the normally closed (NC). The normally open solenoid valve uses a spring to hold the valve open. The normally closed solenoid valve uses a spring to hold the valve closed. Once the electromagnet is with the NC solenoid valve energized, the armature is pulled up lifting the valve disc against the spring. This opens the valve. When power is shut off to the electromagnet the spring forces the valve closed.
• In the water and wastewater industry, solenoid valves are commonly used to control the flow of seal water to a pump, fill a chemical solution tank, control the water level in a fluoride saturator, and operate the lawn sprinkler system.
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Control Relays
• Control relays are electro-mechanical devices used to turn pilot lights on and off, control mechanical equipment, and send signals from a sensing device, such as a float switch, to another component, such as a motor. The control relay is the key component in a motor control system. These relays were discussed in depth in the lesson on electromagnetism and, therefore, will only be reviewed here.
• The coil is identified by a circle with a letter and number placed in the center. The contact(s) are identified by two parallel lines. The contacts may be normally open (NO) or normally closed (NC).
• The symbols below show the relationship between the coil and the contacts as they would be observed in a standard electrical drawing.
• The control relay is composed of a single iron core electromagnet which operates an armature. The armature is used to open and close one or more electrical contacts. A control relay may have all NO or NC contacts or have a combination of normally open and normally closed contacts. The control relay above has one normally-open and one normally-closed set of contacts both operated by the same electromagnet. A spring is usually used to hold the contacts in one position.
• When the electromagnet is energized, the iron core attracts the armature and pulls it to the core. This opens the normally closed contacts and closes the normally open contacts. When the power is shut off, the spring pulls the contacts back to their “off the shelf” or “normal” position.
• The most important feature of the control relay is the fact that the pair of contacts are not electrically connected to the coil or each other. There are independent circuits. The contacts and the coil are connected physically.
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Time Delay Relays
• As the name implies, time delay relays are used to delay the time for a component to start or stop.
• When a pump is controlled by pressure in the system, a sudden change in this pressure could cause the pump to start. However, if the pressure change were the result of a surge in the line, the pump might turn on and off several times until the pressure stabilized. By placing a time delay relay in the system and setting it for 15 seconds, a condition could be established where the pressure would have to rise or drop to a set level and stay at that level for 15 seconds before the pump was energized.
• The standard symbol used to identify the time delay relay is nearly identical to the control relay. The only difference is in the letter indicator associated with the coil and contacts. The letters usually used are TD or TDR. Below is an example of a time delay relay in a pump circuit.
• There are two common types of time delay relays: the pneumatic and the solid state.
• Solid state time delay relays use electrical circuitry to delay the close or open time. The pneumatic timer uses an air bellows that is connected to a small orifice. When the coil is energized, the armature is pressed against the bellows, causing it to collapse and force air out through the orifice. The rate of closing is controlled by a needle valve in the orifice. When the bellows is completely collapsed, it mechanically closes or opens the contacts.
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Motor Starters
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• Motor starters allow low voltage (24 VDC, 48 VDC and 120 VAC) to be used to mechanically close a switch that applies high voltage to an electric motor.
• Motor starters are the link between the power circuit and the control circuit. The two are not connected electrically but are connected physically in the starter.
• As was mentioned in the power system lesson, the typical motor starter is composed of five items. Two of these, the contacts and the heater elements, were discussed in the power system lesson. In this lesson we will discuss the coil, thermal overload contacts, and the auxiliary contacts.
• Since there are five items in the motor starter, there are five symbols used to identify these items.
• Coil
• Contacts
• Heater elements
• Thermal overload contacts
• Auxiliary contacts
• Motor Starters - Electrically operated switches used to connect power to electric motors. Also called a magnetic starter.
• The starter is composed of five basic components, show above. There are a set of contacts placed in the power circuit (2) along with the heat sensing elements (3). The control system contains an electromagnet (1), the thermal overload device contacts (4), and the auxiliary contacts (5). The power system contacts are held in an open position by a spring. This same spring can be used to hold the auxiliary contacts in a power off position. Auxiliary contacts can be either normally open or normally closed. Most starters are constructed with one set of normally open auxiliary contacts.
• Additional normally open or normally closed contacts can be installed.
• The discussion thus far pertains only to the operation of the closing and opening of the power circuit by the starter. The thermal overloads and auxiliary contacts operation are discussed below.
• When there is a demand for the pump to start, power is applied to the magnetic coil of the starter. Once the coil is energized, it moves the armature which is attached to the contacts. This closes the contacts and applies power to the motor.
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The starter is composed of five basic components, show above. There are a set of contacts placed in the power circuit (2) along with the heat sensing elements (3). The control system contains an electromagnet (1), the thermal overload device contacts (4), and the auxiliary contacts (5). The power system contacts are held in an open position by a spring. This same spring can be used to hold the auxiliary contacts in a power off position. Auxiliary contacts can be either normally open or normally closed. Most starters are constructed with one set of normally open auxiliary contacts. Additional normally open or normally closed contacts can be installed.
The discussion thus far pertains only to the operation of the closing and opening of the power circuit by the starter. The thermal overloads and auxiliary contacts
operation are discussed below. When there is a demand for the pump to start, power is applied to the magnetic coil of the starter. Once the coil is energized, it moves the armature which is attached to the contacts. This closes the contacts and applies power to the motor.
Auxiliary Contacts
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Description/Function Most starters are built with one set of extra contacts. These contacts are called the auxiliary contacts and are normally open. They are used in a holding circuit if a start button is used, or to turn on or start equipment and/or status lights that are critical to the operation of the motor. This might include shaft oilers on line shaft turbines, pre-lube lines on line shaft turbines, chemical feed pumps, and status lights.
