THIS DOCUMENT PROVIDES AN EXPLANATION
EXPLANATION OF MANY OF THE TERMS
THAT ARE ASSOCIATED
WITH THE SPECIFICATION AND USE OF MOTORS
Actuator: A device that creates mechanical motion by converting various forms of energy to rotating or linear mechanical energy.
AC/DC Formulas:
|AC/DC Formulas |
|To Find |Direct Current |AC / 1Phase |AC / 1Phase |AC 3 Phase |
| | |115V, 120V |208V,230V, 240V |All Voltages |
|Amps when |HP x 746 |HP x 746 |HP x 746 |HP x 746 |
|Horsepower is known |V x Eff |V x Eff X Pf |V x Eff x Pf |1.73 x V x Eff x Pf |
|Amps when |kW x 1000 |kW x 1000 |kW x 1000 |kW x 1000 |
|kiloWatts is known |V |V x Pf |V x Pf |1.73 x V x Pf |
|Amps when | |kVA x 1000 |kVA x 1000 |kVA x 1000 |
|kVA is known | |V |V |1.73 x V |
|kiloWatts |I x V |I x V x Pf |I x V x Pf |I x V x 1.73 Pf |
| |1000 |1000 |1000 |1000 |
|kVA | |I x V |I x V |I x V x 1.73 |
| | |1000 |1000 |1000 |
|Shaft Horsepower |I x V x Eff |I x V x Eff x Pf |I x V x Eff x Pf |I x V x Eff x 1.73 x Pf |
|(Output) |746 |746 |746 |746 |
V = Voltage / I = Amps /W = Watts / Pf = Power Factor / Eff = Efficiency / HP = Horsepower
kVA = kiloVoltAmps / kW = kiloWatts
Air-Over (AO): Motors for fan or blower service that are cooled by the air stream from the fan or blower. Motor is located in the air stream to cool the motor.
Alternating Current (AC): The standard power supply available from local electric utility companies.
Ambient Temperature (AMB): The temperature of the space (air) around the motor. Most motors are designed to operate in an ambient not to exceed 40°C (104°F).
Ampere (Amp): The standard unit of electric current. The current produced by a pressure of one volt in a circuit having a resistance of one ohm.
Armature: The rotating part of a brush type direct current (DC) motor. In an induction motor, the rotating part is called a rotor.
Bearings: Sleeve: Common in home appliance motors. Normally used in blower applications where noise levels low are important. Ball: Used when high shaft load (radial or axial thrust load) capacity is required. Ball bearings are usually used in industrial and agricultural motors. Occasionally, roller bearings may be used on larger horsepower motors for maximum radial load capacity.
Brush: Current conducting material in a DC motor, usually graphite, or a combination of graphite and other materials. The brush rides on the commutator of a motor and forms an electrical connection between the armature and the power source.
Canadian Standards Association (CSA): The agency that sets safety standards for motors and other electric equipment used in Canada.
Capacitance: As the measure of electrical storage potential of a capacitor, the unit of capacitance is the farad, but typical values are expressed in microfarads (MFD).
Capacitor: A device that stores electrical energy.
Capacitor Start Motor: Or more specifically, Capacitor-Start, induction-run, this provides high starting and break-down torque, medium starting current. Used on hard starting applications such as compressors, positive displacement pumps, farm equipment, etc.
Capacitor-Start, capacitor-run: Similar to capacitor-start, except have higher efficiency, these are generally used in higher HP single phase ratings.
Centrifugal Start Switch: A mechanism that disconnects the starting circuit (start winding) when the rotor reaches approximately 75% of operating speed (usually in two or three seconds).
Commutator: The part of a DC motor armature that causes the electrical current to be switched to various armature windings. Properly sequenced switching creates the motor torque. The commutator also provides the means to transmit the electrical current to the moving armature through the brushes that ride on the commutator.
Design Classification: See Induction Motor Design.
DC Current: The power supply available from batteries, generators (not alternators), or a rectified source used for special purpose applications.
Duty Cycle: The relationship between operating time and the resting time of an electric motor.
Continuous Duty: The operation of loads for over one hour.
Intermittent Duty: The operation during alternate periods of load and rest.
Efficiency: The ration of the useful work performed and the energy expended in producing it.
Enclosure: Term used to describe the motor housing.
ODP: Open Drip Proof, housing has openings in end shields and shell to allow air to cool the motor, normally used in "clean" applications.
TEFC: Totally Enclosed Fan Cooled, housing has no openings. Motor is cooled by an external fan on the non-drive end of the motor shaft. (Motor is not air tight or water proof), normally used in dirty, oily or damp applications.
TENV: Totally Enclosed Non-Ventilated. Not equipped with an external cooling fan, depends on convection air for cooling.
TEAO: Totally Enclosed Air Over. Air flow from the driven or external device provides cooling air flow of the motor.
End Shield: Also referred to as "End Bell", this is the part of the motor that houses the bearing supporting the rotor and acts as a protective guard to the internal parts of the motor.
