7 Transformers Section 26 of the electrical code governs the use and installations of transformers. A transformer is a static device used to transfer energy from one alternating current circuit to another. It is important to understand that the frequency will remain the same in both circuits of a transformer. A step-up transformer is used to increase the voltage. A step-down transformer is used to decrease the voltage. Transformers operate on the principle of mutual induction. Mutual induction occurs when the magnetic field surrounding one conductor cuts across another conductor inducing a voltage in it. This would explain why constant DC voltages cannot be transformed. When any transformer is connected to a supply with no load, a small current will flow in the primary winding. This small current is a result of self induction in the primary and is called the exciting current. The voltage induced in the secondary winding is a result of mutual induction. The voltage and the number of turns are proportional. The voltage and current are inversely proportional. This can be shown as: ~=E.i!=h Ns Es Ip where Np = number of turns in primary winding Ns = number of turns in secondary winding Ep = primary coil voltage Es = secondary coil voltage Is = current in secondary Ip = current in primary Transformer Efficiency The efficiency of electrical equipment is usually stated as a ratio of output over input. This can be shown as: % efficiency = output x 100 input Transformers are usually very efficient in operation ( approx. 97%). There are two main losses of a transformer, they are copper losses and core losses. Copper losses are the losses due to the resistance of the copper windings, dissipated as heat. Core losses include hysterisis and eddy currents. Eddy currents are circulating currents induced in the magnetic core by the alternating flux. These are kept to a minimum with the use of laminated iron cores. Hysterisis loss is the power required to continuously reverse the molecules in the core many times per second. Silicone steel is used to keep hysterisis losses to a minimum. Transformers are rated in kv A rather than kw. This is because the power factor of the load determines the transformer output in kw. As the load power factors vary widely, it is not practical to rate transformers in kw. There are several type of transformers for different applications. Power transformers are generally larger than 500 kv A used at sub-stations and power plants. Distribution transformers range from 5 to 500 kv A. These are used to step-down utility voltages to standard voltages used by consumers. Instrument transformers are used in conjunction with test and measurement equipment. A potential transformer is used to step-down system voltages to the voltage rating of the instrument. A current transformer supplies an instrument with a small current which is proportional to the main current. Current transformers are used for metering purposes and the secondary of a current transformer should never be opened under load. It is important to short the secondary of a CT first. Isolation transformers are used to change a grounded system to an ungrounded system. Isolation transformers have a one to one ratio. Autotransformers have a single tapped winding common to both the primary and secondary. It is used where a relatively small increase or decrease in voltage is required. An example would be in an auto-transformer starter, or to boost the voltage of a long transmission line to compensate for the line loss.
It is standard that transformer leads are marked "H" for the high voltage leads, and "X" for the low voltage leads. Note that "H" can be either the primary or secondary leads depending if the transformer is step-up or step-down. Transformer polarity is the polarity of the high voltage leads with respect to the low voltage leads. The HI and Xl leads will have the same polarity at any instant in the alternating cycle. When facing a single-phase transformer from the low voltage terminal side, the HI lead is always on the left. If the Xl leads are on the left, then the polarity of the transformer is said to be subtractive. If the Xl lead is on the right, then the polarity of the transformer is said to be additive. If the leads are not marked a simple voltage test can be done to determine if the transformer is subtractive or additive. See diagrams below. 1. 2. HI HI 120 volts 132 volts 108 volts X2 12 volts Additive Xl 12 volts Subtractive Single-phase distribution transformers Single-phase distribution transformers usually have a split or double secondary winding, which may be connected for two voltage outputs. If the windings are connected in series a voltage of 240 volts would be read between line 1 and line 2. If the windings are connected in parallel a voltage of 120 volts would be read between line 1 and line 2. Note that when the windings are connected in series there is a grounded center tap common to both lines. This is the neutral point. A voltage of 120 volts should be read from any line to neutral. This is referred to as 120/240-volt 3-wire distribution, and is common in residential applications. See diagrams below. 3. 4. 600 600 X4 HI volts 120 volts H2 Xl X4 HI volts 240 volts H2 Xl