The standard symbol for the auxiliary contacts is the normally open contact symbol with the numbers 2 and 3 placed next to the contacts. Operation These contacts are often smaller in size than the power contacts in the motor starter. The numbers 2 and 3 are normally stamped in the metal next to the wire connections. When the coil of the motor starter is energized, these contacts are closed at the same time the other motor starter contacts are closed. In the drawing below, energizing the starter coil will cause the armature to move upward, closing the contacts. Special Consideration It is possible to purchase extra normally open and normally closed contacts that are fastened to the side of the motor starter and operated when the starter is operated. These extra contacts can be used to operate status lights, send a signal to a remote facility, or start or stop auxiliary equipment.
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Overload Protection
Description/Function Overload protection6 devices are designed to monitor the amperage to a motor. If the amperage exceeds a predetermined level and stays there for an adequate amount of time, the overload device will disconnect the power to the motor starter coil, stopping the motor. The connection between the overload circuit and the power circuit is physical and not electrical.
There are three common overload devices: the bimetallic, melting alloy, and the magnetic device.
Other Names Overload protection devices are also called heaters and thermal overload devices.
Symbols The following symbols are used to identify the two components of the overload device. (The heating element and the contacts):
Bimetallic Type
The bimetallic device is composed of a heating element that is made from two different metals fused at one end. This device is physically connected to a set of normally closed contacts. The contacts are in the control circuit. Operation-Bimetallic Strips-Normal Under normal operating conditions, current flows from the power-in terminal through the heating element (1) to the power-out terminal. The higher the amperage through a metallic device, the greater the heat that the device generates. The control circuit is connected to the control circuit terminals. Current flows from one of these terminals through the contact points (5) to the other terminal.
Overload protection
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The protection of electric motors with the use of a heat-sensitive device placed in a control circuit, typically called a heater.
Bimetallic Strips - Trip As current passes through the heater (1), the bimetallic strip (2), heats and bends. When it heats and bends sufficiently, the plastic plunger (3), is pressed against a spring which causes the metal strip (4), to snap away from the contacts (5), opening the control circuit. By opening these contacts, power is lost to the starter coil, it is de-energized, and the motor stops. To reset the overload, item (6), is pressed, causing the cam (7), to rotate, snapping the spring (4) back into position and closing the contacts at (5).
Once the overload has had an opportunity to cool, which takes a few minutes; it can be reset by pressing the reset button (6). When this button is depressed, a lever reconnects the contacts and the system is ready for operation. Caution: The power to the motor should be turned off before the reset button is pressed. The HO- A switch should be placed in the “Off” position.
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Melting Alloy
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Components The melting alloy device uses a heating coil in the power circuit. This coil is wrapped around a metal tube that contains solder. The shaft is attached to a ratchet wheel which is inserted in the solder. A small seal prevents the solder from leaking out of the tube. A latching assembly is held in place by the ratchet.
When an overload occurs, sufficient heat is generated by the coil to melt the solder. Once melted, the shaft on the ratchet wheel rotates, releasing the ratchet wheel. This in turn releases the overload contacts.
Once power has been disconnected, the solder will cool and reset. After it has reset, the reset button on the starter can be pressed and the overload contacts will be physically reconnected. It is important that power be shut off before the reset button is pressed.
Trip Time
At 600% of full load current (FLC), it takes 10 to 30 seconds for the heating element to heat sufficiently to cause the thermal overload devices to trip. More information on trip time is found in the Normal Operations lesson.
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REFERENCE PAGES
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Two-Wire Control
Two wires connect the control device (which could be a thermostat, float switch, limit switch or other contact device) to the magnetic starter. When the contacts of the control device close, they complete the starter coil circuit, causing it to connect the motor to the power lines. When the control device contacts open, the starter is de-energized, and the motor stops.
Two wire control provides low voltage release, but not low voltage protection. Wired as illustrated, the starter will function automatically in response to the direction of the control device, without the attention of an operator.
The dotted portion shown in the diagram represents the holding circuit interlock furnished on the starter, but it is not used in 2-wire control.
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Single-phase and 3-phase AC squirrel cage induction motors need some type of control circuit to initiate a start or stop function. The two types of ladder control circuits commonly used are the 2-wire control circuit and the 3-wire control circuit. The 2-wire control circuit uses maintained contact devices to control the magnetic motor starter. The 3-wire control circuit uses momentary contact devices to control the magnetic motor starter.
A typical 2-wire control circuit consists of a normally open maintained contact device that, when closed, energizes the coil of a magnetic motor starter, which, in turn, energizes the connected motor load. The 2-wire control circuit provides what is known as “low-voltage release.” In the event of a power failure, the magnetic motor starter will drop out. Once power is restored, the magnetic motor starter will automatically re-energize, provided that none of the maintained contact devices have changed state. This can be very advantageous in applications such as refrigeration or air conditioning where you do not need someone to restart the equipment after a power failure. However, it can be extremely dangerous in applications where equipment starts automatically, placing the operator in danger.
A typical 3-wire control circuit consists of a normally closed stop button (STOP), a normally open start button (START), a sealing contact (M), and the coil of a magnetic motor starter. When the normally open start button is pressed, the coil of the magnetic motor starter is energized. An auxiliary contact seals around the start button to provide a latched circuit. Pressing the normally closed stop button disrupts the circuit. The 3-wire control circuit provides what is known as “low-voltage protection.” In the event of a power failure, the magnetic motor starter will drop out. In this case, however, once power is restored the magnetic motor starter will not automatically re-energize. The operator must press the start button to initiate the sequence of operations once again.
