Electrotechnology
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Use of routine equipment plant technologies in an energy sector environment
UEENEEE141A
Student Resource
Contents
Electrical supply and distribution within a building or premises 3
Arrangement of circuits 5
R.C.D protection requirements 5
Hazards Associated With Electrical Systems And Apparatus 7
Measurement and calculation of voltage, current, resistance and power 11
Work Safely 15
Insulation Resistance Testing 16
Ammeters (measuring current) 17
Difference Between Alternating And Direct Current 20
Basic Alternating Current generation 22
AC and DC Shock Comparison 24
Facts about Electric Shock 25
Inductors & Magnetism 26
Magnetic Induction 31
Practical Task: Determine North end of a coil 33
Practical Task: Generating an E.M.F 34
Practical Task: Motor Effect 35
Practical Task: Effect of air gap in a magnetic circuit 37
Transformers 38
The three basic parts of a transformer 38
Core characteristics 39
Transformer Windings 41
Schematic symbols for transformers 42
Primary and secondary Phase relationship 44
Turns and Voltage ratio 45
Transformer losses 46
Types and applications of transformers 47
Electrical supply and distribution within a building or premises
Definitions
|Point Of Supply |Clause 1.4.6 |The junction of the consumers mains with: |
| |AS/NZS 3000:2007 |Conductors of an electricity distribution system |
| | |Output terminals of an electricity generation system within the premises |
|Consumers Mains |Clause 1.4.6 |Those conductors between the Point Of Supply and the Main Switch Board |
| |AS/NZS 3000:2007 | |
|Main Switch Board |Clause 1.4.6 |A switchboard from which the supply to the whole electrical installation can be controlled |
| |AS/NZS 3000:2007 | |
|Sub Mains |Clause 1.4.6 |A circuit originating at a switchboard to supply another switchboard |
| |AS/NZS 3000:2007 | |
|Distribution Board |Clause 1.4.6 |A switchboard from which the supply to part of electrical installation can be controlled |
| |AS/NZS 3000:2007 | |
|Final Sub Circuit |Clause 1.4.6 |A circuit originating at a switchboard and to which only consuming devices or points will be connected |
| |AS/NZS 3000:2007 | |
|Appliance |Clause 1.4.6 |A consuming device, other than a lamp, in which electricity is converted into heat, motion, or any other form of energy, or |
| |AS/NZS 3000:2007 |is substantially changed in its electrical character |
Arrangement of circuits
The reasons electrical installations are divided into circuits are:
a) Avoid danger
b) Minimise inconvenience in the event of a fault
c) Facilitate safe operation, inspection, testing and maintenance
When deciding the number and types of circuits in an electrical installation, the following needs to be taken into account:
a) The relationship of the equipment, including any requirement for operation as a group and any special need identified by the user
b) The load and operating characteristics of the equipment in relation to the rating of the circuit components.
c) The limitation of consequences of circuit failure including loss of supply to critical equipment, overload and the ability to locate a fault.
d) The facility for maintenance work, and the capacity for alterations and additions, to be performed without interrupting supply to other parts of the installation.
Division of circuits falls logically into several categories, each an individual circuit or group of circuits. Typically, the circuit groups selected are:
a) Lighting
b) Socket-Outlets
c) Heating and/or air conditioning appliances
d) Motor driven plant
e) Auxiliary services, such as indication and control
f) Safety services
R.C.D protection requirements
R.C.Ds must be provided with a maximum residual current of 30mA for the following final sub circuits in residential installations
• One or more socket outlets; and
• Lighting points; and
• Directly connected handheld electrical equipment, e.g. directly connected hair dryers or tools
R.C.Ds must be provided with a maximum residual current of 30mA for the following final sub circuits in residential installations
• Final sub circuits supplying socket outlets where the rated residual current of any individual socket does not exceed 20A; and
• Final sub circuits supplying lighting where any portion of the circuit has a rated current not exceeding 20A; and
• Final sub circuits supplying directly connected hand held electrical equipment, e.g. hair dryers or tools.
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Hazards Associated With Electrical Systems And Apparatus
Regard all electricity as extremely hazardous whether the voltage be high (greater than 1000 volts A.C or 1500 volts D.C), low (greater than 50 volts A.C or 120 volts D.C). The primary hazard of electricity is electric shock effecting normal breathing and heart function. In addition, as current and its duration through the body increase, body tissue is burnt irreversibly.
The second hazard is that of falling or being thrown by an electric shock, even one at very low voltages, causing impact injuries.
The other main hazard of electricity is the amount of energy that is released in an arcing fault when the air ionises becoming a conductor and heating to more than 20000C. This is flowed by an explosion caused by the rapid expansion of the surrounding air. The frightening thing is that all this happens in much less that a second.
Any person within the vicinity of such an event, if not killed, will suffer horrific and disabling burns, damaged eyesight due to the ultraviolet radiation and possibly other physical injuries. Irrespective of the voltage, an electrical source that can supply a high current, even the humble 12 volt car battery, has the potential to produce a hazardous arc fault.
Another situation that poses high risk of electric shock is fallen power lines caused by storms or road accident. Dealing with fallen, broken or sagging power lines is not within the domain of most electricians and should be left to the energy distributor and emergency services to remedy the situation
Measurement and calculation of voltage, current, resistance and power
Safety
Before undertaking any testing, it is imperative that any Hazards be identified, the risks assessed and safe work methods developed and adopted to eliminate or control the risks. This is particularly important when testing energized parts.
Every job will be different; however, there are a number of hazards that are common to electrical testing work. It is important that these hazards are recognized and risk control measures put in place.
|Hazard |Risk |Risk Control Measure |
|Flashover (short circuit) between phases, or phase to |Severe burns |Use only appropriately certified testing devices and test probes |
|earth or phase and neutral, while testing energized |Electric shock |Use only testing devices and test probes that have been checked and |
|circuits |Falls |are not damaged in any way and function correctly |
| | |Before using testing devices and test probes, make sure testing |
| | |devices are correctly set up |
| | |Use appropriate PPE |
|Exposed energized terminals or conductors |Severe burns |Isolate and lock off supply |
| |Electric shock |Use isolation barriers to prevent inadvertent contact with energized |
| | |parts |
| | |Use PPE |
|Periodic energisation of circuits and equipment such as |Electric shock Falls |Understand how a circuit and equipment is controlled and operates |
|with controlled loads or automatic control systems. | |before commencing any tests |
| | |Do not rely on electrical diagrams for accuracy of the “as installed” |
| | |circuits and equipment. |
|Testing associated energy storage such as batteries and |Physical injuries Chemical |Always regard energy stored devices as energized. |
|capacitors |burns Electric shock Falls |Do not assume ELV circuits and equipment to be safe-apply the same |
| | |safety measures used for LV |
| | |Use PPE |
|De-energised cables holding a captive charge from an |Electric shock Falls |Short circuit cable ends immediately after conducting an insulation |
|inulation resistance test | |resistance test. |
Managing the risk
When carrying out electrical testing, always keep in mind the importance of safe working procedures and only test energised circuits when carrying out an isolation procedure or when necessary to locate a fault. Irrespective of whether testing is on an energized or de-energised circuit and equipment; the risk management plan must include the following points:
• Testing is carried out using a “Safe System Of Work”, incorporating a risk assessment and safe work methods that eliminate or control the risk of inadvertent contact with energized parts
• Approved testing devices suitable for the tests being conducted are used; persons conducting the tests are competent to do so.
• Approved clothing and personal protective equipment and personal protective equipment (PPE) suitable for the tests being conducted is provided and used by the persons performing the testing.