Equivalence of Power: The above discussion indicated that electrical power (in Watts) is the same as mechanical power (in HP) with only a conversion factor, (746 Watts per HP), being required to convert between the different units. Indeed, all types of power are equivalent, and the units we speak of are a matter of convention based on the context. Consider a steam boiler. If used for space heating it would be rated in BTU/Hour; for electrical generation, in kilo Watts; for driving steam engines, in HP; and for supplying steam for process consumption, in pounds per hour at a given pressure. All these are equivalent, and the appropriate conversion factors can be found in many texts.
The definition of power is the time rate of doing work. Work is done by a force applied to move an object through a distance. In English units, work is expressed in foot-pounds. This is known as mechanical work and is equivalent to all other work. One HP is the rate of work done when lifting 33,000 pounds at the rate of one foot per minute or 33,000 foot-pounds/minute. For example, consider a geared-down electric motor whose pulley winds up a rope attached to a block and tackle. (The block and tackle is frictionless in this example). The block and tackle is lifting a weight of 33,000 pounds. If the system is sized so that the weight is lifted one foot per minute, then the motor is putting out one HP.
The unit known as the horsepower was defined in the 1790's by James Watt. Up until the later eighteenth century steam engines had been used primarily to drain mines or otherwise pump water. They were rated at so many gallons of water lifted per hour to a given height. As engines started to be used for powering factories, a more directly useful rating was required. Watt estimated what a horse could do and first arrived at 32,400 foot-pounds/minute. The unit was later taken to be 33,000 foot-pounds/minute.
Excitation: The act of creating magnetic lines of force from a motor winding by applying voltage.
Field: The stationary part of a DC motor, commonly consisting of permanent magnets; it is also used to describe the stator of an AC motor.
Frame: Standardized motor mounting and shaft dimensions as established by NEMA or IEC. (See also Induction Motor Design)
Frequency: An expression of how often a complete cycle occurs. Cycles per second describe how many complete cycles occur in a given time increment. Hertz (Hz) has been adopted to describe cycles per second so that time as well as number of cycles is specified. The standard power supply in North and South America is 60Hz. Most of the rest of the world has 50Hz power.
Full Load Amperes (FLA): Line current (Amperage) drawn by a motor when operating at rated load and voltage on motor nameplate. This is important for proper cable size selection, and motor starter or drive selection; also known as full load current. (FLC)
Full Load Torque: The torque a motor produces at its rated horsepower and full-load speed.
Fuse: A piece of metal, connected in the circuit to be protected that melts and interrupts the circuit when excess current flows.
Generator: Any machine that converts mechanical energy into electrical energy.
Hertz: Frequency, in cycles per second, of AC power, named after H.R. Hertz, the German scientist who discovered electrical oscillations.
High Voltage Test: Application of a voltage greater than the working voltage to test the adequacy of motor insulation, often referred to as high potential test or "hi-pot".
Horsepower (HP): A measure of the rate of work. 33,000 pounds lifted one foot in one minute, or 550 pounds lifted one foot in one second. Exactly 746 watts of electrical power equals one horsepower.
International Electrotechnical Commission (IEC): The worldwide organization that promotes international unification of standards or norms. Its formal decisions on technical matters express, as nearly as possible, an international consensus.
Impedance: The total opposition in an electric circuit to the flow of an alternating current, expressed in ohms.
Induction Motor: The simplest and most rugged electric motor, it consists of a wound stator and a rotor assembly. The AC induction motor is named because the electric current flowing in its secondary member (the rotor) is induced by the alternating current flowing in its primary member (stator). The power supply is connected only to the stator. The combined electromagnetic effects of the two currents produce the force to create rotation.
Induction Motor Design: Has a major effect on the behavior and performance of an induction motor.
Induction Motor Stator Design.
The stator is the outer body of the motor which houses the driven windings on an iron core. In a single speed three phase motor design, the standard stator has three windings, while a single phase motor typically has two windings. The stator core is made up of a stack of round pre-punched laminations pressed into a frame which may be made of aluminium or cast iron. The laminations are basically round with a round hole inside through which the rotor is positioned. The inner surface of the stator is made up of a number of deep slots or grooves right around the stator. It is into these slots that the windings are positioned. The arrangement of the windings or coils within the stator determines the number of poles that the motor has. A standard bar magnet has two poles, generally known as North and South. Likewise, an electromagnet also has a North and a South pole. As the induction motor Stator is essentially like one or more electromagnets depending on the stator windings, it also has poles in multiples of two. i.e. 2 pole, 4 pole, 6 pole etc. The winding configuration, slot configuration and lamination steel all have an effect on the performance of the motor. The voltage rating of the motor is determined by the number of turns on the stator and the power rating of the motor is determined by the losses which comprise copper loss and iron loss, and the ability of the motor to dissipate the heat generated by these losses.
The stator design determines the rated speed of the motor and most of the full load, full speed characteristics.
Induction Motor Rotor Design.