Compared to the 2-wire control circuit, the 3-wire control circuit provides much more safety to the operator because machinery will not automatically start once power has been restored. In this circuit, multiple normally closed stop buttons are placed in series, and multiple normally open start buttons are placed in parallel to operate a magnetic motor starter. This is a common application of a 3-wire control circuit in which you need to start and stop the same motor from multiple locations within the facility. The 3-wire control circuit can be used in a variety of ways to meet specific circuit application.
The power circuit
Typically, single-phase motors can be started with full voltage across the line. Smaller horsepower 3-phase motors can also be started with full voltage across the line. However, larger horsepower 3-phase motors require you to use reduced voltage starting techniques.
The power circuit used in full voltage across-the-line starting consists of the overcurrent protective device (OCPD); the line conductors that terminate on the L1, L2, and L3 terminals; the magnetic motor starter or solid-state device; and the load conductors that terminate on the T1, T2, and T3 terminals. The power circuit is sized according to the voltage rating of the motor load (i.e., 115V, 200V, 230V, 460V, or 575V). The control circuit can operate at the same voltage as the power circuit as well as at lower voltages by using a machine tool transformer to step down the voltage to lower levels.
The electric utility typically has rules in place for how large a motor you can start across the line. Once the horsepower of a motor exceeds that rating, reduced voltage starting techniques must be used. Motors are inductive loads; therefore, they have very high starting currents in the range of 2.5 to 10 times the full load running current of the motor. This excessive inrush current (also called locked rotor current) causes voltage fluctuations on the power lines. You probably have observed the effect of inrush current whenever the lights in a building dip when an HVAC piece of equipment comes online. When this excessive inrush current is drawn from the voltage source for a few seconds, it causes a voltage drop. This voltage drop means a lower voltage is available to equipment, causing lighting fixtures, in particular, to reduce their light output.
Reduced voltage starters
There are primarily six styles of reduced voltage starters: primary resistor, reactor, autotransformer, part winding, wye-delta, and solid-state.
Primary resistor starters use resistors in series with the motor leads during the start function. Because this is now a series circuit, the applied voltage between the series resistor and the motor winding drops, causing a lower starting current. A timing relay operates a control relay whose contacts short the series resistors once startup is achieved.
Reactor starters operate in the same manner except reactors are used instead of resistors. The use of reactor-type starters are far less common today than in the past.
Autotransformer starters rely on tapped autotransformers for operation. The taps are typically set at 50%, 65%, or 80% of the available line voltage. Relying on the concept of “turn’s ratio” in a transformer, this type of starter allows for smaller currents on the line side as seen by the electric utility and larger currents on the load side as seen by the motor during startup. An autotransformer is different from a 2-winding transformer in that it does not provide electrical isolation between the primary and secondary windings. A step-up autotransformer is known as a “boosting” transformer, and a step-down autotransformer is referred to as a “bucking” transformer.
Let's review a simple example for illustration of this concept. A 1kVA transformer has a 240V primary and a 120V secondary voltage rating. The primary current is 4.17A at 240V, and the secondary current is 8.33A at 120V. By simply plugging in the values to the aforementioned equation, you can easily see the transformer has a 2:1 turn’s ratio. What does this mean? It simply means the voltage is stepped down by a factor of two while the current is stepped up by a factor of two. This principle allows the autotransformer-type starter to operate.
The part-winding starter is designed to work with a part-winding motor, which features two identical sets of windings. You can use 230V/460V dual-voltage motors, but you must do so with extreme caution. The concept is that a 230V/460V motor operated at 230V does so with its windings in parallel. Therefore, one half of the motor windings are in the circuit during startup. Then, a few seconds later, the other half of the motor windings are brought into the circuit. Serious problems can develop if the timing circuit does not connect the other half of the motor windings immediately after startup. For example, if the control circuit does not connect the delta windings of the motor back together after startup, the motor will fail.
A wye-delta starter operates by allowing the motor to be started in a wye configuration and run in a delta configuration. Using this design configuration allows the inrush current to be lower during startup while still maintaining a starting torque of approximately 33%, a percent rating of rated torque of the motor during startup. Open transition is an important concept to keep in mind with wye-delta starters because there will be a period of time between the wye configuration for start and the delta configuration for run when the motor windings will be disconnected. Closed transition starters overcome this disadvantage but at a much higher cost.
Solid-state starters are often called “soft start” starters because they rely on silicon-controlled rectifiers (SCRs) to accomplish the starting task. The SCR has three elements: anode, cathode, and gate. By applying a signal to the gate element at precisely the right time, you can control how much current the SCR will either pass or block during a cycle. This is known as phase control. The ability of this device to allow either partial conduction or full conduction during a cycle offers much flexibility to the design engineer. This capability allows for precise control of current to a motor during startup. Solid-state reduced voltage starters are in common use today because they interface well with variable-frequency drives (VFDs) and programmable logic controllers (PLCs).
A specialized segment of the electrical construction and maintenance industry, AC motor control is an area that requires specific knowledge in order to troubleshoot motors effectively and ensure smooth operations. This means gaining a clear understanding of ladder diagrams and ladder logic, which enable the automation that drives motors. The combination of input devices that either manually or automatically sense a condition — and the corresponding change in condition performed by the output device — make up the core of motor control. Let's take a closer look at what's involved in learning the symbolic language of motor control.
First, it's important to discuss the term “logic” for a moment. In the study of digital electronics, devices are used that operate in either an ON or OFF state. A specialized branch of mathematics called Boolean algebra analyzes this relationship with two numbers: a zero (representing the OFF state) or a one (representing the ON state). These two numbers comprise the binary number system.