• Testing is carried out with a competent safety observer present when the identified risk requires it.
Note:
It is the employer’s responsibility to ensure the equipment needed for the test is available
Multi-Meter Category Ratings
| |Definition |Examples |
|CAT IV | |Origin of installation’, i.e. low-voltage connection to utility power; |
| | |Electricity meters, primary overcurrent protection equipment; |
| |Three-phase at utility connection, any outdoor |Outside and service entrance, service drop from pole to building between meter and panel; |
| |conductors |Overhead line to detached building. |
|CAT III |Three-phase distribution, including single-phase |Equipment in fixed installations, e.g. switchgear and polyphase motors; |
| |commercial lighting |Bus and feeder in industrial plants; |
| | |Feeders and short branch circuits, distribution panel devices; |
| | |Lighting systems in larger buildings; |
| | |Appliance outlets with short connections to service entrance. |
|CAT II |Single-phase receptacle connected loads |Appliances, portable tools, household and similar loads; |
| | |Outlet and long branch circuits; |
| | |Outlets at more than 10 m from CAT III source; |
| | |Outlets at more than 20 m from CAT IV source. |
|CAT I |Electronic |Protected electronic equipment; |
| | |Equipment connected to (source) circuits where measures are taken to limit transient over |
| | |voltages to appropriately low level; |
| | |High-voltage, low-energy source derived from a high winding resistance transformer, e.g. |
| | |high-voltage section of copier. |
Safety Standards
IEC 1010 is the international safety standard for electrical test equipment. Locally, this was recently adopted as AS 61010. Meters designed to this new standard offer a high level of safety.
The most important concept to understand is the Overvoltage Installation Categories, or CAT I, II, III, IV, qualifying power distribution systems into categories, based on the fact that dangerous high-energy transients will be attenuated as they travel through the impedance of a system. A higher CAT number refers to an electrical environment with higher power available and higher-energy transients.
Within a category, a higher voltage rating denotes a higher transient withstand rating; eg, a CAT III-1000 V meter has superior protection compared with a CAT III-600 V rated meter. The real misunderstanding occurs if someone selects a CAT II-1000 V rated meter thinking that it is superior to a CAT III-600 V meter.
It’s not just the voltage level
A technician working on office equipment in a CAT I location could encounter DC voltages much higher than the power line voltages measured by the motor electrician in a CAT III location. Yet transients in CAT I electronic circuitry, whatever the voltage, are a lesser threat because the energy available to an arc is limited. This doesn’t mean that there’s no electrical hazard in CAT I or CAT II equipment. The primary hazard is electric shock, not transients and arc blast. Shocks can be just as lethal as an arc blast.
Transients — the hidden danger
Consider a worst-case scenario of a technician performing measurements on a live three-phase motor circuit, using a meter with inadequate safety precautions:
• A lightning strike causes a transient on the power line, in turn striking an arc between the input terminals inside the meter. The components to prevent this event have just failed or perhaps the meter isn’t CAT III rated, resulting in a direct short between the measurement terminals through the meter and test leads.
• A high-fault current — possibly several thousand amps — flows in the short-circuit just created. When the arc forms inside the meter, the tech hears a loud bang and sees bright blue arc flashes at the test lead tips — fault currents superheating the probe tips, which start to burn away and draw an arc from the contact point to the probe.
• As the tech’s hands pull back to break contact with the hot circuit, an arc is drawn from the motor terminal to each probe. If these two arcs join to form a single arc, there is now another direct phase-to-phase short, this time directly between the motor terminals.
• This arc can approach 6000°C and as it grows, superheats the surrounding air, creating a shock blast and plasma fireball. If the technician is lucky, the blast blows him away, removing him from the proximity of the arc, only injured. In the worst case, he’s subjected to fatal burn injuries from the fierce heat of the arc or plasma blast.
Anyone working on live power circuits should also be protected with flame-resistant clothing, insulated gloves and safety glasses or, better still, a safety face shield.
Transients aren’t the only source of possible short-circuit and arc blast hazards. One of the most common misuses of handheld multimeters can cause a similar chain of events.
Let’s say a tech is making current measurements on signal circuits. The procedure is to select the ‘Amps’ function, insert the leads in the ‘mA’ or ‘Amps’ input terminals, open the circuit and take a series measurement. In a series circuit, current is always the same. The input impedance of the amps circuit must be low enough so it doesn’t affect the series circuit’s current. The input impedance on the 10 A terminal of a Fluke meter is 0.01 O, whereas the input impedance on the voltage terminals is 10 MO.
If the test leads are left in the Amps terminals and then accidentally connected across a voltage source, the low input impedance becomes a short circuit! It doesn’t matter if the selector dial is turned to ‘Volts’; the leads are still physically connected to a low-impedance circuit. That’s why the Amps terminals must be protected by fuses, the only thing standing between an inconvenience — a blown fuse — and a potential disaster. Only multimeters with high-energy fuses protecting the Amps inputs should be used. Never replace a blown fuse with the wrong fuse. Use only the high-energy fuses specified by the manufacturer.
Overload Protection
Overvoltage protection is provided by a circuit that clamps high voltages to an acceptable level. Additionally, a thermal protection circuit detects an overvoltage condition, protects the meter until the condition is removed and then automatically returns to normal operation. This protects the multimeter from overloads when it’s in ‘ohms’ mode. This way overload protection with automatic recovery is provided for all measurement functions as long as the leads are in the voltage input terminals.
Work Safely
Combining the right tools with safe work practices gives maximum protection. Here are some tips:
Work on de-energised circuits whenever possible. Use proper lock-out/tag out procedures. If these procedures aren’t in place, the circuit is considered to be live.
• On live circuits, use protective gear:
• Insulated tools;
• Safety glasses or face shield;
• Flame-resistant clothing, covering the full body;
• Insulated gloves (remove watches and jewellery);
• Stand on an insulated mat.
When making measurements on live circuits:
• Hook the ground clip first, and then make contact with the hot lead. Remove the hot lead first and then the ground lead last;
• Hang or rest the meter if possible. Avoid holding it to minimise personal exposure to transients;
• Use the three-point test method, especially when checking to see if a circuit is dead. First, test a known live circuit. Second, test the target circuit. Third, test the live circuit again. This verifies the meter worked properly before and after the measurement;
• Use the old electricians’ trick of keeping one hand in a pocket, lessening the chance of a closed circuit across the chest and through the heart.
Measuring circuit resistance
There are a few instances in an electrical installation circuit where a resistance test is used:
1. Polarity testing
2. Earth continuity testing
3. Insulation resistance testing
4. Fault loop impedance testing
5. Fault finding elements
Continuity testing is the most used and versatile testing technique. Some applications for continuity testing are as follows:
1. Checking that the earthing system is electrically continuous and of sufficiently low resistance in accordance with table 8.2 of the AS/NZS 3000:2007.
2. Identifying the active and neutral conductors in a cable prior to connecting them to an accessory or appliance.
3. Checking that the wiring of a circuit is connected to the correct terminals at an accessory or appliance.
4. Checking that there are no short circuits in a new installation or locating a short circuit that might have developed in an existing circuit or equipment.
