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ECE 4501 Power Systems Laboratory Manual Rev 1.0

4.0 TRANSFORMERS

4.1 THE SINGLE PHASE TRANSFORMER

4.1.1 OBJECTIVE

To study the voltage and current ratios of a transformer and learn about exciting currents, volt-ampere capacity and short-circuit currents.

4.1.2 DISCUSSION

Transformers are arguably the most universally applied pieces of electrical equipment. As such, they range in size from miniature units weighing ounces to huge units weighing tons. All transformers, however, exhibit the same basic properties.

When mutual induction is permitted between two coils or windings, a change in current flowing through one coil induces a voltage upon the other coil. All transformers have a primary winding and one or more secondary windings. The electromagnetic coupling between the primary and secondary windings allows electrical energy to be transferred from the primary winding to the secondary winding. Electrical current entering the primary winding appears as an electromotive force (emf) at the secondary. Connecting the secondary winding to a load allows the energy to be transferred to the load. Since there is no electrical connection between primary and secondary windings (only a magnetic connection), the source and load can be electrically isolated from each other by means of a transformer.

When a transformer is energized and loaded, AC current flowing in its windings creates an alternating magnetic field in its iron core. A small portion of the current, called the magnetizing current, is dedicated to the magnetic circuit in the creation of the magnetic field. Losses associated with the magnetizing current are reactive power (VARs). In addition, there are real power losses (Watts) in the transformer, associated with the inherent resistance in the windings (copper losses) and with eddy currents and hysteresis in the core (iron losses). For these reasons, the total power delivered to the primary side of the transformer is always larger than the total power available at the secondary side. Even so, it is still reasonable to say that energy is conserved in the transformer and that the real, reactive and apparent power applied to the primary of almost any transformer equals the real, reactive and apparent power available at the secondary.

When the voltage applied to the primary winding is raised above rated value, the iron core begins to saturate, which leads to a rapid increase in the magnitude of the magnetizing current. Saturation of the core also distorts the sinusoidal voltage and current waveforms. The resulting harmonics can lead to mechanical resonances which, in large transformers, can be damaging. Transformers are also very susceptible to damage from short circuit currents.

4.1.3 INSTRUMENTS AND COMPONENTS

Power Supply Module EMS 8821Transformer Module EMS 8341AC Metering Module (2.5 A) EMS 8425AC Metering Module (250 V) EMS 8426

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Ohmmeter ---

4.1.4 PROCEDURE

CAUTION! – High voltages are present in this Experiment. DO NOT make any connections with the power supply ON. Get in the habit of turning OFF the power supply after every measurement.

1) Examine the construction of the Transformer Module, EMS 8341, paying particular attention to the laminated steel core and the three windings.

2) List the rated voltage for each of the three windings, list the rated voltage between each of the identified connection terminals, and list the rated current for each of the listed connections:

Rated Voltage for Each Winding:

Rated Voltage Between Connections:

Rated Current Between Connections:

Terminals 1 to 2: _______ V rms










Terminals 1 to 2: _______ A rms





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3) Using an Ohmmeter, measure and record the DC resistance of each of the following windings:

DC Resistance of each Transformer Winding:

Terminals 1 to 2: _______ Ohms








4) Now connect the following transformer circuit:

Output Voltages:

V 1 to 2: _______ V rms

V 3 to 4: _______ V rms

V 5 to 6: _______ V rms

V 3 to 7: _______ V rms

V 7 to 8: _______ V rms

V 8 to 4: _______ V rms

V 5 to 9: _______ V rms

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V 9 to 6: _______ V rms

5) Turn on the power supply and adjust the voltage control until the voltage, V4N, reads 120 V rms on the voltmeter module.

6) Measure and record each of the output voltages listed above. Be sure to TURN OFF the power supply BEFORE changing the voltmeter leads.

7) Measure and record the Ammeter reading: _________________ Amps

8) Turn the voltage control knob to zero percent and turn OFF the power supply.