The Rotor comprises a cylinder made up of round laminations pressed onto the motor shaft, and a number of short-circuited windings. The rotor windings are made up of rotor bars passed through the rotor, from one end to the other, around the surface of the rotor. The bars protrude beyond the rotor and are connected together by a shorting ring at each end. The bars are usually made of aluminium or copper, but sometimes made of brass. The position relative to the surface of the rotor, shape, cross sectional area and material of the bars determine the rotor characteristics. Essentially, the rotor windings exhibit inductance and resistance, and these characteristics can effectively be dependant on the frequency of the current flowing in the rotor. A bar with a large cross sectional area will exhibit a low resistance, while a bar of a small cross sectional area will exhibit a high resistance. Likewise a copper bar will have a low resistance compared to a brass bar of equal proportions. Positioning the bar deeper into the rotor, increases the amount of iron around the bar, and consequently increases the inductance exhibited by the rotor. The impedance of the bar is made up of both resistance and inductance, and so two bars of equal dimensions will exhibit different A.C. impedance depending on their position relative to the surface of the rotor. A thin bar which is inserted radialy into the rotor, with one edge near the surface of the rotor and the other edge towards the shaft, will effectively change in resistance as the frequency of the current changes. This is because the A.C. impedance of the outer portion of the bar is lower than the inner impedance at high frequencies lifting the effective impedance of the bar relative to the impedance of the bar at low frequencies where the impedance of both edges of the bar will be lower and almost equal. The rotor design determines the starting characteristics.
Induction Motor Equivalent Circuit.
The induction motor can be treated essentially as a transformer for analysis. The induction motor has stator leakage reactance, stator copper loss elements as series components, and iron loss and magnetising inductance as shunt elements. The rotor circuit likewise has rotor leakage reactance, rotor copper (aluminium) loss and shaft power as series elements. The transformer in the centre of the equivalent circuit can be eliminated by adjusting the values of the rotor components in accordance with the effective turn’s ratio of the transformer. From the equivalent circuit and a basic knowledge of the operation of the induction motor, it can be seen that the magnetising current component and the iron loss of the motor are voltage dependant, and not load dependant. Additionally, the full voltage starting current of a particular motor is voltage and speed dependant, but not load dependant. The magnetising current varies depending on the design of the motor. For small motors, the magnetising current may be as high as 60%, but for large two pole motors, the magnetising current is more typically 20 - 25%. At the design voltage, the iron is typically near saturation, so the iron loss and magnetising current do not vary linearly with voltage with small increases in voltage resulting in a high increase in magnetising current and iron loss.
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Induction Motor Starting Characteristics.
In order to perform useful work, the induction motor must be started from rest and both the motor and load accelerated up to full speed. Typically, this is done by relying on the high slip characteristics of the motor and enabling it to provide the acceleration torque. Induction motors at rest, appear just like a short circuited transformer, and if connected to the full supply voltage, draw a very high current known as the "Locked Rotor Current". They also produce torque which is known as the "Locked Rotor Torque". The Locked Rotor Torque (LRT) and the Locked Rotor Current (LRC) are a function of the terminal voltage to the motor, and the motor design. As the motor accelerates, both the torque and the current will tend to alter with rotor speed if the voltage is maintained constant. The starting current of a motor, with a fixed voltage, will drop very slowly as the motor accelerates and will only begin to fall significantly when the motor has reached at least 80% full speed. The actual curves for induction motors can vary considerably between designs, but the general trend is for a high current until the motor has almost reached full speed. The LRC of a motor can range from 500% Full Load Current (FLC) to as high as 1400% FLC. Typically, good motors fall in the range of 550% to 750% FLC.
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The starting torque of an induction motor starting with a fixed voltage, will drop a little to the minimum torque known as the pull up torque as the motor accelerates, and then rise to a maximum torque known as the breakdown or pull out torque at almost full speed and then drop to zero at synchronous speed. The curve of start torque against rotor speed is dependant on the terminal voltage and the motor/rotor design. The LRT of an induction motor can vary from as low as 60% Full Load Torque (FLT) to as high as 350% FLT. The pull-up torque can be as low as 40% FLT and the breakdown torque can be as high as 350% FLT. Typical LRTs for medium to large motors are in the order of 120% FLT to 280% FLT. The power factor of the motor at start is typically 0.1 - 0.25, rising to a maximum as the motor accelerates, and then falling again as the motor approaches full speed.
A motor which exhibits a high starting current, i.e. 850% will generally produce a low starting torque, whereas a motor which exhibits a low starting current will usually produce a high starting torque. This is the reverse of what is generally expected. The induction motor operates due to the torque developed by the interaction of the stator field and the rotor field. Both of these fields are due to currents which have resistive or in phase components and reactive or out of phase components. The torque developed is dependant on the interaction of the in phase components and consequently is related to the I2R of the rotor. A low rotor resistance will result in the current being controlled by the inductive component of the circuit, yielding a high out of phase current and a low torque.
Figures for the locked rotor current and locked rotor torque are almost always quoted in motor data, and certainly are readily available for induction motors. Some manufactures have been known to include this information on the motor name plate. One additional parameter which would be of tremendous use in data sheets for those who are engineering motor starting applications, is the starting efficiency of the motor. By the starting efficiency of the motor, I refer to the ability of the motor to convert Amps into Newton meters. This is a concept not generally recognised within the trade, but one which is extremely useful when comparing induction motors. The easiest means of developing a meaningful figure of merit, is to take the locked rotor torque of the motor (as a percentage of the full load torque) and divide it by the locked rotor current of the motor (as a percentage of the full load current).
|Starting efficiency = |Locked Rotor Torque |
| |Locked Rotor Current |
If the terminal voltage to the motor is reduced while it is starting, the current drawn by the motor will be reduced proportionally. The torque developed by the motor is proportional to the current squared, and so a reduction in starting voltage will result in a reduction in starting current and a greater reduction in starting torque. If the start voltage applied to a motor is halved, the start torque will be a quarter; likewise a start voltage of one third will result in a start torque of one ninth.