5. Checking there are no interconnections between circuits
6. Identifying a circuit by measuring it’s resistance; and
7. Locating high resistance connections that might have developed in a circuit while in service.
Insulation Resistance Testing
An insulation resistance test is conducted to ensure that the insulation resistance between all live conductors and earth or, as the case may be, all live parts and earth is adequate to ensure the integrity of the insulation. This is to prevent:
a) Electric shock hazards from inadvertent contact; and
b) Fire hazards from short circuits; and
c) Equipment damage
(AS/NZS 3000:2007 Clause 8.3.6.1)
In an electrical installation, the insulation resistance of any circuit including cabling, cannot be lower that 1M Ω when applying a D.C voltage of 500 volts from an insulation resistance tester. A sheathed heating element can have an insulation resistance value no lower than 10 000 Ω.
The insulation resistance (IR) test (also commonly known as a Megger Test) is a spot insulation test which uses an applied DC voltage (typically 250Vdc, 500Vdc or 1,000Vdc for low voltage equipment less than 600V and 2,500Vdc and 5,000Vdc for high voltage equipment) to measure insulation resistance in either kΩ, MΩ or GΩ. The measured resistance is intended to indicate the condition of the insulation or dielectric between two conductive parts, where the higher the resistance, the better the condition of the insulation. Ideally, the insulation resistance would be infinite, but as no insulators are perfect, leakage currents through the dielectric will ensure that a finite (though high) resistance value is measured.
Because IR testers are portable, the IR test is often used in the field as the final check of equipment insulation and also to confirm the reliability of the circuit and that there are no leakage currents from unintended faults in the wiring (e.g. a shorted connection would be obvious from the test results).
One of the advantages of the IR test is its non-destructive nature. DC voltages do not cause harmful and/or cumulative effects on insulation materials and provided the voltage is below the breakdown voltage of the insulation, does not deteriorate the insulation. IR test voltages are all well within the safe test voltage for most (if not all) insulation materials.
Ammeters (measuring current)
The most common type of instrument for measuring current in the field is the clamp meter. This test device allows the user to measure circuit current without the need to break the circuit. Clamp meters can measure circuit current by the use of a current transformer or, a Hall Effect sensor. The advantage of the Hall Effect sensor is that it can measure both A.C and D.C currents.
A clamp meter that utilizes a Hall Effect sensor consists of a ferrite core which is clamped around a conductor with a Hall Effect sensor positioned in the air gap. A magnetic flux is induced in the ferrite core whenever current flows through the conductor. The magnetic flux passes through the Hall Effect Sensor which then generates a proportional output voltage.
Measurement and calculation of voltage, current, resistance and power in practical circuits
The coil element shown on the right has a resistance of 28.1Ω when measured with an ohm meter. The rated voltage of this element is 240 volts. To work out the power consumed by this element, we would use the following formula:
Therefore:
The following formulas can be used in fault finding, so they are worth learning. They are all derived from two base formulas:
• I=V/R
• P=VxI
Example
Using the formulas given on the previous page, calculate how much current the coil stove element would draw from the supply.
Calculate the voltage at the cord extension socket if a load of 10A was connected
Difference Between Alternating And Direct Current
Edison, Tesla and Westinghouse
In the late 19th century, DC, which was championed by Thomas Edison, inventor of the electric light bulb, appeared to be in the position to become the dominant means of electric power in the United States. However, at the turn of the 20th Century, Nikola Tesla invented AC, along with the means to transmit AC by generators and motors. He then licensed his patents to George Westinghouse, who went head-to-head with Edison. Westinghouse and Tesla eventually prevailed with AC for the electrical grid system, although DC and battery power are still essential for portable electronic appliances and machinery.
How AC Power Works
AC power gets its name because of the way the electrons that form its current flow. AC flows in one direction, then alternates to flow in the opposite direction. AC has the capacity to be carried over long distances via power cords without a significant loss of power. This is the main reason AC prevailed over DC (much like VHS prevailed over Beta because VHS tapes had a longer recording capacity).
In the modern electric grid, AC power is transmitted to and from transformers by high-capacity wires, which then connect to the household or commercial establishment that draws from the power through its own internal wiring system. Power cords then connect appliances and electronics to the AC power source by being plugged into outlets. Some businesses and homes also have generators that can be used as a backup system in case of a failure of the main AC power source.
How DC Power Works
DC power works by generating a flow of direct current, or current which flows in a single direction. Sometimes DC is called "dry cell" when referring to batteries, although this is technically incorrect. With a battery, when the stored energy has been depleted, it ceases to generate electron flow. In popular terms, the battery is "dead." With rechargeable batteries, this process can repeat itself several times, although eventually, even rechargeable batteries fail to have the capacity to generate electron flow.
With Edison's system, DC power was produced by a generator, which actually initially produced AC power, which was then converted to DC by a device called a commutator. However, DC power cannot be carried over long distances without voltage (power) loss. There is also the danger of long distance wires overheating and catching fire unless they were made of very expensive super-conductive materials. DC power was eventually dismissed as not being commercially viable.
D.C currents can be produced chemically by a battery or mechanically through a generator. What is the difference between Electricity produced by a battery, and Electricity produced by a generator?
| |In the case of a battery, current flows in one direction, from positive |
| |to negative. Everything is straight forward. In the case of a generator,|
| |however, things get a bit more complicated. It is possible to generate |
| |voltage & current flow by spinning a coil within a magnetic field. The |
| |coil is in constant motion within the magnetic field, and a voltage is |
| |being generated in the coil. The resulting current flows by way of the |
| |brushes and commutator, is pulsating and the commutator ensures flow in |
| |one direction. |
| | |
| |dc Voltage/Current produced by a generator |
| |[pic] |
| | |
|dc Voltage/Current produced by a battery | |
|[pic] | |
| | |
| | |
|If we look at the current leaving the battery, it is constantly moving | |
|in the same direction. We call this DIRECT CURRENT. But if we attach a | |
|generator, by way of the brushes and slip-rings in the same circuit, we | |
|notice a major change. The meter would swing back and forth from | |
|negative to positive. This seems strange until we examine what is going | |
|on inside the generator. | |
Basic Alternating Current generation
The figures below show a suspended loop of conductor being rotated in a clockwise direction through the magnetic field between the poles of a permanent magnet. For ease of explanation, the loop has been divided into a dark half and light half. Notice in position (A) the dark half is moving along (parallel to) the lines of force. Consequently, it is cutting NO lines of force. The same is true of the light half, which is moving in the opposite direction. Since the conductors are cutting no lines of force, no E.M.F is induced. As the loop rotates toward the position shown in (B), it cuts more and more lines of force per second (inducing an ever-increasing voltage) because it is cutting more directly across the field (lines of force). At (B), the conductor is shown completing one-quarter of a complete revolution, or 90°, of a complete circle. Because the conductor is now cutting directly across the field, the voltage induced in the conductor is maximum. When the value of induced voltage at various points during the rotation from (A) to (B) is plotted on a graph (and the points connected), a curve appears as shown below.
The loop has now been rotated through half a circle (one alternation or 180°). If the preceding quarter-cycle is plotted, it appears as shown below. When the same procedure is applied to the second half of rotation (180° through 360°), the curve appears as a negative value. Notice the only difference is in the polarity of the induced voltage. Where previously the polarity was positive, it is now negative. As the loop continues to be rotated toward the position shown below in (C), it cuts fewer and fewer lines of force. The induced voltage decreases from its peak value. Eventually, the loop is once again moving in a plane parallel to the magnetic field, and no E.M.F is induced in the conductor.
Simple alternating-current generator:
Basic alternating-current cycle generation:
The sine curve shows the value of induced voltage at each instant of time during rotation of the loop. Notice that this curve contains 360°, or two alternations. Two alternations represent one complete cycle of rotation.