9) Do the measured voltages correspond with the rated values? ________________

10) What does the current measurement taken in step 7) represent? __________________

______________________________________________________________________

11) Windings 1 to 2 and 5 to 6 each have 500 turns of wire. Winding 3 to 4 has 865 turns. Calculate the following turns ratios:

Turns ratio = N1/N2 = Winding 1 to 2 / Winding 5 to 6 = ____________

Turns ratio = N1/N3 = Winding 1 to 2 / Winding 3 to 4 = ____________

12) Now connect the circuit shown below. Notice that the second Ammeter provides a very low impedance (short circuit) path between terminals 5 and 6.

Short Circuit Test 1:

V1 = _______ V rms

I1 = _______ A rms

I2 = __0.4 __ A rms

Current Ratio:

I1/I2 = ________

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13) Turn on the power supply and CAREFULLY increase the voltage until the short circuit current, I2, is 0.4 A rms.

14) Measure and record V1, I1.


16) Calculate the current ratio, I1/I2.

17) Is this current ratio equal to the turns ratio calculated previously? __________

18) Now connect the second ammeter across winding 3 to 4 as shown below:

Short Circuit Test 2:

V1 = _______ V rms

I1 = __0.4__ A rms

I3 = _______ A rms

Current Ratio:

I1/I3 = ________

19) Turn on the power supply and CAREFULLY increase the voltage until the short circuit current, I1, is 0.4 A rms.

20) Measure and record V1 and I3.


22) Calculate the current ratio, I1/I3.

23) Is this current ratio equal to the turns ratio calculated previously? ___________

Explain why or why not: ____________________________________________

_________________________________________________________________

_________________________________________________________________

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24) To determine the effect of core saturation on magnetizing (exciting) current in the transformer, connect the following circuit. NOTE that this circuit uses line to line voltage, V45.

Core Saturation Test:

V1, Volts:

I1, Amps:

V2, Volts:

25 V mA V

50 V

75 V

100 V

125 V

150 V

175 V

200 V

25) Turn on the power supply and adjust the voltage control for 25 V rms as indicated by the meter, V1.

26) Measure and record the exciting current, I1, and the output voltage, V2.

27) Increase the V1 voltage to 50 V rms and repeat the measurements. Continue until the table is complete.

28) Turn the voltage control to zero percent and turn off the power supply.

29) Plot the recorded current values of I1 versus the secondary voltage, V2, on the graph below and connect them with a smooth curve. Note that the exciting current increases rapidly after a certain threshold voltage.

(See next page)

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30) Compare the recorded values of V1 and V2 . Does the voltage ratio change as the core becomes saturated?

_______________ Explain: __________________________________________________

_________________________________________________________________________

_________________________________________________________________________

4.1.5 CONCLUSIONS

1. If the short-circuit current through winding 9 to 6 is 1.0 A rms, what should the current through the primary winding, 1 to 2, be?

_________________________________________________________________________

_________________________________________________________________________

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2. If winding 7 to 8 is shorted and winding 5 to 6 is considered the primary and is carrying 0.5 A rms, calculate the short circuit current in winding 7 to 8. Also, why should this test be performed very quickly?

_________________________________________________________________________

_________________________________________________________________________

3. If 120 V rms is applied across winding 3 to 4, what will the following voltages read?

V 1 to 2: _______ V rms

V 5 to 9: _______ V rms

V 7 to 8: _______ V rms

4. If 120 V rms is applied to winding 1 to 2 while winding 5 to 6 is short-circuited, as in Short Circuit Test 1, what is the resulting short circuit current? (Try to extrapolate from Test 1 data)

_________________________________________________________________________

_________________________________________________________________________

How much larger is this current than the rated value? (In percent? In per unit?)

_________________________________________________________________________

_________________________________________________________________________

How much more heat than normal is generated in winding 5 to 6 for this condition?

_________________________________________________________________________

_________________________________________________________________________

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4.2 TRANSFORMER POLARITY

4.2.1 OBJECTIVE

To determine transformer winding polarity and explore the impact of connecting windings in series aiding and series opposing configurations.

