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EE 231
Electro Mechanical System Lab
Experiment # 6 Single Phase Transformers Fall 2011
Objectives:
To determine the polarity of a transformer using a dc source. To determine the polarity of a transformer using an ac source. To determine the current ratio of a transformer. To determine the impedance of a transformer. To perform open circuit and short circuit test to evaluate transformer equivalent circuit parameters. To observe the voltage regulation of a transformer for resistive, inductive and capacitive loads.
Equipment:
Description Model Three phase transformer bank EMS 8348 Three phase inductive load EMS 8321 Variable Resistance EMS 83 11 Mag-Tran parts EMS 8355 Three phase WATT/VAR meter EMS 8446 DC Voltmeter/Ammeter EMS 84 12 AC Voltmeter EMS 8426 AC Ammeter EMS 8425 Power Supply EMS 8821 Connection Leads EMS 8941
Table 1
The Mag Tran parts you will use are the following:
Qty Description Item* 3 133 mm Laminated Bar 1 1 133 mm Laminated Bar with Hook 3 1 Mounting Base 9 2 Coil 10
Table 2
*Before proceeding, consult Appendix D for the identification of the Mag Tran parts.
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Discussion:
Polarity of a Transformer: The primary and secondary voltages of a transformer increase and decrease in steps reaching their maximum values at practically the same instant. Because the voltages are alternating, the polarity of the terminals is continually changing. In Figure 1, because of the ac source Es, the terminal 1 is momentarily positive with respect to terminal 2, and an instant later its polarity is reversed. The same is true for the polarity of terminal 3 with respect to terminal 4.
The ‘polarity’ of a transformer tells us which terminals of the transformer are positive at the same instant. For example, when terminal 1 in Figure 1 is momentarily positive with respect to terminal 2, the polarity of the transformer indicates whether terminal 3 is momentarily positive or negative with respect to terminal 4.
One quick way to determine the polarity is to momentarily connect the primary winding P to a dc source, as shown in Figure 2. This causes a momentary current to flow in primary winding P so a flux is created in the core. The change in flux induces a momentary voltage across terminals 3 and 4 whose polarity can be found by means of a dc voltmeter E.
Figure 1. Transformer. Figure 2. Circuit used to determine the transformer polarity using a dc source.
In Figure 2, terminal 3 is connected to the positive terminal of the voltmeter, and terminal 1 is connected to the positive side of the dc source. When the switch is closed, terminal 1 becomes positive with respect to terminal 2. If at the same time the pointer on the voltmeter E moves upscale, it shows that terminal 3 is positive with terminal 4. In this case, we can state that the polarity of the transformer is such that terminal 1 is positive with respect to 2 at the same instant as terminal 3 is positive with respect to 4.
If the pointer shows a negative reading when the switch is closed, the terminal 3 is negative with respect to 4. In this case, the polarity of the transformer is such that 4 is positive with respect to 3 when terminal 1 is positive with respect to 2.
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Figure 4. Circuit used to measure the current ratio.
A practical transformer can be represented as an ideal transformer with a resistance R and a reactance connected in series with each of the windings, as shown in Figure 5.
Figure 5. Equivalent circuit of a transformer
Since a transformer can transform impedances, the resistance and reactance of one winding in the equivalent circuit can be transferred, or referred, to the other winding by multiplying their value by the square of the turns ratio. Figure 6 shows the same equivalent circuit as in Figure 7, but with all the impedances referred to the primary winding.
Figure 6. Equivalent circuit with all impedances referred to the primary winding.
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Figure 7. Equivalent circuit of a transformer.
Open Circuit Test: The rated voltage is applied to the primary side. Then voltage, current and power is measured.
Figure 8. Open Circuit Test
Short Circuit Test: The input voltage is adjusted until the current in the short circuited windings is equal to its rated value.
Figure 9. Short Circuit Test.
Voc YE Ioc
V ocIoc p.f cos θ Poc
θ V oc YE Ioc
I sc ZSE Vsc
V scIsc p.f cosθ Psc
I scsc θ Req jXeq Z V SE
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- Set the voltmeter switch on the power supply to position 7-N. Turn on the power. Observing the voltmeter on the power supply, set the voltage to 100 V dc. Turn switch S on and off several times.