Assuming a closed path is provided across the ends of the conductor loop, you can determine the direction of current in the loop by using the left-hand rule for generators. Refer to the figure below. The left-hand rule is applied as follows: First, place your left hand on the illustration with the fingers as shown. Your thumb will now point in the direction of rotation (relative movement of the wire to the magnetic field); your forefinger will point in the direction of magnetic flux (north to south); and your middle finger (pointing out of the paper) will point in the direction of electron current flow.
Left-hand rule for generators:
By applying the left-hand rule to the dark half of the loop in (B) in figure on page 8, you will find that the current flows in the direction indicated by the heavy arrow. Similarly, by using the left-hand rule on the light half of the loop, you will find that current therein flows in the opposite direction. The two induced voltages in the loop add together to form one total E.M.F. It is this E.M.F which causes the current in the loop.
When the loop rotates to the position shown in (D), the action reverses. The dark half is moving up instead of down, and the light half is moving down instead of up. By applying the left-hand rule once again, you will see that the total induced E.M.F and its resulting current have reversed direction. The voltage builds up to maximum in this new direction, as shown by the sine curve on page 8. The loop finally returns to its original position (E), at which point voltage is again zero. The sine curve represents one complete cycle of voltage generated by the rotating loop. All the illustrations used in this chapter show the wire loop moving in a clockwise direction. In actual practice, the loop can be moved clockwise or counterclockwise. Regardless of the direction of movement, the left-hand rule applies.
If the loop is rotated through 360° at a steady rate, and if the strength of the magnetic field is uniform, the voltage produced is a sine wave of voltage, as indicated in the figure below. Continuous rotation of the loop will produce a series of sine-wave voltage cycles or, in other words, an ac voltage. As mentioned previously, the cycle consists of two complete alternations in a period of time. Recently the hertz (hz) has been designated to indicate one cycle per second. If one cycle per second is one hertz, then 100 cycles per second are equal to 100 hertz, and so on
AC and DC Shock Comparison
What are the physical differences between shocks by AC and DC current? It is commonly taught that AC current fibrillates the heart, but DC current causes deep tissue burns as it causes continuous muscle contraction and not letting go.
Difference between AC and DC Current
AC current is alternating in nature and follows a sine curve. It is continuously changing direction and passing through zero to a maximum positive value and then to a maximum negative value. The voltage AC current is a RMS or root mean square value, and the peak or maximum value is 1.4 times the RMS value. It means that a 230 V AC supply is going to 325 Volts before coming down to zero and changing direction. This characteristic of the AC current must be considered before making a comparative study of the AC and DC shock. This same principle also stands for the AC current also.
DC current is direct current and does not change in magnitude, though it can be negative or positive depending on the direction of the circuit. DC current is ideal for electronic circuits and is now quite common with photovoltaic installations whereas AC is ideal for electrical installation and motors, etc.
Effects of AC and DC Current on Human Body
The three basic factors that ascertain the kind of shock are the amplitude of the current, the duration of the current passing through the body, and the frequency. In direct current the frequency is not there. However the passing of direct current is the flow of electric energy through the body, and it would have its physiological effects during electrocution no matter what type of current is there.
The factor deciding the effects of the AC and DC current is the path the current takes through the body. If it is from the hand to the foot, it does not pass through the heart, and then the effects are not so lethal. However DC current will make a single continuous contraction of the muscles compared to AC current, which will make a series of contractions depending on the frequency it is supplied at. In terms of fatalities, both kill but more milliamps are required of DC current than AC current at the same voltage.
If the current takes the path from hand to hand thus passing through the heart it can result in fibrillation of the heart. Fibrillation is a condition when all the heart muscles start moving independently in a disorganized manner rather than in a state of coordination. It affects the ability of the heart to pump blood, resulting in brain damage and eventual cardiac arrest. If AC current is passed through the heart it induces fibrillation, whereas DC current just freezes the heart and makes it stand still. In terms of recovery once the offending current is removed a frozen heart has greater chances of recovery over a fibrillating heart. A fibrillating heart would require one or more shocks from a defibrillating machine to reestablish a rhythm.
The reason why all defibrillating machines have switched over from AC to DC is that the DC stops the fibrillation thus allowing the heart to recover.
Though both AC and DC currents and shock are lethal, more DC current is required to have the same effect as AC current. For example, the let-go threshold in AC a current of 5 to 10 mA is required against 15 to 88 of DC current.
However deep tissue burns are more common in DC currents as it causes a continuous muscle contraction that does not allow one to let go. In AC currents, fibrillation of the heart followed by cardiac arrest is more likely.
AC current is more dangerous to the heart than DC current. The incidence of ventricular fibrillation is ten times more frequent after AC than DC shock.
Facts about Electric Shock
• It is the magnitude of current and the time duration that causes any effect. That means a low value current for a long duration can also be fatal.
• The voltage of the electric supply is only important as it ascertains the magnitude of the current. As Voltage = Current x Resistance, the bodily resistance is an important factor. Sweaty or wet persons have a lower body resistance and so they can be fatally electrocuted at lower voltages.
• Let-go current is the highest current at which subject can release a conductor. Above this limit, involuntary clasping of the conductor is present.
• Apart from electric shock the other equally dangerous hazards of playing (or working) with electricity are electrical arc flash and electrical arc blast.
• Hand in the pocket policy is good as it does not allow the current to pass through the heart and makes the shock non-lethal.
• The severity of the electric shock depends on the following factors: body resistance, circuit voltage, amplitude of current, path of the current, area of contact, and duration of contact.
• Death may also occur from falling in case of electric shock.
• Burn injury may occur at both the entrance and exit of the current.
• Low frequency AC is more dangerous than high frequency AC.
• AC and DC both kill so treat them with respect.
Inductors & Magnetism
Energy, according physical law of "Conservation of Energy", is never lost nor gained. It may be changed from one form to another, but it never just "disappears". Just like in our resistor, we had energy being used which was dissipated as heat. The electrical energy is transformed into heat energy. It doesn't disappear, it merely changes form. There are many other forms of energy. Some other forms of energy are light, sound, momentum, and magnetism.
We are all familiar with magnets, and their peculiar properties which make them seem almost magical. A magnet can be used to hold a screw onto a screwdriver, to lift a car, or find your way in the forest. But what is it that makes a magnet do what it does?
If we take a magnet, and mark one end of it, we can identify one end from the other. If we then suspend the magnet from a string, so that it is free to rotate, we will notice that one end will always point toward the north, and that it will always be the same end of the magnet that points north.
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From this, we have concluded that there is a North Pole and a South Pole on every magnet. Typically the North Pole is marked with an N, and South Pole is marked with an S.
Now if we take two magnets with known, marked poles, and bring the North Pole of one magnet close to the South Pole of the second magnet, the two magnets will pull towards one another until they are connected. If we reverse the experiment, and bring the North Pole of one magnet, near the North Pole of the second magnet, they will push away from each other. This effect is called the law of poles which states:
OPPOSITE POLES ATTRACT each other, whereas LIKE POLES REPEL each other.
[pic]
Why is it that magnets act this way? And why do magnets have poles? These are questions which science has found difficult to answer. It is believed, though, that according to the Molecular Theory of Magnetism inside of all magnets, the tiny molecules that the magnet are made of, are all little tiny magnets in themselves, and that they are all lined up in a row.
[pic]
In a normal piece of steel, for instance, the molecules are arranged in random order, with positive and negative poles scattered about in all directions.
[pic]
But when magnetized, the tiny magnetic molecules line up, allowing the whole piece of steel to act like one big magnet.