Induction Motor Running Characteristics.
Once the motor is up to speed, it operates at low slip, at a speed determined by the number of stator poles. The frequency of the current flowing in the rotor is very low. Typically, the full load slip for a standard cage induction motor is less than 5%. The actual full load slip of a particular motor is dependant on the motor design with typical full load speeds of four pole induction motor varying between 1420 and 1480 RPM at 50 Hz. The synchronous speed of a four pole machine at 50 Hz is 1500 RPM and at 60 Hz a four pole machine has a synchronous speed of 1800 RPM. The induction motor draws a magnetising current while it is operating. The magnetising current is independent of the load on the machine, but is dependant on the design of the stator and the stator voltage. The actual magnetising current of an induction motor can vary from as low as 20% FLC for large two pole machines to as high as 60% for small eight pole machines. The tendency is for large machines and high speed machines to exhibit a low magnetising current, while low speed machines and small machines exhibit a high magnetising current. A typical medium sized four pole machine has a magnetising current of about 33% FLC. A low magnetising current indicates a low iron loss, while a high magnetising current indicates an increase in iron loss and a resultant reduction in operating efficiency. The resistive component of the current drawn by the motor while operating, changes with load, being primarily load current with a small current for losses. If the motor is operated at minimum load, i.e. open shaft, the current drawn by the motor is primarily magnetising current and is almost purely inductive. Being an inductive current, the power factor is very low, typically as low as 0.1. As the shaft load on the motor is increased, the resistive component of the current begins to rise. The average current will noticeably begin to rise when the load current approaches the magnetising current in magnitude. As the load current increases, the magnetising current remains the same and so the power factor of the motor will improve. The full load power factor of an induction motor can vary from 0.5 for a small low speed motor up to 0.9 for a large high speed machine. The losses of an induction motor comprise: iron loss, copper loss, windage loss and frictional loss. The iron loss, windage loss and frictional losses are all essentially load independent, but the copper loss is proportional to the square of the stator current. Typically the efficiency of an induction motor is highest at 3/4 load and varies from less than 60% for small low speed motors to greater than 92% for large high speed motors. Operating power factor and efficiencies are generally quoted on the motor data sheets.
Induction Motor Design Classification.
There are a number of design/performance classifications which are somewhat uniformly accepted by different standards organisations. These design classifications apply particularly to the rotor design and hence affect the starting characteristics of the motors. The two major classifications of relevance here are Design A, and Design B. Design A motors have a shallow bar rotor, and are characterised by a very high starting current and a low starting torque. Typical values are 850% current and 120% torque. Shallow bar motors usually have a low slip, i.e. 1480 RPM. Design B motors have a deeper bar rotor and are characterised by medium start current and medium starting torque. Typical Design B values are 650% current and 180% torque. The slip exhibited by Design B motors is usually greater than the equivalent Design A motors. i.e. 1440 RPM. Design F motors are often known as Fan motors having a high rotor resistance and high slip characteristics. The high rotor resistance enables the fan motor to be used in a variable speed application where the speed is reduced by reducing the voltage. Design F motors are used primarily in fan control applications with the motor mounted in the air flow. These are often rated as AOM or Air Over Motor machines.
Induction Motor Frame Classification.
Induction motors come in two major frame types, these being Totally Enclosed Forced air Cooled (TEFC), and Drip proof. The TEFC motor is totally enclosed in either an aluminium or cast iron frame with cooling fins running longitudinally on the frame. A fan is fitted externally with a cover to blow air along the fins and provide the cooling. These motors are often installed outside in the elements with no additional protection and so are typically designed to IP55 or better. Drip proof motors use internal cooling with the cooling air drawn through the windings. They are normally vented at both ends with an internal fan. This can lead to more efficient cooling, but requires that the environment is clean and dry to prevent insulation degradation from dust, dirt and moisture. Drip proof motors are typically IP22 or IP23.
Induction Motor Temperature Classification.
There are two main temperature classifications applied to induction motors. These being Class B and Class F. The temperature class refers to the maximum allowable temperature rise of the motor windings at a specified maximum coolant temperature. Class B motors are rated to operate with a maximum coolant temperature of 40 degrees C and a maximum winding temperature rise of 80 degrees C. This leads to a maximum winding temperature of 120 degrees C. Class F motors are typically rated to operate with a maximum coolant temperature of 40 degrees C and a maximum temperature rise of 100 degrees C resulting in a potential maximum winding temperature of 140 degrees C. Operating at rated load, but reduced cooling temperatures gives an improved safety margin and increased tolerance for operation under an overload condition. If the coolant temperature is elevated above 40 degrees C then the motor must be derated to avoid premature failure. Note: Some Class F motors are designed for a maximum coolant temperature of 60 degrees C, and so there is no derating necessary up to this temperature. Operating a motor beyond its maximum limit will not cause an immediate failure, rather a decrease in the life expectancy of that motor. A common rule of thumb applied to insulation degradation, is that for every ten degree C rise in temperature, the expected life span is halved. Note: the power dissipated in the windings is the copper loss which is proportional to the square of the current, so an increase of 10% in the current drawn, will give an increase of 21% in the copper loss, and therefore an increase of 21% in the temperature rise which is 16.8 degrees C for a Class B motor, and 21 degrees C for a Class F motor. This approximates to the life being reduced to a quarter of that expected if the coolant is at 40 degrees C. Likewise operating the motor in an environment of 50 degrees C at rated load will elevate the insulation temperature by 10 degrees C and halve the life expectancy of the motor.