Q1) On closing the switch, answer the following questions. a) Does the pointer of voltmeter E move upscale?
Yes No
b) What is the instantaneous polarity of terminal 3 with respect to terminal 4?
Positive Negative
c) What is the instantaneous polarity of terminal 1 with respect to terminal 2?
Positive Negative
d) Which of the following terminals are positive at the same time?
1 and 3 1 and 4
Turn off the power.
- Keep the circuit of Figure 10 but connect the DC Voltmeter E to terminals 3 and 4 of the coil B. Be sure terminal 3 is connected to the positive side of the DC Voltmeter.
- Turn on the power. Using the meter on the power supply (position 7-N), set the voltage to 100 V dc. Turn switch S on and off several times.
Q2) On closing the switch, answer the following questions.
a) Does the pointer of voltmeter E move upscale?
Yes No
b) What is the instantaneous polarity of terminal 3 with respect terminal 4?
Positive Negative
c) What is the instantaneous polarity of terminal 1 of coil A with respect to terminal 2?
Positive Negative
d) Which of the following terminals are positive at the same time?
1 and 3 1 and 4
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Although the coils A and B are identical, why is the polarity in this step not the same as that in step 2?
Turn off the power.
- Set up the circuit of Figure 11. Tighten the screws to minimize the air gaps. Note that the jumper between terminals 2 and 3 connects the two windings in series. The AC Voltmeters Es and E will enable us to determine the polarity of the transformer.
Figure 11. AC Circuit used to determine the polarity of a transformer.
- Turn on the power and adjust Es to 100 V ac. Measure the value of the series voltage E. E = _______ V ac
Q3a) Do terminals 2 and 3 have the same or opposite instantaneous polarities?
Same Opposite
Q3b) When terminal 3 is positive with respect to 4, then terminal 1 is ______ with respect to terminal 2. When terminal 2 is negative with respect to terminal 1, then terminal 4 is ________ with respect to terminal 3.
Turn off the power.
- Set up the circuit as shown in Figure 12. The secondary winding is short- circuited through ammeter I 2. Tighten the screws in order to minimize the air gaps in the magnetic circuit.
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- Connect one transformer from the three-phase transformer bank (EMS 8348) for open-circuit as illustrated in Figure 13. Note that each transformer is rated at 380/380-220-160 V Use the 4-5 line voltage from the three-phase power supply (EMS8821) to supply an input voltage of approximately 220V (Line voltage). Record the input voltage, current and real power. Then turn the voltage control knob fully CCW (0 position) and turn off the main power supply.
Figure 13. Circuit used to perform open circuit test of a transformer.
Quantity Value Units V I P Table 3
- Connect the same transformer used in step 9 for the short circuit as shown in Figure 14. Turn on the power supply. Gradually increase the input voltage with the help of Voltage control knob until the current reaches 0.60 A on the secondary side ( meter I2 ) ( you must connect Ammeter in series with the short circuit at the secondary side ). Record the voltage, current and real power. Then turn the voltage control knob fully CCW (0 position) and turn off the main power supply.
Figure 14. Circuit used to perform short-circuit test of a transformer.
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Quantity Value Units V I P Table 4 Q5) From the data obtained in steps 9 and 10, compute and draw the equivalent circuit of the form provided.
- Connect the same transformer used in step 9 for the short circuit as shown in Figure 14. Turn on the power supply. Gradually increase the input voltage until the current reaches 0.60A. Record the voltage, current and real power. Then turn the Voltage control knob fully CCW (0 position) & turn off the main power supply.
Figure 14.Short Circuit test of a transformer with a different configuration.
Quantity Value Units V I P Table 5
Q6) From the data obtained in steps 9 - 11, compute and draw the equivalent circuit of the form provided.
- Connect voltmeter to the secondary side of the same transformer used in step 9 and adjust the input voltage at primary side to 220 V (phase voltage) then measure the no load voltage. Vnl= _________ V
- Connect resistive load load (R = 685.7 Ω) to the secondary side of the transformer in step 12 and two Watt/Var meter one to the primary side and another one to the secondary side. Then record your measurements of Vfl, Pin and Pout in the following table.
Vfl Pin Pout VR η
Table 6