[pic]
If we place a magnet beneath a piece of paper, and place iron filings on top of the piece of paper, the result would look something like the example to the right. The iron filings will arrange themselves to look like the invisible magnetic force which surrounds the magnet. This invisible magnetic force which exists in the air or space around the magnet, is known as a magnetic field, and the lines are called magnetic lines of force.
[pic]
Now if we take a non-magnetic object, such as a glass rod, and place it within the path of a magnetic field, the lines of force produced by the field would pass right through the object.
If, however, we wrap a magnetically conductive layer around the object, such as a soft iron, the iron will cause the lines of force to bend, and go around the object instead of through it. This is called a shielding effect.
There are actually 2 types of magnetism:
• Temporary
• Permanent
Soft iron can be easily magnetized by placing it inside a magnetic field. However, as soon as the iron is removed from the field, most of its magnetism fades away. A negligible amount of magnetism is, however, retained. This type of magnet is called a temporary magnet. The small amount of magnetism that does remain is called residual magnetism.
Steel or hard iron, which is difficult to magnetize, retains the majority of its magnetism long after it has been removed from the magnetic field. This type of magnet is called a permanent magnet. Permanent magnets are generally made in the shape of a bar or a horseshoe. Of the two shapes, the horseshoe type has the stronger magnetic field because the magnetic poles are closer to each other. Horseshoe magnets are used in the construction of headphones. Loudspeakers, on the other hand, generally use a type of Bar magnet.
It has been found that when a compass is placed in close proximity to a wire, and an electrical current flows through the wire, the compass needle will turn until it is at a right angle to the conductor. Since a compass needle lines up in the direction of a magnetic field, there must be a magnetic field around the wire, which is at right angles with the conductor! Science has discovered then, that wires which carry current have the same type of magnetic field that exists around a magnet! We say that an electric current induces a magnetic field.
If you closely examine the picture on the right, you will find that there are "rings" circling about the wire. These rings represent the magnetic lines of force which exist around a wire which carries an electric current. They are strongest directly around the wire, and extend outward from the wire, gradually decreasing in intensity. You will also note that the compass needle is steady, and not spinning. This indicates that the magnetic field goes in a ring around the wire. It also travels in a specific direction.
The direction of the magnetic field can be predicted by use of what we call the left hand rule. According to the left hand rule, if you wrap your left hand around the wire that is carrying the current, with your thumb following the direction of current flow (thumb points positive), your fingers will show you what direction the magnetic field will turn. Note that when the current flows from negative to positive, it induces a magnetic field in a specific direction, such that the North Pole is always at right angles with the electrical current flow.
No matter which way we turn or twist the wire, the left hand rule applies. But what happens if we put a loop in the wire? When the wire is looped, as you will see from the picture on the right, the little magnetic fields that wrap around the wire cross through each other's path. If you use the left hand rule, and follow around the coils of the wire, you will find that the magnetic field acts as if it is running through the hole inside of the loop. (If the loop were a donut, the magnetic field would go through the hole in the donut). Thinking along these lines... if we put a dozen donuts side to side, with a stick going through the holes, the magnetic field would follow the stick.
[pic] [pic]
Through experimentation, it was found that if a wire is wound in the form of a coil (coiled up), the total strength of the magnetic field around the coil will be magnified. This is because the magnetic fields of each turn add up to make one large resulting magnetic field. Furthermore, it was found that the direction of the magnetic field could be predicted. The POSITIVE end of the battery is ALWAYS connected to the NORTH POLE of the coil, regardless of whether the coil is wound clockwise or counterclockwise. The coil of wire, because of their properties and capabilities, makes up one of the main components in electronics. For this reason, it has taken on many names, to include:
• Electromagnet
• Inductor
• Solenoid
• Coil
Coils have been given their own schematic symbol. So far we have discussed the schematic symbol for the resistor, lamp and battery. The schematic symbol for the coil is on the left. Note that there can be many variations of this, which will be discussed in more detail later. In Australia we are using the symbols based on AS1102
[pic]
There are several factors which determine the strength of a given electromagnet. They are:
• The amount of current - the greater the current, the greater the field.
• The number of turns - the greater the number of turns in a coil, the greater the field.
• The permeability of the core.
The core of a coil is the material that the coil is wrapped around. It can be glass, wood, metal, air, or even a vacuum. If the coil is wound upon an iron core, the strength of the electromagnet is increased several hundred times over what it would be with an air core. We say that iron is more permeable than air. Permeability is the ability of a given substance to conduct magnetic lines of force. It is similar to the effect of conductance with respect to electrical current flow. The standard for permeability is air, which is given a permeability of one. All other substances are compared to air. Some examples of substances with high permeability are transformer steel and iron.
To the right is a picture of a “variable “air core coil. This particular coil is adjustable in value, based on a moving “tap” in the coil, which rolls along the outside of the coil as the spindle is turned. Sometimes this is called a “roller inductor ". As the spindle is turned, the coil itself rotates, and the tap moves along the length of the coil, changing its “electrical length ". Of course this is just one example of the many types and shapes of coils that exist.
Just as resistance is the opposition to current flow; permeability also has an opposition called reluctance. Reluctance is mathematically the reciprocal of permeability.
Voltage is the measurement for Amplitude of an electrical circuit. Magnetism also has a counterpart for this, which is called Magnetomotive Force. Magnetomotive Force is the force which produces the magnetic lines of force or flux.
Magnetomotive Force
The force that establishes and maintains the magnetic field in the magnetic circuit. This force can be created by a permanent magnet or by an electric current flowing in the electric circuit of a coil.
Permeability
The measure of the ease with which lines of force pass through a substance
Reluctance
The opposition a substance offers to the passage of flux in a magnetic circuit
Magnetic Induction
A wire conducting an electric current generates a magnetic field around it. Along the same lines, when a magnetic field, radiating from a permanent magnet, passes through a wire or coil of wire, it induces an electrical current on the wire. To state this another way, just as a current in a wire generates a magnetic field - a magnetic field passing through a wire generates a current.
We can monitor this action by placing a meter across the wire. When we approach a wire with a magnet, the wire cuts the magnetic field and we see the meter needle move.
In this way, we can "generate" electricity by moving a magnet in close proximity to a wire. The stronger the magnetic field, the more current flows through the wire. There is a catch though.
If we stop the movement of the wire, right in the middle of the field, one would think that electrical current would continue to be generated. Actually, this is not the case. The magnetic field must be moving in relation to the wire in order for a current to be generated in the wire. In other words, either the magnet, or the wire must be moving. And the faster the wire passes through the field, the more current is generated.
Now we know that according to the physical law of CONSERVATION OF ENERGY that no energy is ever lost or gained. So the energy generated in the wire can't just come out of the blue. It must be transformed from some other sort of energy. The question being, does it come from the magnetic field, or from the motion? The answer is that the energy is transformed from mechanical momentum into electrical current.
This is the principle behind an electric generator. If we take a wire coil, and place it on a rotating shaft, then we can spin the coil. If the shaft runs midway between two permanent magnets, we can control the movement of a coil of wire between two magnetic poles. Thus, it is possible to generate electricity by spinning the coil upon the shaft, because the wire is in constant motion within the magnetic field. The motion is transformed into electricity via the magnets. The electricity goes out to the world from the terminals, by way of the brushes and slip rings. The important point to remember is that we can generate magnetism with a wire conducting electricity, and we can generate electricity with magnets.