Insulation: In motors, classified by maximum allowable operating temperature. NEMA Classifications include:
Class A=105°C, Class B=130°C, Class F=155°C and Class H=180°C.
Integral Horsepower Motor: A motor rated one horsepower or larger at 1800RPM. By NEMA definitions, this is any motor having a three digit frame, for example 143T.
Kilowatt: A unit of power equal to 1000 watts and approximately equal to 1.34 horsepower.
Load: The work required of a motor to drive attached equipment; this expressed in horsepower or torque at a certain motor speed.
Locked Rotor Current: Measured current with the rotor locked and with rated voltage and frequency applied to the motor.
Locked Rotor Torque: Measured torque with the rotor locked and with rated voltage and frequency applied to the motor.
Magnetic Polarity: Distinguishes the location of North and South Poles of a magnet. Magnetic lines of force emanate from the North Pole of a magnet and terminate at the South Pole.
Motor Electrical Ratings: In addition to horsepower, a motor's nameplate will specify the operating Voltage and the current draw at rated horsepower. You have to operate the motor at close to its specified Voltage, but the current (Amps) will vary with the actual load. A lightly loaded motor will draw less current. However, since the power factor and efficiency (both described below) decrease with decreasing load, the current will not drop as much as expected.
Since all types of power (mechanical, electrical) are equivalent, these electrical ratings can be related to the horsepower rating. Most generally, electrical power in Watts is equal to the Voltage in Volts times the current in Amps. 1 horsepower (mechanical power) is approximately equal to 746 Watts (electrical power.) Thus a perfect motor (the type used in physics classes, which do not exist in the real world) rated at 120 Volts and 1 HP would draw about 6.2 Amps ([1 HP = 746 Watts]/120 Volts gives 6.2 Amps.) However, in the real world things are not quite so simple, and corrections must be applied.
Efficiency. Electric motors, and everything else involved with power production or conversion, have an efficiency factor. The efficiency of a motor is the ratio of useful mechanical power at the pulley to the electrical power input. A perfect motor would have an efficiency of 1.0 or 100%, meaning that all the electrical power input would appear as mechanical power. All electrical power that does not contribute to mechanical power is loss. The loss is from several sources, including heat and friction. Full-load motor efficiencies usually range from 50% to 95%. Fractional horsepower motors usually have efficiencies under 75%. Standard 1-10 HP motors have efficiencies between approximately 75% and 85%. Note that efficiency decreases as the load decreases from maximum, and drops severely for less than 50% of full load.
Note that the rated horsepower takes this into account; you do not derate the nameplate rating by the efficiency. What the efficiency tells you is how much electrical power you are wasting. Spending more money on a higher efficiency motor will reduce your electric bill. For example, a 1 HP 75% efficient motor would require 746W/0.75 or 995W of electrical power. An 85% efficient motor would require 878W.
Power Factor. Motors typically do not have a rating in Watts. Instead you get Amps at rated Voltage and full load. To relate the Voltage and Amperage ratings to the horsepower, they must be converted to Watts. As stated above, Watts is Volts times Amps. However, this is not true for AC (alternating current) systems as we typically describe them. Typical "AC Voltages" are only averages, known as RMS or Root-Mean-Square averages. Thus "120 Volts" really means that the average Voltage, when computed by this particular method, is 120. The actual Voltage is 170sin(21600t), where t is in seconds and the sin is of degrees. The current is Ipeaksin(21600t + a), where Ipeak is the peak current in Amps, t is in seconds, and a is the phase angle in degrees. Clearly we want to avoid multiplying these two and instead figure out how to use the averages. This is done with the power factor, which is the cosine of the phase angle:
Power (RMS Watts) = Voltage (RMS Volts) x Current (RMS Amps) x Power-Factor
RMS power is what your power meter measures and what converts to horsepower, so that is the figure you want. For resistive loads such as light bulbs and toasters the power factor is 1.0. Thus a light bulb rated at 120 VAC and drawing 1 Amp will draw a power of (120 Volts) x (1 Amp)(1.0) or 120 Watts. For reactive loads, which include motors, the power factor is always less than 1. Motor power factors typically range from 0.5 to 0.95. For motors from 1 to 10 HP the power factor would typically increase from 0.75 to 0.85 for single phase induction motors. Like efficiency, the power factor is only valid at full load. It drops significantly for a partial load.
As an example, consider a motor rated at 15 Amps for 120 VAC with an efficiency of 0.75 and a power factor of 0.7. The net power in Watts would be (15 A) x (120 V) x (0.75) x (0.7) or 945 Watts. At 746 Watts per horsepower the proper rating would be about 1 1/4 HP.