8 It is sometimes necessary to parallel transformers in order to distribute peak loads and reduce light flicker caused by heavy surge currents. In order to accomplish this a few important things need to be considered. They are: - voltage rating of the transformers must be identical - The percent impedance should be the same (%Z) - The frequency ratings should be the same for proper sharing of the loads It is important that the above conditions are met before transformers are connected in parallel. If the conditions are not met, circulating currents between transformers could occur, or improper sharing of the loads may occur causing damage to the transformers or the equipment connected. Three Phase Configurations Closed Delta The Delta connection is when three single-phase windings are connected end to end to form a triangle resembling the Greek letter D that is called delta. The end of one phase is connected to the start of the next phase. This would be the connection referred to in a 3 phase, 3-wire system. This system is not grounded. When a ground occurs in one line, the voltage from the grounded phase to ground will be zero. The voltage on the remaining two lines to ground would be the same as the line-to-line voltage of the system. In this connection, it would take at least two lines to ground before any overcurrent device would trip, because the system is not grounded. In the closed delta connection it is important to keep the phases balanced, otherwise different voltages may occur between lines. The advantage in a closed delta system is that the coil current is less than the line current. The value of the coil or phase current is found by dividing the line current by 1.73. The coil voltage and the line voltage are equal. See diagrams below. 5. Line current = coil current x 1.73 Delta - Delta Connection Line voltage = coil voltage 4160 volts 4160 V 4160 V 240V H2 X2 240 HI volts Xl
9 Open Delta Configuration This connection was made when the customer did not require the full capacity of the closed delta system. Ifsinglephase transformers were used to make a three-phase bank, the open delta connection required only two single-phase transformers. Therefore reducing the cost of the installation. The open delta configuration was also used where one single-phase transformer in a three phase closed delta system became defective. The remaining two single phase transformers could be connected open delta until a replacement could be installed. The open delta configuration has some disadvantages. The line current and coil current are equal. The line voltage and coil voltage are equal. Because the line current and coil current are equal, the output capacity is reduced. For example. Three 50 kv A single phase transformers (150 kv A output) were originally connected closed delta but one became damaged. We could connect the remaining two in an open delta configuration. The remaining two transformers (50 kv A each) connected open delta would have an output of ISO kv A (the original kv A rating of the transformers connected closed delta) divided by 1.73 because the line current and coil current are the same. The connection uses two single-phase windings to produce a three-phase connection. This also helps to explain why the open delta connection has a reduced capacity. Another way to explain: is that the output of an open delta system is equal to 86.6% of the two single phase transformers connected, or 57.7% of the output of the original three transformers used in the closed delta configuration. See diagrams below. 6. 7. Line voltage = coil voltage Line current = coil current Open Delta - Open Delta Connection 4160 volts 4160 V 240V HI H2 H2 X2 X2 Xl 240 volts
1 0 The Wye or Star Connection This connection is made when three single-phase coils have one end connected to a common point. Typically, the start of each separate winding is connected to a three-phase line, then the finishes of all three windings are connected together to form the wye point or common point. If a neutral is required, it is connected to the common point. This configuration refers to a three phase, 4-wire system. The common point is grounded for safety. The advantage of a Wye configuration is that the coil voltage is less than the line voltage. The current in the coil is equal to the current in the line. The voltage in the coil is found by dividing the line voltage by 1.73. In this connection the neutral will carry the unbalanced loads between the three lines, which prevents voltage fluctuations between lines. See diagrams below. Line current = coil current Line voltage = coil voltage x 1.73 Wve - Wve Connection 8. 9. 10. A 4160 Volts B 4160 V 4160 V 208V X2 C N N 208 Volts A B C
1 1 i\utotransformers The main difference between conventional transformers an autotransformers are the arrangement of the windings. The autotransformer has a single tapped winding serving as both the primary and the secondary of the transformer. This transformer has a very high efficiency because both the primary and the secondary are in the came winding. The total power consists of two parts, conducted power and transformed power. The conductive power is the portion of the load that is contained in the primary winding and not transferred or transformed. The transformed portion of the load is the power that is transferred or passed by the secondary winding. The autotransformer can be used to stepdown or step-up voltages. See diagram below. 11. 2 4 0 v o l t s Power In = E x I = 300 V x 16 A =4800 VA Conducted Power = E x I = 240 V x 16 A = 3840 VA S e c o n d a r y W i n d i n g s P o w e r 1 O U T = E x l 300 Volts = 2 4
0Vx20A =4800 VA Transformed Power = E x I =240Vx4A = 960 VA LO;\D
1 2 Transformer Practice Questions: 12. A transformer is used to step-down 600 volts to 120 volts. If the secondary coil has 600 turns, how many turns are there on the primary coil? 13. A transformer has 900 turns on the primary and 1557 turns on the secondary. If the secondary voltage is 208 volts, what is the primary voltage? 14. A transformer has a rating of75 KV A. If the two secondary coils of 120 volts each are connected in parallel, what is the full load output in amps? 15. If the transformer in question # 3 had a primary voltage of 600 volts, what is the load input to the primary winding if the percent load is 73 %? 16. If a 250 KV A transformer has two secondary coils rated at 120 volts each and they are connected in series, what is the full load output in amps? 17. Using the transformer from question # 5 with a primary voltage of 600 volts, calculate the full load input to the primary winding.