[pic]
In the early days of Electrical experimentation it was assumed that the current was flowing from the positive pole to a negative pole. Today we know that this is not the case, the Negative pole has a ready supply of Electrons and the Positive pole is starved of Electrons that is the “Real” current flow is from Negative to Positive. Having maintained to use the two Theories has led to much confusion, but in the Capacitive and Inductive explanations above we have been applying the conventional current flow
[pic]
Practical Task: Determine North end of a coil
For a coil, e.g. relay coil the right-hand thumb rule can be applied to establish the magnetic polarity.
1. Check the rotation of the coil winding supplied. Predict the polarity and enter N & S into the relevant boxes.
2. Connect the coil to a D.C supply and place a compass at the end of the coil to show which end is north
3. Dose this match what the right hand thumb rule indicated?
Practical Task: Generating an E.M.F
Task instructions
1) Connect the analogue multi-meter to the coil with 4 turns.
2) Move the coil through the magnetic field.
3) Take note of what you see occur when you move the coil in both directions through the magnetic field.
4) Repeat steps 1 to 3 using the coil with 8 turns.
5) Repeat steps 1 to 3 using the coil with 4 turns and the coil with 8 turns connected in series.
6) Repeat steps 1 to 3 using 2 coils with 8 turns connected in series.
7) Using the coil in step 6, move the coil through the magnetic field, but change the angle that the coil moves through the field.
8) Record your observations below.
Observations
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Practical Task: Motor Effect
Task Instructions
Observation Questions
5) Explain in your own words what causes a conductor in a magnetic field carrying current to move.
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Practical Task: Back E.M.F
Materials required
Task instructions
1. Measure the resistance of the armature winding as shown in figure 1.
2. Using the formula derived from ohms law, calculate how much current the motor should draw when connected to a 32 volt d.c supply.
3. Re-assemble the motor and connect to the 32 volt d.c supply with a fluke 117 multi meter connected in series to measure circuit current.
4. Turn on the 32 volt d.c supply and take note of the circuit current being drawn by the motor.
5. Complete the questions below.
Questions
1) What was the resistance of the armature winding?
________________
2) What current would you expect this motor to draw if it were connected to a 32 volt d.c supply?
(Show working)
__________________________________________________________________________________________________________________________________________________________________
3) What was the actual current draw when connected to the 32 volt supply?
_________________
4) What is the value of back e.m.f being generated in the motor and what is the effective voltage pushing current? (show working)
Back e.m.f _____________________________________________________________
Effective Voltage _____________________________________________________________
Practical Task: Effect of air gap in a magnetic circuit
Task Instructions
Transformers
In its most basic form a transformer consists of:
• A primary coil or winding.
• A secondary coil or winding.
• A core that supports the coils or windings.
Refer to the transformer circuit in figure1 as you read the following explanation: The primary winding is connected to a 50 hertz ac voltage source. The magnetic field (flux) builds up (expands) and collapses (contracts) about the primary winding. The expanding and contracting magnetic field around the primary winding cuts the secondary winding and induces an alternating voltage into the winding. This voltage causes alternating current to flow through the load. The voltage may be stepped up or down depending on the design of the primary and secondary windings.
Figure 1 - Basic transformer action.
The three basic parts of a transformer
Two coils of wire (called windings) are wound on some type of core material. In some cases the coils of wire are wound on a cylindrical or rectangular cardboard form or ceramic material. In effect, the core material is air and the transformer is called an Air-core transformers. Transformers used at low frequencies, such as 50 hertz and 400 hertz, require a core of low-reluctance magnetic material, usually iron. This type of transformer is called an Iron-core transformers. Most power transformers are of the iron-core type. The principle parts of a transformer and their functions are:
• The Core, which provides a path for the magnetic lines of flux.
• The Primary Winding, which receives energy from the ac source.
• The Secondary Winding, which receives energy from the primary winding and delivers it to the load.
• The Enclosure, which protects the above components from dirt, moisture, and mechanical damage. The enclosure is in addition to the three basic components. Some transformers do not have an enclosure or are being installed within an enclosure containing other electronic parts.
Core characteristics
The composition of a transformer core depends on such factors as voltage, current, and frequency. Size limitations and construction costs are also factors to be considered. Commonly used core materials are air, soft iron, and steel. Each of these materials is suitable for particular applications and unsuitable for others. Generally, air-core transformers are used when the voltage source has a high frequency (above 20 kHz). Iron-core transformers are usually used when the source frequency is low (below 20 kHz).
A soft-iron-core transformer is very useful where the transformer must be physically small, yet efficient. The iron-core transformer provides better power transfer than does the air-core transformer. A transformer whose core is constructed of laminated sheets of steel dissipates heat readily; thus it provides for the efficient transfer of power. The majority of transformers you will encounter in equipment contain laminated-steel cores. These steel laminations (see figure 2) are insulated with a non-conducting material, such as varnish, and then formed into a core. It takes about 50 such laminations to make a core an inch thick.
The purpose of the laminations is to reduce certain losses which will be discussed later in this chapter. An important point to remember is that the most efficient transformer core is one that offers the best path for the most lines of flux with the least loss in magnetic and electrical energy.
Figure 2 - Hollow-core construction.
Hollow-Core Transformers
There are two main shapes of cores used in laminated-steel-core transformers. One is the Hollow core, so named because the core is shaped with a hollow square through the centre. Figure 2 illustrates this shape of core. Notice that the core is made up of many laminations of steel. Figure 3 illustrates how the transformer windings are wrapped around both sides of the core.
Figure 3. - Windings wrapped around laminations.
Shell-Core Transformers
The most popular and efficient transformer core is the Shell core, as illustrated in figure 4. As shown, each layer of the core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations. The laminations are insulated from each other and then pressed together to form the core.
Figure 4 Shell-type core construction.
Transformer Windings
As stated above, the transformer consists of two coils called WINDINGS which are wrapped around a core. The transformer operates when a source of ac voltage is connected to one of the windings and a load device is connected to the other. The winding that is connected to the source is called the PRIMARY WINDING. The winding that is connected to the load is called the SECONDARY WINDING. (Note: In this chapter the terms "primary winding" and "primary" are used interchangeably; the term: "secondary winding" and "secondary" are also used interchangeably.)
Figure 5 shows an exploded view of a shell-type transformer. The primary is wound in layers directly on a rectangular cardboard form.
Figure 5. - Exploded view of shell-type transformer construction.
In the transformer shown in the cutaway view in figure 6, the primary consists of many turns of relatively small wire. The wire is coated with varnish so that each turn of the winding is insulated from every other turn. In a transformer designed for high-voltage applications, sheets of insulating material, such as paper, are placed between the layers of windings to provide additional insulation.
Figure 6 - Cutaway view of shell-type core with windings.
When the primary winding is completely wound, it is wrapped in insulating paper or cloth. The secondary winding is then wound on top of the primary winding. After the secondary winding is complete, it too is covered with insulating paper. Next, the E and I sections of the iron core are inserted into and around the windings as shown.
The leads from the windings are normally brought out through a hole in the enclosure of the transformer. Sometimes, terminals may be provided on the enclosure for connections to the windings. The figure shows four leads, two from the primary and two from the secondary. These leads are to be connected to the source and load, respectively.
Schematic symbols for transformers
Figure 7 shows typical schematic symbols for transformers. The symbol for an air-core transformer is shown in (A). (B) & (C) show iron-core transformers. The bars between the coils are used to indicate an iron core. Frequently, additional connections are made to the transformer windings at points other than the ends of the windings. These additional connections are called Taps. When a tap is connected to the centre of the winding, it is called a Centre tap. Figure (C) shows the schematic representation of a centre-tapped iron-core transformer.