4.2.2 DISCUSSION

Exciting the primary winding of a transformer with an AC source establishes an alternating magnetic flux in the transformer core. The alternating flux links the turns of the primary and secondary windings, inducing a voltage across the secondary terminals. Since the AC source voltage is continually changing, the voltage applied to the primary is also continually changing. On the positive half of a sinusoidal voltage cycle, H1, the primary winding terminal of the transformer shown below, is of positive polarity with respect to H2. Likewise, on the negative half of a sinusoidal voltage cycle, the polarity of H1 becomes negative with respect to H2.

In addition, the transformer windings may be considered to have polarity marks on each winding to signify all terminals that have the same polarity at any given time. For example, when H1 is positive with respect to H2 in the diagram above, X1 will also be positive with respect to X2 and T1 will be positive with respect to T2 and also to T3. The convention is that current entering a polarity mark on one winding will leave the other winding at its polarity mark.

Transformers are an extremely efficient means for changing the voltage level in power systems, allowing the use of extremely high voltages for low loss transmission of electric energy while maintaining low utilization voltages. Two-winding transformers also offer electric isolation between source and load. Large power transformers are often not built as two-winding transformers however. Due to cost per KVA of transformation, large transformers are usually

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built as autotransformers, a one-winding transformer with a tap for the low voltage side. See the figure below:

It is possible to connect the windings of a two-winding transformer in series to create an autotransformer. As with batteries wired in series, where connecting the positive terminal of one battery to the negative terminal of the other will cause the two battery voltages to add, the transformer windings will “add” if the polarity marked terminal of one winding is connected to the unmarked terminal of the other winding. Likewise, connecting both marked terminals will cause the voltage across the secondary winding to be subtracted from the primary.


Power Supply Module EMS 8821Transformer Module EMS 8341DC Metering Module (20/200 V) EMS 8412AC Metering Module (250 V) EMS 8426

4.2.4 PROCEDURE

CAUTION! – High voltages are present in this Experiment. DO NOT make any connections with the power supply ON. Get in the habit of turning OFF the power supply after every measurement.

1) Connect the 0 – 20 V DC (or 0 – 150 V DC) Meter across terminals 7 and N of the power supply module. Turn on the power supply and adjust the voltage control dial for 10 V DC output. Leave the dial at that setting and turn OFF the power supply.

2) Connect the circuit shown below:

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3) Now turn ON the power supply and immediately note any deflection in the DC meter.

4) Turn OFF the power supply, but do not change the voltage control knob.

If the meter deflects to the right (positive direction), then terminals 1 and 3 of the transformer have the same polarity mark since terminal 1 is connected to the positive terminal of the dc supply and 3 is connected to the positive terminal of the voltmeter. If the meter deflects to the left, then 1 and 4 are of the same polarity mark.

Did the meter deflect right or left? _______________ Which terminal, 3 or 4, is of the

same polarity as 1? _______________

5) Disconnect the DC Voltmeter from terminals 3 and 4 and reconnect it to terminals 5 and 6.

6) Repeat steps 4) and 5) above.

7) Which terminal, 5 or 6, is of the same polarity as terminal 1? ________________

8) To demonstrate the importance of polarity, connect the following circuit (Note that terminals 1 and 5 are wired together):

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Vs = 104 V rms

V 1to 2 = ____________ V rms

V 5 to 6 = ___________ V rms

V 2 to 6 = ___________ V rms

9) Turn on the power supply and adjust the voltage control for 104 V RMS.

10) Measure, then record the voltages in spaces provided next to the circuit diagram.

11) Return the voltage control to zero percent and turn OFF the power supply.

12) Remove the wiring between terminals 1 and 5, wire terminal 1 to terminal 6 and complete the circuit shown below:

Vs = 104 V rms

V 1to 2 = ____________ V rms

V 5 to 6 = ___________ V rms

V 2 to 5 = ___________ V rms

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13) Turn on the power supply and adjust the voltage control for 104 V RMS.