Smaller motors usually will not have either the efficiency or the power factor on the nameplate. You can usually get their product and hence if one is given you can get the other. For example, consider a motor rated at 1 1/2 HP, 18 Amps at 120VAC with 63% efficiency. Its power output is (1.5 HP) x (746 Watts/HP) or 1119 Watts. Based on the efficiency alone we would expect to get (18 A) x (120 V) x (0.63) or 1361 Watts. Hence the power factor is (1119 W) / (1361 W) or 0.82. Note that power factors do not affect residential electric bills; you only pay for the Wattage. However, if you size a circuit breaker for a low power factor motor based on Wattage rather than volt-Amps, it might turn out to be too small. And if you are an industrial customer, you can expect to pay more for low power factor machines (since it costs the utility more to transmit the extra current).
How to Verify the Horsepower. The above calculations show how to check the claimed horsepower ratings on equipment. For example, the horsepower ratings on some air compressors are done by the marketing people and substantially overstate how much useful work you can get from the motor. Consider a typical "home" air compressor on the market today. It is labeled as 5HP, 120 VAC and 15 Amps. Removal of the covers to inspect the motor nameplate reveals that the horsepower space is blank, and the efficiency and power factor are missing altogether. However, you can estimate the combined efficiency and power factor at around 0.5 for motors around 1 HP. So the net motor power would be estimated at (15 A)(120 V)(0.5) or 900 Watts, or about 1 1/4 HP. Discussion with the manufacturer revealed that that the 5HP came from the break-down torque. A true 5 HP motor (as measured with a dynamometer) with a typical power factor and efficiency of 0.85 each would consume (5 HP)(746 Watts/HP)/(0.85)(0.85) or 5160 Watts. At 120 VAC this would require 43 Amps. (The home compressor calls for a 20 Amp circuit and says it will operate on a 15 Amp circuit under ideal conditions.)
The point is that such a device will only supply about 1 1/4 continuous horsepower, no matter what that "rating" says. Presently the Voltage and current ratings must still be "true numbers," so these should form your basis for calculations.
NEMA states that the dynamometer measurement is used for "general purpose" motors that are sold stand-alone. When a motor is sold as part of equipment the equipment manufacturer can negotiate any design and rating system desired with the motor manufacturer, and this information can be kept private. Hence it might not be known how a horsepower rating was arrived at.
Motor Power Ratings:
Rated Horsepower. The most important power rating of a motor is the "rated horsepower," which is the continuously available net shaft power. This is the power available at the pulley for doing useful work elsewhere. It is typically measured by a dynamometer. This is a braking devices attached to the shaft used to measure the net available power. A motor can always produce more than its rated power, but continuously overloading it will reduce its life. Excessive overload will cause it to shut down, or catch fire. An overload will also cause the motor to draw more than its rated current, which risks tripping the circuit breaker.
Service Factor. Design standards for motors permit operation with Voltages and line frequencies that vary somewhat from their nominal values. When both are at their nominal values, the motor can then be continuously overloaded to a degree. This overload is specified by the service factor (SF). The SF is the ratio of the maximum permissible continuous power to the rated power. If the SF is 1.0 then the rated power is the maximum. Otherwise the motor can be run at up to the SF times the rated power. Standard motors from around 1 HP to 200 HP have a SF of 1.15, meaning you can safely get 15% more than their rated power on a continuous basis.
Torque Ratings. The rated horsepower is what is important for continuous service. Additional ratings may be important if the motor must start against a large load, or may sometimes be significantly overloaded. Since the primary problem with an overload is thermal, a properly designed motor can tolerate an overload if it is short enough and the motor then has sufficient time to cool down before the next one. These ratings are specified as torques instead of power, were torque is rotational force. The start-up torque and pull-up torque are related to the motors ability to overcome inertia when starting loads. The break-down torque tells how much overload can be tolerated before the motor stalls. Note that the break-down torque only applies to a transient load; the continuous load still may not exceed the rated power times the service factor. And if the break-down torque is used, the current will be quite high for that time.
Mounting, Basic Types: The most common motor mounts include: rigid base, resilient base C face or D flange, and extended through bolts.
Mush Coil: A coil made with round wire.
National Electric Code (NEC): A safety code regarding the use of electricity. The NEC is sponsored by the National Fire Protection Institute. It is also used by insurance inspectors and by many government bodies regulating building codes.
NEMA (National Electrical Manufactures Association): A non-profit trade organization, supported by manufacturers of electrical apparatus and supplies in the USA. Its standards alleviate misunderstandings and help buyers select the proper products. NEMA standards for motors cover frame sizes and dimensions, horsepower ratings, service factors, temperature rises and performance characteristics.
Ohms Law
|Ohm's Law / Power Formulas |
|[pic] |P = Watts |
| |I = Amps |
| |R = Ohms |
| |E = Volts |
Open Circuit: A break in an electrical circuit that prevents normal current flow.
Output Shaft: The shaft of a speed reducer assembly that is connected to the load, this may also be called the drive shaft or the slow speed shaft.
Permanent Split Capacitor (PSC) Motor: This motor has a performance and applications similar to shaded pole motors, but more efficient, with lower line current and higher horsepower capabilities.