The most common logic functions are the AND, OR, and NOT functions. Think of a single-pole light switch in your home that controls a 100W light bulb. The switch can either be off or on, thereby representing a zero in the off state and a one in the on state. Now imagine placing two single-pole switches in series to control the same 100W light bulb. In this condition, switch No. 1 and switch No. 2 have to both be on to light the 100W bulb. This is an example of an AND operation. Figure 1 represents the AND circuit just mentioned. Logic relates to ladder diagrams because input functions in series constitute an AND operation, while input functions in parallel constitute an OR operation.
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Fig.1. Switch S1 and S2 must both be closed for the light to come on.
You will encounter two types of ladder diagrams: the 2-wire control circuit and the 3-wire control circuit. The 2-wire control circuit is shown in Fig. 2. This circuit is used to start a motor for some industrial process. The components in a 2-wire control circuit are a maintained contact switching device (S1), a relay coil (M1), and the thermal overload relay contact (OL). The sequence of operations is fairly simple. When S1 is closed, the coil of magnetic motor starter M1 is energized and the motor starts, provided the running overload current is within the values of the overload relay OL. To stop the motor, S1 is simply opened.
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Fig.2. Typical 2-wire control circuit for starting a motor.
A 3-wire control circuit is shown in Fig. 3. Again, this circuit is used to start a motor for some industrial process. The components in a 3-wire control circuit are a momentary pushbutton (STOP), a momentary pushbutton (START), a normally open relay contact (M1), a relay coil (M1), and the thermal overload relay contact (OL). The sequence of operations here is a little more complex. When the start button is pressed, the coil of magnetic motor starter M1 is energized and the motor starts, provided the running overload current is within the values of the overload relay OL. However, there is one very important difference: A normally open contact of magnetic motor starter M1 seals around the start button to latch the circuit. To stop the motor, the STOP button is pressed, which, in turn, breaks the latch and de-energizes the coil of magnetic motor starter M1, stopping the motor.
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Fig.3. Typical 3-wire control circuit for starting a motor (M1).
What components make up a full-blown ladder diagram? There are several types of input and output devices. For the purpose of this article, we will focus on conventional electromechanical devices. See Fig. 4 for a list of common symbols used in ladder diagrams and motor control circuits.
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Fig.4. Common symbols used in ladder diagrams and motor control circuits.
Input devices can first be classified as momentary contact and maintained contact devices. Momentary contact devices are spring-loaded and are classified as normally open and normally closed devices. The designation “normally” refers to the state of the device in its resting position — when no external stimulus is acting upon it. The contact arrangement of switching devices can also be classified as SPST, SPDT, DPST, DPDT, 3PDT, etc. The first two letters refer to the number of “poles,” and the last two letters refer to the number of “throws.” For example, SPST refers to a single-pole, single-throw contact while 3PDT refers to a 3-pole, double-throw contact. A fractional manual motor starter useful for single-phase motors (1 hp and lower) can either be an SPST for 120V applications or DPST for 240V applications. A green start button is an example of a normally open momentary pushbutton, while a red stop button is an example of a normally closed momentary pushbutton.
Maintained contact devices are not spring-loaded. Instead, they remain in either an ON or OFF state. They can also be classified as normally open and normally closed. An emergency stop is an example of a maintained contact device.
Temperature-sensing devices commonly used in motor control applications are thermostats and thermocouples. A thermostat relies on the thermal expansion/contraction of a bimetal, while a thermocouple relies on a principle known as the Seebeck effect. Two dissimilar metal wires are joined together in a loop with one end being the hot junction; the other being the cold junction. A difference of potential is generated in the loop in response to temperature change. Each of these devices sense temperature change and then presents a contact closure for use in a control circuit.
Motion-sensing devices commonly used are photoelectric controls and proximity controls. Early versions of photoelectric controls had an incandescent lamp transmitter and a cadmium sulfide photocell receiver. Modern versions of the photoelectric control have pulsed infrared transmitters and solid-state photo-detector receivers. They work on the principle of beam interruption to sense motion and then present a contact closure to the control circuit.
Proximity controls sense motion when an object passes by the sensing target on the device. They can detect metallic as well as non-metallic objects. They operate on the principles of magnetism and capacitance, and then present a contact closure to the control circuit.
Limit switches are the most versatile devices in terms of motion detection. Available in a variety of operator mechanisms and contact arrangements, they work on the principle of physical contact between an object and the operator mechanism to present a contact closure to the control circuit.
The most commonly used liquid level sensing device is the float switch, which operates on the principle of buoyancy. The float is suspended in a liquid bath. As levels of the liquid rise and fall, the float moves. This movement presents a contact closure to the control circuit.
The most commonly used pressure-sensing device is the pressure switch. The diaphragm in a pressure switch monitors the change in pressure and presents a contact closure to the control circuit.
Other types of input devices include the foot switch, the selector switch, or even the contact of a control relay or a timing relay. These are all mechanical devices that present a contact closure to the control circuit.