14) Measure, then record the voltages in spaces provided next to the circuit diagram.

15) Return the voltage control to zero percent and turn OFF the power supply.

16) Explain why connecting the same two transformer windings in series (as an autotransformer) produced two different readings in the previous two circuits:

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

17) Examine the circuit shown below:

Vs = 104 V rms

Predicted Voltages:

Measured Voltages:

V 1to 2 = ____________ V rms (two winding operation)

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V 2 to 4 = ___________ V rms (series, subtractive polarity)

V 2 to 3 = ___________ V rms (series, additive polarity)

18) Try to predict three possible output voltages for this transformer, depending upon whether its windings are configured as shown or in series using additive polarity or subtractive polarity. Write them in the space provided above.

19) Wire the circuit, turn on the power supply and adjust the voltage control for 104 V.

20) Measure the voltage between terminals 1 and 2 and record it above.

21) Turn OFF the power supply (leaving the voltage control untouched).

22) Now remove the voltmeter lead from terminal 1 and connect it to terminal 4. Then connect terminal 1 to terminal 3, placing the two windings in series.

23) Turn on the power supply, measure the voltage between terminals 4 and 2 and record it above.

24) Turn OFF the power supply (leaving the voltage control untouched).

25) Remove the voltmeter lead from terminal 4 and connect it to terminal 3. Remove the lead between terminals 1 and 3 and re-connect it between 1 and 4. Now the windings are again in series, but with reverse polarity.

26) Turn on the power supply, measure the voltage between terminals 3 and 2 and record it above.

27) Turn the voltage control to zero percent and turn OFF the power supply.

28) Do the measured voltages agree with your predicted values? ________________________

Explain: _______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

4.2.5 CONCLUSIONS

1. Given a 120 V ac source and the transformer used in this lab, draw four circuit diagrams, each of which has 120 Volts on the primary side of the transformer and the following secondary voltages:

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a.) 240 Voltsb.) 88 Voltsc.) 180 Voltsd.) 92 Volts

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4.3 THREE-PHASE TRANSFORMERS

4.3.1 OBJECTIVE

To explore both types of three-phase connections, DELTA and WYE, by using three single-phase transformers and to compare voltage and current relationships between them.

4.3.2 DISCUSSION

A three-phase transformer may consist of three single-phase windings on the same core inside a single tank or three single-phase transformers wired externally in WYE or DELTA. It is also common practice to construct valid three-phase transformation using only two single-phase transformers. This configuration is called “open-DELTA”.

In three-phase systems, the default voltage designation is the Line-to-Line value, VL-L. Commercial and light industrial systems utilizing three-phase are usually served at 208 Volts (with 120 Volts line-to-neutral). Larger industrial systems utilize 480 Volts (277 Volts line-to-neutral). Very large industrial plants may use 4160 Volts (2400 Volts line-to-neutral) or even higher ranges. A high voltage system will transmit a given amount of power at a lower current than a lower voltage system. This is important for many reasons, not the least of which is I2R losses (efficiency).

There are really only four ways to configure a standard three-phase transformer bank: DELTA-DELTA, WYE-WYE, DELTA-WYE and WYE-DELTA. The most commonly used configuration is DELTA-WYE.

When creating a DELTA winding using single-phase transformers, great care must be taken to ensure proper phasing and proper polarity. If a DELTA is completed with the wrong polarity, it is possible to create a dead short between phases, resulting in great damage to the transformer (and perhaps yourself). NEVER CLOSE A DELTA TRANSFORMER CONNECTION WITHOUT MEASURING THE VOLTAGE ACROSS THE OPEN CORNER. The life you save may be your own.

When a three-phase load is served by two single-phase transformers, the connection is called open-DELTA. The third phase is “constructed” across the open Vee of the delta. Open-DELTA transformer banks will carry less than rated capacity, since both transformers carry not only their own phase current, but also a portion of the current for the third phase. For two transformers of equal size (kVA rating), wired in open-DELTA and serving a balanced three-phase load, the de-rating factor is 58% of a corresponding three-phase bank. For example, two 33.3 kVA transformers that are wired in open-DELTA would have a three-phase capacity of 58 kVA, whereas three transformers in DELTA would have a capacity of 100 kVA.