The AS/NZS symbols are applicable to Australia and New Zealand
Figure 7 - Schematic symbols for various types of transformers.
No-Load Condition
You have learned that a transformer is capable of supplying voltages which are usually higher or lower than the source voltage. This is accomplished through mutual induction, which takes place when the changing magnetic field produced by the primary voltage cuts the secondary winding.
A no-load condition exists when a voltage is applied to the primary, but no load is connected to the secondary, as illustrated by figure 8. Because of the open switch, there is no current flowing in the secondary winding. With the switch open and an ac voltage applied to the primary, there is, however, a very small amount of current called magnetising current flowing in the primary. Essentially, what the magnetising current does is "excite" the coil of the primary to create a magnetic field. The amount of magnetising current is determined by three factors: (1) the amount of voltage applied (Ep), (2) the resistance (R) of the primary coil's wire and core losses, and (3) the XL which is dependent on the frequency of the magnetising current. These last two factors are controlled by transformer design.
Figure 8 - Transformer under no-load conditions.
This very small amount of magnetising current serves two functions:
• Most of the magnetizing energy is used to maintain the magnetic field of the primary.
• A small amount of energy is used to overcome the resistance of the wire and core losses which are dissipated in the form of heat (power loss).
Magnetising current will flow in the primary winding at all times to maintain this magnetic field, but no transfer of energy will take place as long as the secondary circuit is open.
Producing a Counter E.M.F
When an alternating current flows through a primary winding, a magnetic field is established around the winding. As the lines of flux expand outward, relative motion is present, and a counter E.M.F is induced in the winding. This is the same counter E.M.F that you learned about in the chapter on inductors. Flux leaves the primary at the North Pole and enters the primary at the South Pole. The counter E.M.F induced in the primary has a polarity that opposes the applied voltage, thus opposing the flow of current in the primary. It is the counter E.M.F that limits magnetising current to a very low value.
Inducing a Voltage into the Secondary Winding
To visualize how a voltage is induced into the secondary winding of a transformer, again refer to figure 8. As the magnetising current flows through the primary, magnetic lines of force are generated.
During the time current is increasing in the primary, magnetic lines of force expand outward from the primary and cut the secondary. As you remember, a voltage is induced into a coil when magnetic lines cut across it. Therefore, the voltage across the primary causes a voltage to be induced across the secondary.
Primary and secondary Phase relationship
The secondary voltage of a simple transformer may be either in phase or out of phase with the primary voltage. This depends on the direction in which the windings are wound and the arrangement of the connections to the external circuit (load). Simply, this means that the two voltages may rise and fall together or one may rise while the other is falling.
Transformers in which the secondary voltage is in phase with the primary are referred to as LIKE-WOUND transformers, while those in which the voltages are 180 degrees out of phase are called UNLIKE-WOUND transformers.
Dots are used to indicate points on a transformer schematic symbol that have the same instantaneous polarity (points that are in phase).
The use of phase-indicating dots is illustrated in figure 5-9. In part (A) of the figure, both the primary and secondary windings are wound from top to bottom in a clockwise direction, as viewed from above the windings. When constructed in this manner, the top lead of the primary and the top lead of the secondary have the SAME polarity. This is indicated by the dots on the transformer symbol. A lack of phasing dots indicates a reversal of polarity.
Figure 9 - Instantaneous polarity depends on direction of winding.
Part (B) of the figure illustrates a transformer in which the primary and secondary are wound in opposite directions. As viewed from above the windings, the primary is wound in a clockwise direction from top to bottom, while the secondary is wound in a counter clockwise direction. Notice that the top leads of the primary and secondary have OPPOSITE polarities. This is indicated by the dots being placed on opposite ends of the transformer symbol. Thus, the polarity of the voltage at the terminals of the secondary of a transformer depends on the direction in which the secondary is wound with respect to the primary.
Turns and Voltage ratio
The total voltage induced into the secondary winding of a transformer is determined mainly by the Ratio of the number of turns in the primary to the number of turns in the secondary, and by the amount of voltage applied to the primary. Refer to figure 1. Part (A) of the figure shows a transformer whose primary consists of ten turns of wire and whose secondary consists of a single turn of wire. You know that as lines of flux generated by the primary expand and collapse, they cut both the ten turns of the primary and the single turn of the secondary. Since the length of the wire in the secondary is approximately the same as the length of the wire in each turn in the primary,
E.M.F induced into the Secondary will be the same as the E.M.F induced into each turn in the Primary.
This means that if the voltage applied to the primary winding is 10 volts, the counter E.M.F in the primary is almost 10 volts. Thus, each turn in the primary will have an induced counter E.M.F of approximately one-tenth of the total applied voltage, or one volt. Since the same flux lines cut the turns in both the secondary and the primary, each turn will have an E.M.F of one volt induced into it. The transformer in part (A) of figure 10 has only one turn in the secondary, thus, the E.M.F across the secondary is one volt.
The transformer represented in part (B) of figure 10 has a ten-turn primary and a two-turn secondary. Since the flux induces one volt per turn, the total voltage across the secondary is two volts. Notice that the volts per turn are the same for both primary and secondary windings.
Since the counter E.M.F in the primary is equal (or almost) to the applied voltage, a proportion may be set up to express the value of the voltage induced in terms of the voltage applied to the primary and the number of turns in each winding. This proportion also shows the relationship between the number of turns in each winding and the voltage across each winding. This proportion is expressed by the equation:
The transformer in each of the above problems has fewer turns in the secondary than in the primary. As a result, there is less voltage across the secondary than across the primary. A transformer in which the voltage across the secondary is less than the voltage across the primary is called a STEP-DOWN transformer. The ratio of a four-to-one step-down transformer is written as 4:1. A transformer that has fewer turns in the primary than in the secondary will produce a greater voltage across the secondary than the voltage applied to the primary. A transformer in which the voltage across the secondary is greater than the voltage applied to the primary is called a STEP-UP transformer. The ratio of a one-to-four step-up transformer should be written as 1:4. Notice in the two ratios that the value of the primary winding is always stated first.
Transformer losses
Copper Loss
Whenever current flows in a conductor, power is dissipated in the resistance of the conductor in the form of heat. The amount of power dissipated by the conductor is directly proportional to the resistance of the wire, and to the square of the current through it. The greater the value of resistance or current, the greater is the power dissipated. The primary and secondary windings of a transformer are usually made of low-resistance copper wire.
The resistance of a given winding is a function of the diameter of the wire and its length. Copper loss can be minimized by using the proper diameter wire. Large diameter wire is required for high-current windings, whereas small diameter wire can be used for low-current windings.
Eddy-Current Loss
The core of a transformer is usually constructed of some type of ferromagnetic material because it is a good conductor of magnetic lines of flux.
Whenever the primary of an iron-core transformer is energized by an alternating-current source, a fluctuating magnetic field is produced. This magnetic field cuts the conducting core material and induces a voltage into it. The induced voltage causes random currents to flow through the core which dissipates power in the form of heat. These undesirable currents are called
Eddy currents.
To minimize the loss resulting from eddy currents, transformer cores are LAMINATED. Since the thin, insulated laminations do not provide an easy path for current, eddy-current losses are greatly reduced.
Hysteresis Loss
When a magnetic field is passed through a core, the core material becomes magnetized. To become magnetized, the domains within the core must align themselves with the external field. If the direction of the field is reversed, the domains must turn so that their poles are aligned with the new direction of the external field.