Outputs of the control circuit can be relay coils, pilot indicating lights, or audible devices. To use the generic term “relay coil,” we need further classification into magnetic motor starter, contactor, and relay. A magnetic motor starter is a relay with a coil and contacts as well as running overload protection by means of thermal overload relays. Bi-metallic thermal overload relays are units made of a heater coil that heats a coil of wire to a specified temperature based on overload current, and a bi-metal unit that expands/contracts and operates a contact. Solder pot thermal overload relays use a similar heater coil and a eutectic solder that melts under overload conditions and correspondingly turns a ratchet wheel to operate a contact. The contact arrangement on the thermal overload relay is normally closed. However, it will open under excessive current conditions and de-energize the coil of the magnetic motor starter and consequently disconnect the motor. Contactors are also relays that switch high load currents but do not provide running overload protection via the thermal overload relay. Control relays are usually designed to switch small control circuit currents. Common types of timing relays are ON-delay (delay on operate), OFF-delay (delay on release), interval delay, and repeat cycle delay. Time delay relays are used for timing in a control circuit.
Pilot indicating lights are used to provide visual indication of a function or to verify that a certain operation is either on or off. Audible sounding devices are used to indicate trouble with a process or alert the user to a particular situation.
Now that you've been introduced to the more common input and output devices that make up a ladder diagram, next time we'll explore in more depth how 2- and 3- wire control circuits tie the control circuit with ladder diagrams into motor operation. Look for the next installment of “Motor Facts” in the June 2007 issue.
How to Wire a Motor Starter
by Keri Schieber,
AutomationDirect
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A motor starter is a combination of devices used to start, run, and stop an induction motor based on commands from an operator or a controller. In North America, an induction motor will typically operate at 230V or 460V, 3-phase, 60 Hz and has a control voltage of 115 VAC or 24 VDC. Several other combinations are possible in North America and other countries, and are easily derived from the methods shown in this document.
The motor starter must have at least two components to operate: a contactor to open or close the flow of energy to the motor, and an overload relay to protect the motor against thermal overload. Other devices for disconnecting and short-circuit protection may be needed, typically a circuit breaker or fuses. Short-circuit protection will not be shown in the examples that follow.
The contactor is a 3-pole electromechanical switch whose contacts are closed by applying voltage to a coil. When the coil is energized, the contacts are closed, and remain closed, until the coil is de-energized. The contactor is specifically designed for motor control, but can be used for other purposes such as resistive and lighting loads. Since a motor has inductance, the breaking of the current is more difficult so the contactor has both a horsepower and current rating that needs to be adhered to.
The overload relay is a device that has three current sensing elements and protects the motor from an overcurrent. Each phase going from the contactor to the motor passes through an overload relay current-sensing element. The overload relay has a selectable current setting based on the full load amp rating of the motor. If the overload current exceeds the setting of the relay for a sufficient length of time, a set of contacts opens to protect the motor from damage.
There are four basic wiring combinations :
a) Full-voltage non-reversing 3-phase motors.
b) Full-voltage reversing 3-phase motors
c) Single-phase motors
d) Wye-delta open transition 3-phase motors
You must supply a disconnect switch, proper sized wire, enclosures, terminal blocks and any other devices needed to complete your circuit.
WARNING! Use the instructions supplied for each specific device. Failure to do so may result in electrical shock or damage.
The following components will be used:
Main Contact Block [pic]
Overload Relays
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Aux. Contacts
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Normally Open Pushbutton Normally Closed Pushbutton
Full-voltage non-reversing 3-phase motors
The following diagram depicts 3-phase non-reversing motor control with 24 VDC control voltage and manual operation. We will use a contactor, an auxiliary contact block, an overload relay, a normally open start pushbutton, a normally closed stop pushbutton, and a power supply with a fuse. The start and stop circuits can also be controlled using PLC inputs and outputs.
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Full-voltage reversing 3-phase motors
This diagram is for 3-phase reversing motor control with 24 VDC control voltage. It uses two contactors, two auxiliary contact blocks, an overload relay, a mechanical interlock, two normally open start pushbuttons, a normally closed stop pushbutton, and a power supply with a fuse. The forward, reverse, and stop circuits can also be controlled using PLC inputs and outputs.
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Full-voltage single-phase motors
This diagram is for single-phase motor control. It uses a contactor, an overload relay, one auxiliary contact block, a normally open start pushbutton, a normally closed stop pushbutton, and a power supply with a fuse. The start and stop circuits can also be controlled using PLC inputs and outputs.
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Wye-delta open transition 3-phase motors
The following diagram is shown for 3-phase motor control of a delta-star connection. It uses three contactors, an overload relay, one auxiliary contact block, a normally open start pushbutton, a normally closed stop pushbutton, an on delay timer of 0-20 seconds and a power supply with a fuse. The start, stop, and timing circuits can also be controlled using PLC inputs and outputs.
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THIS INFORMATION PROVIDED BY TECHNICAL SUPPORT IS SUPPLIED "AS IS" WITHOUT A GUARANTEE OF ANY KIND. We do not guarantee that the data is suitable for your particular application, nor we do assume any responsibility for them in your application.
A typical 3-wire control circuit consists of a normally closed stop button (STOP), a normally open start button (START), a sealing contact (M), and the coil of a magnetic motor starter. When the normally open start button is pressed, the coil of the magnetic motor starter is energized. An auxiliary contact seals around the start button to provide a latched circuit. Pressing the normally closed stop button disrupts the circuit. The 3-wire control circuit provides what is known as “low-voltage protection.” In the event of a power failure, the magnetic motor starter will drop out. In this case, however, once power is restored the magnetic motor starter will not automatically re-energize. The operator must press the start button to initiate the sequence of operations once again.
Compared to the 2-wire control circuit, the 3-wire control circuit provides much more safety to the operator because machinery will not automatically start once power has been restored. In this circuit, multiple normally closed stop buttons are placed in series, and multiple normally open start buttons are placed in parallel to operate a magnetic motor starter. This is a common application of a 3-wire control circuit in which you need to start and stop the same motor from multiple locations within the facility. The 3-wire control circuit can be used in a variety of ways to meet specific circuit application.