Phase: The number of individual voltages applied to an AC motor. A single-phase motor has one voltage in the shape of a sine wave applied to it. A three-phase motor has three individual voltages applied to it. The three phases are at 120 degrees with respect to each other so that peaks of voltage occur at even time intervals to balance the power received and delivered by the motor throughout its 360 degrees of rotation.
Plugging: A method of braking a motor that involves applying partial or full voltage in reverse in order to bring the motor to zero speed.
Polarity: As applied to electric circuits, polarity indicates which terminal is positive and which is negative. As applied to magnets, it indicates which pole is North and which pole is South.
Poles: Magnetic devices set up inside the motor by the placement and connection of the windings. Divide the number of poles into 7200 to determine the motor's normal speed. For example, 7200 divided by 2 poles equals 3600RPM, by 4 poles equals 1800RPM, by 6 poles equals 900RPM, by 8 poles equals 450RPM. (Remember that frequency also affects the speed of rotation of the motor shaft)
Power Factor: Is the cosine of the phase angle:
Power (RMS Watts) = Voltage (RMS volts) x Current (RMS Amps) x Power-Factor
|To Find |Single Phase |Three Phase |
|Efficiency |746 x HP |746 x HP |
| |V x I x Pf |V x I x Pf x 1.732 |
|Power Factor |Input Watts |Input Watts |
| |V x A |V x I x 1.732 |
Power Factor Correction: Power factor correction is achieved by the addition of capacitors across the supply to neutralize the inductive component of the current. The power factor correction may be applied either as automatic bank correction at the main plant switchboard, or as static correction installed and controlled at each starter in such a fashion that it is only in circuit when the motor is on line. Automatic bank correction consists of a number of banks of power factor correction capacitors, each controlled by a contactor which in turn is controlled by a power factor controller. The power factor controller monitors the supply coming into the switchboard and adds sufficient capacitance to neutralize the inductive current. These controllers are usually set to adjust the power factor to 0.9 - 0.95 lagging, (inductive). Static correction is controlled by a contactor when the motor is started and when the motor is stopped. In the case of a Direct on Line starter, the capacitors are often controlled by the main DOL contactor which is also controlling the motor. With static correction, it is important that the motor is under corrected rather than over corrected. This is because the capacitance and the inductance of the motor form a resonant circuit. While the motor is connected to the supply, there is no problem. Once the motor is disconnected from the supply, it begins to decelerate. As it decelerates, it generates voltage at the frequency at which it is rotating. If the capacitive reactance equals the inductive reactance, i.e. unity power factor, we have resonance. If the motor is critically corrected (Pf = 1) or over corrected, then as the motor slows, the voltage it is generating will pass through the resonant frequency set up between the motor and the capacitors. If this happens, major problems can occur. There will be very high voltages developed across the motor terminals and capacitors causing insulation damage, high resonant currents can flow, and transient torques generated can cause mechanical equipment failure. The correct method for sizing static correction capacitors is to determine the magnetizing current of the motor being corrected, and connect sufficient capacitance to give 80% current neutralization. Charts and formula based on motor size alone can be totally erroneous and should be avoided if possible. There are some power authorities who specify a fixed amount of kVAR per kilowatt, independent of the size or speed. This is a dangerous practice.
Relay: A device have two separate circuits, it is constructed so that a small current in one of the circuits controls a large current in the other circuit. A motor starting relay opens or closes the starting circuit under predetermined electrical conditions in the main circuit (run winding).
Reluctance: The characteristics of a magnetic field which resists the flow of magnetic lines of force in it.
Resistor: A device that resists the flow of electrical current for the purpose of operation, protection or control. There are two types of resistors-fixed and variable. A fixed resistor has a fixed value of ohms while a variable resistor is adjustable.
Rotation: The direction in which a shaft turns is either clockwise (CW) or counterclockwise (CCW). When specifying rotation, also state if viewed from the shaft end or the opposite shaft end of the motor.
Rotor: The rotating component of an induction AC motor. It is typically constructed of a laminated, cylindrical iron core with slots of cast-aluminum conductors. Short-circuiting end rings complete the "squirrel cage," which rotates when the moving magnetic field induces current in the shorted conductors.
Service Factor: A measure of the overload capacity built into a motor. A 1.15 SF means the motor can deliver 15% more than the rated horsepower without injurious overheating. A 1.10 SF motor should not be loaded beyond its rated horsepower. Service factors will vary for different horsepower motors and for different speeds.
Shaded Pole Motor: (Single Phase) Motor has low starting torque, low cost. Usually used in direct-drive fans and small blowers, and in small gear motors.
Short Circuit: A fault or defect in a winding causing part of the normal electrical circuit to be bypassed, frequently resulting in overheating of the winding and burnout.
Single phase motors: In order for a motor to develop a rotating torque in one direction, it is important that the magnetic field rotates in one direction only. In the case of the three phase motor, there is no problem and the field follows the phase sequence. If voltage is applied to a single winding, there are still multiples of two poles which alternate between North and South at the supply frequency, but there is no set rotation for the vectors. This field can be correctly considered to be two vectors rotating in opposite directions. To establish a direction of rotation for the vector, a second phase must be added. The second phase is applied to a second winding and is derived from the first phase by using the phase shift of a capacitor in a capacitor start motor, or inductance and resistance in an induction start motor, (also known as a split phase motor.) Small motors use techniques such as a shaded pole to set the direction of rotation of the motor.