Power transformers normally operate from either 60 Hz, or 400 Hz alternating current. Each tiny domain must realign itself twice during each cycle, or a total of 120 times a second when 60 Hz alternating current is used. The energy used to turn each domain is dissipated as heat within the iron core. This loss, called HYSTERESIS LOSS, can be thought of as resulting from molecular friction. Hysteresis loss can be held to a small value by proper choice of core materials.
Transformer efficiency
To compute the efficiency of a transformer, the input power to and the output power from the transformer must be known. The input power is equal to the product of the voltage applied to the primary and the current in the primary. The output power is equal to the product of the voltage across the secondary and the current in the secondary. The difference between the input power and the output power represents a power loss.
Types and applications of transformers
Types of transformers used in the Electrotechnology Industry are:
• Distribution Transformers
• Power Transformers
• Control Transformers
• Auto Transformers
• Isolation Transformer
• Potential Transformer
• Current Transformer
• Toroidal Transformer
Isolating transformer
Most transformers isolate, meaning the secondary winding is not connected to the primary. But this isn't true of all transformers.
However the term 'isolating transformer' is normally applied to mains transformers providing isolation rather than voltage transformation. They are simply 1:1 laminated core transformers. Extra voltage tapings are sometimes included, but to earn the name 'isolating transformer' it is expected that they will usually be used at 1:1 ratio.
Current transformer
A current transformer (CT) is a series connected measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary. Current transformers are commonly used in metering and protective relays in the electrical power industry. Current transformers used in metering equipment for three-phase 400 ampere electricity supply
Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. The CT is typically described by its current ratio from primary to secondary. For example, a 1000:1 CT would provide an output current of 1 amperes when 1000 amperes were passing through the primary winding. Standard secondary current ratings are 5 amperes or 1 ampere, compatible with standard measuring instruments. The secondary winding can be single ratio or have several tap points to provide a range of ratios.
Care must be taken that the secondary winding is not disconnected from its low-impedance load while current flows in the primary, as this may produce a dangerously high voltage across the open secondary and may permanently affect the accuracy of the transformer.
Potential transformer
Voltage transformers (VT) (also called potential transformers (PT)) are a parallel connected type of instrument transformer, used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have an accurate voltage ratio to enable accurate metering.
Autotransformer
A transformer that enables voltage transformation with only one winding is called an autotransformer. It consists of a single continuous winding that is tapped on one side to provide a step up or step down purpose. The autotransformer winding is both electrically and magnetically interconnected. Because this transformer does not have the advantage of double wound transformers (dividing up circuits into electrically isolated sections) a person coming into contact with either of the secondary leads may receive an electric shock equal to the primary voltage if the lead is common to both open circuits. Applications for autotransformers include; motor speed control, transformer impedance testing, laboratory applications.
Control Transformer
A control transformer is a device used to transform or "step down" a high main circuit voltage to a lower voltage which is then used to operate the control or switching components of the main circuit. These devices are commonly used in industrial starter circuits where the main circuit voltage is not suitable for use in the control circuit and where a separate control circuit feed would not be practical.
Toroidal Transformer
Toroidal transformers are made with tape-wound circular ferromagnetic cores. This class of transformer offers many advantages over a conventional laminated transformer.
The type of core provides an almost perfect magnetic circuit having its primary and secondary windings uniformly distributed around the core. The ring core minimises losses, fringing, leakage and distortion, and provides good magnetic shielding.
These transformers also decrease the magnetising force required to produce a given flux density and they are much more efficient than the E type lamination cores. It is these attributes that make the toroidal transformer very quiet and efficient.
Toroidal transformers are smaller and lighter that the lamination type transformers making them perfect for use in power supplies and amplifier power supplies.
Distribution Transformer
A distribution transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. If mounted on a utility pole, they are called pole-mount transformers. If the distribution lines are located at ground level or underground, distribution transformers are mounted on concrete pads and locked in steel cases, thus known as pad-mount transformers.
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D.B
Distribution Board
M.S.B
Main Switch Board
P.O.S
Point Of Supply
Final Sub Circuit
Sub Mains
Consumers Mains
Appliance
Final Sub Circuit
Appliance
Domestic Meter Box
Fuse Board
Circuit Breaker Board
Commercial Switch Board
IEC Category III at 1000 volts
IEC Category IV at 600 volts
Insulation Resistance ircuit
Appliance
Domestic Meter Box
Fuse Board
Circuit Breaker Board
Commercial Switch Board
IEC Category III at 1000 volts
IEC Category IV at 600 volts
Insulation Resistance Functions:
1000 volt
500 volt
250 volt
Low Reading Ohm-meter:
3Ω scale
500 Ω scale
Digital Clamp Meter
Analog Clamp Meter
Calculations
Cable Resistance:
• Active conductor 1.5Ω
• Neutral conductor 1.5Ω
• Total cable resistance 3 Ω
230v supply voltage
Connected load 10A
Active 1.5Ω
Load 10A
V1
230 V
V2
V3
Neutral 1.5Ω
Calculations
Right-hand thumb rule – Grasp the coil, so that the fingers point in the direction of the conventional current flowing through the coil. Extend the thumb at right angles to the fingers and the thumb will point in the direction of the North-pole.
Conductor
North
Pole
South
Pole
8 Turns
4 Turns
8 Turns
1. Set up your circuit as shown in figure 1. There are 2 d.c power supplies used for this practical task. One power supply is to establish a magnetic field and the other is to supply current to the conductor used (figure 2).
2. Energise the d.c power supply to establish a magnetic field in the transformer. Ensure that the voltage is set to 32 volts and the current control is set to maximum.
3. Set the current to minimum and the voltage to 32 volts on the second power supply.
4. Energise the second power supply and slowly wind up the current control.
5. Take note of what you see happening
6. Set the current to minimum on the second power supply.
7. Change the direction of current flow through the conductor within the magnetic field by twisting the conductor as shown in figure 4
8. Energise the second power supply and slowly wind up the current control.
9. Take note of what you see happening
10. Answer the observation questions below
Figure 1
Figure 2
1) What occurred when current was passed through the conductor within the magnetic field?
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2) What occurred when the direction of current was reversed in the conductor within the magnetic field?
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3) What rule is used to determine the direction a conductor will move carrying current within a magnetic field?
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Figure 3
Figure 4
Fluke 117 Multi-meter
32 Volt D.C Supply
Permanent Magnet D.C Motor
Ω
1. Measure the resistance of the transformer coil shown in figure 1.
2. Calculate what current you would expect to see drawn from the supply by the transformer.
The supply will be 240 volts.
3. Connect the transformer up to a variac and fit a clamp meter over one of the conductors to measure circuit current.
4. Energise the supply and wind the variac up to 240 volts as shown in figure 3.
5. Take note of the current reading on the clamp meter.
6. Wind the variac down to 0 volts
7. Introduce an air gap into the magnetic circuit of about 10 mm as shown in figure 2.
8. Slowly wind the variac voltage up and take note of the current draw.
DO NOT GO PAST 5 AMPS
9. Adjust the variac voltage so that you have a current draw of 1 amp with the 10mm air gap.
10. Slowly adjust the air gap distance greater than 10mm and take note of what happens to the current.
11. Answer the observation questions below.
Resistance of transformer coil: ______________
Expected current draw: ______________
I = V/R
Figure 1
1) What was the current draw with no gap in the magnetic circuit?
________________
2) What happened to the circuit current when an air gap was introduced into the magnetic circuit?
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3) What effect did increasing the air gap have on the circuit current?
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4) In your own words, describe why this circuit behaved this way. What was causing these results?
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Observation Questions
Figure 3
Figure 2
Figure 1
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