A 3-wire control circuit is shown in Fig. 3. Again, this circuit is used to start a motor for some industrial process. The components in a 3-wire control circuit are a momentary pushbutton (STOP), a momentary pushbutton (START), a normally open relay contact (M1), a relay coil (M1), and the thermal overload relay contact (OL). The sequence of operations here is a little more complex. When the start button is pressed, the coil of magnetic motor starter M1 is energized and the motor starts, provided the running overload current is within the values of the overload relay OL. However, there is one very important difference: A normally open contact of magnetic motor starter M1 seals around the start button to latch the circuit. To stop the motor, the STOP button is pressed, which, in turn, breaks the latch and de-energizes the coil of magnetic motor starter M1, stopping the motor.
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Fig. 3. Typical 3-wire control circuit for starting a motor (M1).
What components make up a full-blown ladder diagram? There are several types of input and output devices. For the purpose of this article, we will focus on conventional electromechanical devices. See Fig. 4 for a list of common symbols used in ladder diagrams and motor control circuits.
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Fig. 4. Common symbols used in ladder diagrams and motor control circuits.
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Input devices can first be classified as momentary contact and maintained contact devices. Momentary contact devices are spring-loaded and are classified as normally open and normally closed devices. The designation “normally” refers to the state of the device in its resting position — when no external stimulus is acting upon it. The contact arrangement of switching devices can also be classified as SPST, SPDT, DPST, DPDT, 3PDT, etc. The first two letters refer to the number of “poles,” and the last two letters refer to the number of “throws.” For example, SPST refers to a single-pole, single-throw contact while 3PDT refers to a 3-pole, double-throw contact. A fractional manual motor starter useful for single-phase motors (1 hp and lower) can either be an SPST for 120V applications or DPST for 240V applications. A green start button is an example of a normally open momentary pushbutton, while a red stop button is an example of a normally closed momentary pushbutton.
Maintained contact devices are not spring-loaded. Instead, they remain in either an ON or OFF state. They can also be classified as normally open and normally closed. An emergency stop is an example of a maintained contact device.
Temperature-sensing devices commonly used in motor control applications are thermostats and thermocouples. A thermostat relies on the thermal expansion/contraction of a bimetal, while a thermocouple relies on a principle known as the Seebeck effect. Two dissimilar metal wires are joined together in a loop with one end being the hot junction; the other being the cold junction. A difference of potential is generated in the loop in response to temperature change. Each of these devices sense temperature change and then presents a contact closure for use in a control circuit.
Motion-sensing devices commonly used are photoelectric controls and proximity controls. Early versions of photoelectric controls had an incandescent lamp transmitter and a cadmium sulfide photocell receiver. Modern versions of the photoelectric control have pulsed infrared transmitters and solid-state photo-detector receivers. They work on the principle of beam interruption to sense motion and then present a contact closure to the control circuit.
Proximity controls sense motion when an object passes by the sensing target on the device. They can detect metallic as well as non-metallic objects. They operate on the principles of magnetism and capacitance, and then present a contact closure to the control circuit.
Limit switches are the most versatile devices in terms of motion detection. Available in a variety of operator mechanisms and contact arrangements, they work on the principle of physical contact between an object and the operator mechanism to present a contact closure to the control circuit.
The most commonly used liquid level sensing device is the float switch, which operates on the principle of buoyancy. The float is suspended in a liquid bath. As levels of the liquid rise and fall, the float moves. This movement presents a contact closure to the control circuit.
The most commonly used pressure-sensing device is the pressure switch. The diaphragm in a pressure switch monitors the change in pressure and presents a contact closure to the control circuit.
Other types of input devices include the foot switch, the selector switch, or even the contact of a control relay or a timing relay. These are all mechanical devices that present a contact closure to the control circuit.
Outputs of the control circuit can be relay coils, pilot indicating lights, or audible devices. To use the generic term “relay coil,” we need further classification into magnetic motor starter, contactor, and relay. A magnetic motor starter is a relay with a coil and contacts as well as running overload protection by means of thermal overload relays. Bi-metallic thermal overload relays are units made of a heater coil that heats a coil of wire to a specified temperature based on overload current, and a bi-metal unit that expands/contracts and operates a contact. Solder pot thermal overload relays use a similar heater coil and a eutectic solder that melts under overload conditions and correspondingly turns a ratchet wheel to operate a contact. The contact arrangement on the thermal overload relay is normally closed. However, it will open under excessive current conditions and de-energize the coil of the magnetic motor starter and consequently disconnect the motor. Contactors are also relays that switch high load currents but do not provide running overload protection via the thermal overload relay. Control relays are usually designed to switch small control circuit currents. Common types of timing relays are ON-delay (delay on operate), OFF-delay (delay on release), interval delay, and repeat cycle delay. Time delay relays are used for timing in a control circuit.
Pilot indicating lights are used to provide visual indication of a function or to verify that a certain operation is either on or off. Audible sounding devices are used to indicate trouble with a process or alert the user to a particular situation.
Now that you've been introduced to the more common input and output devices that make up a ladder diagram, next time we'll explore in more depth how 2- and 3- wire control circuits tie the control circuit with ladder diagrams into motor operation. Look for the next installment of “Motor Facts” in the June 2007 issue.