Slip Ring Motor: Slip ring motors or wound rotor motors are a variation on the standard cage induction motors. The slip ring motor has a set of windings on the rotor which are not short circuited, but are terminated to a set of slip rings for connection to external resistors and contactors. The slip ring motor enables the starting characteristics of the motor to be totally controlled and modified to suit the load. A particular high resistance can result in the pull out torque occurring at almost zero speed providing a very high locked rotor torque at a low locked rotor current. As the motor accelerates, the value of the resistance can be reduced altering the start torque curve in a manner such that the maximum torque is gradually moved towards synchronous speed. This results in a very high starting torque from zero speed to full speed at a relatively low starting current. This type of starting is ideal for very high inertia loads allowing the machine to get to full speed in the minimum time with minimum current draw. The down side of the slip ring motor is that the slip rings and brush assemblies need regular maintenance which is a cost not applicable to the standard cage motor. If the rotor windings are shorted and a start is attempted, i.e. the motor is converted to a standard induction motor, it will exhibit an extremely high locked rotor current, typically as high as 1400% and a very low locked rotor torque, perhaps as low as 60%. In most applications, this is not an option.
Another use of the slip ring motor is as a means of speed control. By modifying the speed torque curve, by altering the rotor resistors, the speed at which the motor will drive a particular load can be altered. This has been used in winching type applications, but does result in a lot of heat generated in the rotor resistors and consequential drop in overall efficiency.
Split Phase Start - Induction Run Motor: A single phase motor which has moderate starting torque, high breakdown torque and used on easy-starting equipment, such as belt-driven fans and blowers, grinders and centrifugal pumps.
Split Phase Start - Capacitor Run Motor: A single phase motor which has high starting torque and high breakdown toque, used on high starting torque loads such as water pumps and compressors.
Stator: The fixed part of an AC motor, consisting of copper windings within steel laminations.
Temperature Classification: See Induction Motor Design.
Temperature Rise: The amount by which a motor, operating under rated conditions, is hotter than its surrounding ambient temperature.
Temperature Tests: These determine the temperature of certain parts of a motor, above the ambient temperature, while operating under specific environmental conditions.
Thermal Protector: A device, sensitive to current and heat, which protects the motor against overheating due to overload or failure to start. Basic types include automatic rest, manual reset and resistance temperature detectors.
Thermostat: A protector, which is temperature-sensing only, that is mounted on the stator winding. Two leads from the device must be connected to control circuit, which initiates corrective action. The customer must specify if the thermostats are to be normally closed or normally open.
Thermocouple: A pair of dissimilar conductors joined to produce a thermoelectric effect and used to accurately determine temperature. Thermocouples are used in laboratory testing of motors to determine the internal temperature of the motor winding.
Three Phase Standard Value:
|Three Phase Values |
|For 208 volts x 1.732, use 360 |
|For 230 volts x 1.732, use 398 |
|For 240 volts x 1.732, use 416 |
|For 440 volts x 1.732, use 762 |
|For 460 volts x 1.732, use 797 |
|For 480 Volts x 1.732, use 831 |
Torque: The turning effort or force applied to a shaft, usually expressed in inch-pounds or inch-ounces for fractional and sub-fractional HP motors. (See graph in Induction Motor Design).
Starting Torque: Force produced by a motor as it begins to turn from standstill and accelerate (sometimes called locked rotor torque).
Full-Load Torque: The force produced by a motor running at rated full-load speed at rated horsepower.
Breakdown Torque: the maximum torque a motor will develop under increasing load conditions without an abrupt drop in speed and power, sometimes called pull-out torque.
Pull-Up Torque: The minimum torque delivered by a motor between zero and the rated RPM, equal to the maximum load a motor can accelerate to rated RPM.
Transformer: Used to isolate line voltage from a circuit or to change voltage and current to lower or higher values, typically constructed of primary and secondary windings around a common magnetic core.
Underwriters Laboratories (UL): Independent United States testing organization that sets safety standards for motors and other electrical equipment.
Voltage: A unit of electromotive forces that, when applied to conductors, will produce current in the conductors.
Voltage Drop:
|Voltage Drop Formulas |
|Single Phase |VD = |2 x K x I x L |K = Ohms per mil foot (Copper = 12.9 at 75°) |
|(2 or 3 wire) | |CM |(Aluminium = 21.2 at 75°) |
| | | |L = Length of conductor in feet |
| | | | |
| | | |I = Current in the conductor in Amps |
| | | | |
| | | |CM = Circular area of conductor |
| |CM= |2K x L x I | |
| | |VD | |
|Three Phase |VD= |1.73 x K x I x L | |
| | |CM | |
| |CM= |1.73 x K x L x I | |
| | |VD | |
Watt: The amount of power required to maintain a current of 1 Ampere at a pressure of one volt when the two are in phase with each other. One horsepower is equal to 746 watts.
Winding: Typically refers to the process of wrapping coils of copper wire around a core, usually of steel. In an AC induction motor, the primary winding is a stator consisting of wire coils inserted into slots within steel laminations. The secondary winding of an AC induction motor is usually not a winding at all, but rather a cast rotor assembly. In a permanent magnet DC motor, the winding is the rotating armature.
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