Power Supply Module EMS 8821Transformer Module (3 of them) EMS 8341AC Metering Module (250 V) EMS 8426

4.3.4 PROCEDURE

CAUTION! – High voltages are present in this Experiment. DO NOT make any connections with the power supply ON. Get in the habit of turning OFF the power supply

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after every measurement.

1.) The transformer circuit shown below has three single-phase transformers connect in a

_________________ configuration.

CALCULATED VALUES MEASURED VALUESV1= 120V V2= 120V V3= 120V V1= 120V V2= 120V V3= 120VV4= V5= V6= V4= V5= V6=

V7= V8= V9= V7= V8= V9=

V10= V11= V12= V10= V11= V12=

2.) Calculate the voltages that should be produced by this circuit and record them in the spaces below the circuit diagram.

3.) Connect the circuit as shown and place voltmeters across the terminals in order to measure V4, V5, and V6.

4.) Turn on the power supply and adjust the voltage control for a Line Voltage of 120 Volts.

5.) Measure and record the indicated voltages in the spaces provided.

6.) Turn the voltage control to zero percent and turn OFF the power supply.

7.) Repeat steps 3, 4 and 5 until all line and phase voltages on the primary and secondary have been measured and recorded.

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V7= V8= V9= V7= V8= V9=







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V7= V8= V9= V7= V8= V9=


17.) Connect the circuit as shown.

18.) IMPORTANT: Open one corner of the DELTA and connect a voltmeter between terminal 5 of one transformer and terminal 6 of the other, as shown.

19.) Turn on the power supply and SLOWLY increase the voltage control while watching the voltmeter placed across the corner of the DELTA. If the circuit is connected properly, the voltmeter will read a small fraction (ideally zero) of the input voltage. If the voltmeter indicates a significant amount of voltage across the corner, turn OFF the power supply and re-check the circuit.

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20.) When the circuit is correct and the voltage across the corner of the DELTA is minimal, make sure the power supply is off, remove the voltmeter from the DELTA, close the DELTA corner and place voltmeters in a manner to record voltages V4, V5 and V6.









27.) Connect the circuit as shown.

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28.) IMPORTANT: Open one corner of the DELTA and connect a voltmeter between terminal 5 of one transformer and terminal 6 of the other, as shown.

29.) Turn on the power supply and SLOWLY increase the voltage control while watching the voltmeter placed across the corner of the DELTA. If the circuit is connected properly, the voltmeter will read a small fraction (ideally zero) of the input voltage. If the voltmeter indicates a significant amount of voltage across the corner, turn OFF the power supply and re-check the circuit.

30.) When the circuit is correct and the voltage across the corner of the DELTA is minimal, make sure the power supply is off, remove the voltmeter from the DELTA, close the DELTA corner and place voltmeters in a manner to record voltages V4, V5 and V6.








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4.3.5 CONCLUSIONS

1. When the results of the DELTA-DELTA and the Open-DELTA are compared, is there any difference in operating voltage between the two configurations? ______________

Are the Volt-Ampere ratings of the two configurations the same? ______________

Explain: _________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

If the current windings of the two transformers used in the Open-DELTA were increased, would it work as well as the DELTA-DELTA? ________________

Explain: _________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

_________________________________________________________________________

2. If one of the secondary winding polarities were reversed in the WYE-WYE configuration:

a) Would the transformer short circuit? _________

b) Would the transformer heat up? ____________

c) Would the primary voltages become unbalanced? ___________

d) Would the secondary voltages become unbalanced? __________

3. If one of the secondary winding polarities were reversed in the DELTA-DELTA configuration:

a) Would the transformer short circuit? _________

b) Would the transformer heat up? ____________

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c) Would the primary voltages become unbalanced? ___________

e) Would the secondary voltages become unbalanced? __________

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