1 3 Transformer Practice Ouestions: 7. Using the transformer from questions # 5 & 6, calculate the secondary current if the primary current was 347 amps. 8. A single phase transformer has the following nameplate data: Primary voltage- 600 KVA - 333 Secondary voltage - 120/240 Polarity - Subtractive It is connected for 240 volt two wire service. a) Draw a schematic diagram of the connection showing all terminal identification and voltages. If the transformer has a total load of 265 KW at a power factor of.80, find the following: a) Secondary line current b) Primary line current c) Percent loading of the transformer
14 Questions on transformers: 1. The primary and secondary of a distribution transformer are tied together: a. Electrically b. Magnetically c. They are not connected d. Pneumatically 2. The primary side of a transformer carries a small current when the secondary is not connected because of: a. Resistance b. Conductance c. Self inductance d. Mutual inductance 3. The EMF in the secondary winding is a result of: a. Self inductance b. Mutual inductance c. Conductance d. Resistance 4. Laminated cores are used in transformers to reduce: a. Hysterisis b. Eddy currents c. Core losses d. Copper losses 5. Silicone steel is used in transformers to reduce: a. Hysterisis b. Eddy currents c. Copper losses d. Resistance 6. A 10-kV A single-phase transformer supplies a 7.5 kv A load. The input voltage is 600 volts. What is the input current? Note: unity power factor and 100% efficiency a. 16.7 amps b. 12.5 amps c. 7.5 amps d. 10 amps 7. The lead designation "H" on a transformer always refers to: a. High voltage leads b. Low voltage leads c. Primary winding d. Secondary winding 8. A transformer used to change the voltage from 600 volts to 120 volts would be referred to as a: a. Step-up transformer b. Step-down transformer c. High voltage transformer d. Step-up current transformer
1 5 9. Transformers used for metering purposes are referred to as: a. Instrument transformers b. Control transformers c. Signal transformers d. Isolation transformers 10. The secondary of a transformer should never be opened under load. a. Potential transformer b. Current transformer c. Auto-transformer d. Distribution transformer 11. If the secondary winding of an auto-transformer were to become open, the output voltage would: a. Increase b. Decrease c. Stay the same d. Approach the supply voltage 12. Three 50-kV A single-phase transformers are connected to form a three phase transformer bank. What is the total kv A rating of the system? a. 150kVA b. 100kVA c. 259 kva d. 86.6 kva 13. How is the rating ofa power or distribution transformer given? a. InkVA b. InkVARS c. In watts d. In amps 14. If due to a fault, one of the phases ofa 600 volt, three phase, three wire system was grounded, the voltage of the remaining two phases to ground would be: a. Zero b. 208 c. 347 d. 600 15. On a 600 volt, three phase three wire system the approximate voltage to ground is: a. 347 volts b. 208 volts c. 415 volts d. Zero 16. A transformer with part of the primary winding serving as the secondary winding is a/an: a. Current transformer b. Auto-transformer c. Potential transformer d. Open delta transformer
18. 19. Fire Alarm Week #11 1 6 Fire Alarm installations and methods are governed by four codes. The electrical code (section 32), the Ontario building code, the Ontario Fire Code, and the ULC standards. It is important to consult the appropriate codes when completing an installation. For your exam, it is section 32 of the electrical code that you should be familiar with. There are several parts to a fire alarm system. These are: -Alarm initiating devices -Signal devices -Fire alarm control panel (F ACP) -Annunciator panel (when required) -Ancillary and auxiliary devices (fan shut down) -End of the line devices (class B) Class of wirine: There are two methods or classes of wiring fire alarm circuits. Class A circuits are those that have two wires leaving the panel (positive and negative) loop to every device and then return to the fire alarm panel (positive and negative). In class A circuits no end of the line device is required because the fire alarm module internally supervises the circuit. This is the safest type of fire alarm wiring method. If a break were to occur in the circuit, the module would sense a trouble, but would also be able to receive an alarm signal from both directions. Note that all fire alarm circuits must be electrically supervised. See diagram below. Initiating Module Class "A" Heat Detector Heat Detector Heat Detector + Class B circuits Note: 2 wires leave the module and 2 wires return to the module, return wires are to be in a separate cable or raceway. Class B circuits require and end of the line device to limit the supervisory current. The end of the line device is usually located in a separate box beyond the last device of that circuit. This is the most common type of fire alarm circuit. See diagram below. ELR Initiating Module Class "B" Heat Detector Heat Detector Heat Detector Note: Only 2 wires are required from the module to the first device, but sometimes the ELR is located in the F ACP however, it is still considered to be a class "B" circuit.