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ONE POWER FEED WITH MULTIPLE MOTORS
NAME: LEVEL: DATE:
CHECK LIST FOR 2 & 3 WIRE CONTROL PACKET
STEPS/TASKS
|1) The student completed all vocabulary associated with this learning guide to 80% accuracy. |10 | |
|2) The student completed all written work associated with this learning guide to 80% accuracy. |10 | |
|3) The student completed post-test associated with this learning guide to 80% accuracy. |10 | |
|4) The student completed project #1 to industry standards. |25 | |
|5) The student completed project #2 to industry standards. |25 | |
|6) The student completed project #3 to industry standards. |25 | |
|7) The student completed project #4 to industry standards. |25 | |
|8) The student completed project #5 to industry standards. |25 | |
|9) The student completed project #6 to industry standards. |25 | |
|4) The student completed project #7 to industry standards. |25 | |
|5) The student completed project #8 to industry standards. |25 | |
|6) The student completed project #9 to industry standards. |25 | |
|7) The student completed project #10 to industry standards. |25 | |
|8) The student completed project #11 to industry standards. |25 | |
|9) The student completed project #12 to industry standards. |25 | |
|10) The student completed project #13 to industry standards. |25 | |
|11) The student completed project #14 to industry standards. |25 | |
|12) The student used the FLIR to check contacts in motor starter for each product. |60 | |
|13) The student completed the identification worksheets associated with this learning guide to 80% accuracy. |10 | |
|Total Points |450 | |
*ALL STEPS/TASKS MUST MEET THE STANDARDS IN ORDER TO ACHIEVE MASTERY.*
COMMENTS:
INSTRUCTOR SIGNATURE: DATE:
NAME: LEVEL: DATE:
1. What are some advantages of using two wire controls?
2. What is a possible safety hazard of two wire control circuits?
3. What is the advantage of a three wire control system compared to a two wire system?
4. Explain how holding contacts operate:
5. Explain what would happen if a forward and reversing starter was not interlocked:
6. Explain how the symbols are drawn in the schematic diagram…then explain why a control circuit is drawn in a “ladder” type schematic:
Name: Date:
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NAME: DATE:
ELECTRIC MOTOR CONTROLS POST TEST
Multiple Choice
Identify the letter of the choice that best completes the statement or answers the question.
____ 1. In a particular application, the motor, machine, and motor controller are
|a. |unrelated |c. |AC powered |
|b. |DC powered |d. |interrelated |
____ 2. Which is not an organization of concern to those working with motor controls?
|a. |UL |c. |NEMA |
|b. |NASA |d. |OSHA |
____ 3. Which is not a factor to consider when selecting and installing motor control components?
|a. |Jumping |c. |Reversing |
|b. |Stopping |d. |Running |
____ 4. A manual control is one whose operation is accomplished by
|a. |electrical means |c. |mechanical means |
|b. |electronic means |d. |none of the above |
____ 5. The use of a safety switch requires
|a. |electrical operation |c. |pneumatic operation |
|b. |manual operation |d. |hydraulic operation |
____ 6. Drum controllers are
|a. |rotary switching devices |c. |automatic switching devices |
|b. |linear switching devices |d. |dc switching devices |
____ 7. Magnetic switch control is accomplished by
|a. |mechanical means |c. |electronic means |
|b. |electrical means |d. |electromagnetic means |
____ 8. A two-wire pilot device often used as an overtravel stop for machines, equipment, and products in process is a
|a. |pressure switch |c. |float switch |
|b. |limit switch |d. |timer switch |
____ 9. A pilot device sensitive to temperature changes is the
|a. |limit switch |c. |pressure switch |
|b. |timer switch |d. |thermostat |
____ 10. When the final operation of one or more motors depends upon the electrical position of each individual control device
|a. |an interlocking system is usually in place |
|b. |a timing system is usually in place |
|c. |a counting system is usually in place |
|d. |a jogging system is usually in place |
____ 11. Using counter torque to brake a dc motor is known as
|a. |plugging |c. |dynamic braking |
|b. |static braking |d. |counter braking |
____ 12. When a load must be held, such as with a crane or hoist,
|a. |mechanical braking is required |c. |counter torque braking is required |
|b. |dynamic braking is required |d. |induction braking is required |
____ 13. To handle frequent reversals of motor rotation,
|a. |large diodes are often inserted in line with the motor |
|b. |an ac voltage must be used |
|c. |a heavy duty drum switch-controller is often used |
|d. |a dc voltage must be used |
____ 14. The simplest method of changing motor speed is by
|a. |reversing voltage |c. |reducing or increasing voltage |
|b. |rectifying voltage |d. |gearing |
____ 15. In a varying speed motor application, such as with a crane or hoist,
|a. |the motor speed slows as the load increases and speeds up as the load decreases |
|b. |the motor speed slows as the load decreases and speeds up as the load increases |
|c. |the motor reverses with an increase in load |
|d. |the motor speed is uneffected by the load |
____ 16. Another term for overload protection is
|a. |phase protection |c. |overspeed protection |
|b. |running protection |d. |overtravel protection |
____ 17. When a motor’s torque decreases, possibly to the point of motor “stall,” the condition is known as
|a. |run away torque |c. |drop torque |
|b. |reverse torque |d. |breakdown torque |
____ 18. A typical overcurrent device is the
|a. |fuse |c. |toggle switch |
|b. |diode |d. |limit switch |
____ 19. Which of the following timers and timing systems are not used for motor acceleration control?
|a. |Pneumatic timing |c. |Inductive timing |
|b. |Capacitor timing |d. |Electronic timing |
____ 20. The primary tool used to trace a circuit is the
|a. |VOM |c. |Logic Probe |
|b. |VTVM |d. |Oscilloscope |
| | | | |
NAME: LEVEL: DATE:
Identify each symbol list below:
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Residential & Industrial Electricity
K-W-L WORKSHEET
NAME: LEVEL: DATE:
ARTICLE TITLE:
TIME START: TIME FINISH:
| | |
|K What do I already KNOW | |
|about this topic? | |
| | |
|W What do I WANT to know | |
|about this topic? | |
| | |
|L What did I LEARN after | |
|reading ABOUT this | |
|topic? | |
I checked the following before reading:
➢ Headlines and Subheadings
➢ Italic, Bold, and Underlined words
➢ Pictures, Tables, and Graphs
➢ Questions or other key information
I made predictions AFTER previewing the article.
Comments:
• Instructor Signature:
NAME: DATE:
ELECTRIC MOTOR CONTROLS PRE TEST
Multiple Choice
Identify the letter of the choice that best completes the statement or answers the question.
____ 1. In a particular application, the motor, machine, and motor controller are
|a. |unrelated |c. |AC powered |
|b. |DC powered |d. |interrelated |
____ 2. Which is not an organization of concern to those working with motor controls?
|a. |UL |c. |NEMA |
|b. |NASA |d. |OSHA |
____ 3. Which is not a factor to consider when selecting and installing motor control components?
|a. |Jumping |c. |Reversing |
|b. |Stopping |d. |Running |
____ 4. A manual control is one whose operation is accomplished by
|a. |electrical means |c. |mechanical means |
|b. |electronic means |d. |none of the above |
____ 5. The use of a safety switch requires
|a. |electrical operation |c. |pneumatic operation |
|b. |manual operation |d. |hydraulic operation |
____ 6. Drum controllers are
|a. |rotary switching devices |c. |automatic switching devices |
|b. |linear switching devices |d. |dc switching devices |
____ 7. Magnetic switch control is accomplished by
|a. |mechanical means |c. |electronic means |
|b. |electrical means |d. |electromagnetic means |
____ 8. A two-wire pilot device often used as an over travel stop for machines, equipment, and products in process is a
|a. |pressure switch |c. |float switch |
|b. |limit switch |d. |timer switch |
____ 9. A pilot device sensitive to temperature changes is the
|a. |limit switch |c. |pressure switch |
|b. |timer switch |d. |thermostat |
____ 10. When the final operation of one or more motors depends upon the electrical position of each individual control device
|a. |an interlocking system is usually in place |
|b. |a timing system is usually in place |
|c. |a counting system is usually in place |
|d. |a jogging system is usually in place |
____ 11. Using counter torque to brake a dc motor is known as
|a. |plugging |c. |dynamic braking |
|b. |static braking |d. |counter braking |
____ 12. When a load must be held, such as with a crane or hoist,
|a. |mechanical braking is required |c. |counter torque braking is required |
|b. |dynamic braking is required |d. |induction braking is required |
____ 13. To handle frequent reversals of motor rotation,
|a. |large diodes are often inserted in line with the motor |
|b. |an ac voltage must be used |
|c. |a heavy duty drum switch-controller is often used |
|d. |a dc voltage must be used |
____ 14. The simplest method of changing motor speed is by
|a. |reversing voltage |c. |reducing or increasing voltage |
|b. |rectifying voltage |d. |gearing |
____ 15. In a varying speed motor application, such as with a crane or hoist,
|a. |the motor speed slows as the load increases and speeds up as the load decreases |
|b. |the motor speed slows as the load decreases and speeds up as the load increases |
|c. |the motor reverses with an increase in load |
|d. |the motor speed is unaffected by the load |
____ 16. Another term for overload protection is
|a. |phase protection |c. |over speed protection |
|b. |running protection |d. |over travel protection |
____ 17. When a motor’s torque decreases, possibly to the point of motor “stall,” the condition is known as
|a. |run away torque |c. |drop torque |
|b. |reverse torque |d. |breakdown torque |
____ 18. A typical overcurrent device is the
|a. |fuse |c. |toggle switch |
|b. |diode |d. |limit switch |
____ 19. Which of the following timers and timing systems are not used for motor acceleration control?
|a. |Pneumatic timing |c. |Inductive timing |
|b. |Capacitor timing |d. |Electronic timing |
____ 20. The primary tool used to trace a circuit is the
|a. |VOM |c. |Logic Probe |
|b. |VTVM |d. |Oscilloscope |
-----------------------
Schuylkill Technology Center-
South Campus
15 Maple Avenue
Marlin, Pennsylvania 17951
(570) 544-4748
Name:
Date:
Learning Guide Due Date:
Pre Test Due Date:
Post Test Due Date:
RESIDENTIAL & INDUSTRIAL ELECTRICITY
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Level 3
Task 1900
POS # 1900
Total Hours-92
Level(s)-3
Project(s) are ADDITIONAL to this learning guide.
Project(s) are ADDITIONAL to this learning guide.
*CORE CURRICULUM STANDARDS*
*ACADEMIC STANDARDS*
[pic]
Points
Available
Points
Earned
Correct/Out of 450
Grade Percentage
Check One Percentage Task Grade
← Below Basic 0%-69% 0-6
← Basic 70%-85% 7
← Competent 86%-92% 8-9
← Advanced 93%-100% 10
Correct/Out of
Grade Percentage
Check One Percentage Task Grade
← Below Basic 0%-69% 0-6
← Basic 70%-85% 7
← Competent 86%-92% 8-9
← Advanced 93%-100% 10
Correct/Out of
Grade Percentage
Check One Percentage Task Grade
← Below Basic 0%-69% 0-6
← Basic 70%-85% 7
← Competent 86%-92% 8-9
← Advanced 93%-100% 10
................
................
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