Single-Phase Transformer Calculations

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Objectives:

• Discuss transformer excitation current.

• Compute values of primary current using the secondary current and the turns-ratio.

• Compute the turns-ratio of a transformer using measured values.

• Connect a step-down or step-up isolation transformer.

LABORATORY EXERCISE:

Name _______Date __________

Ill. 1 Connecting the high-voltage winding for parallel operation.

Ill. 2 Two lamps are connected in parallel to the secondary winding.

Ill. 3 Measuring primary current.

Materials Required:

480-240/1volt, 0.5-kVA control transformer 4 100-watt incandescent lamps AC voltmeter AC ammeter, in-line or clamp-on. (If a clamp-on type is used, the use of a 10:1 scale divider is recommended.) In this experiment the excitation current of an isolation transformer will be measured.

The transformer will then be connected as both a step-down and a step-up transformer.

The turns-ratio will be determined from measured values and the primary current will be computed and then measured.

1. Connect the high-voltage windings of the transformer in parallel for 240 volt operation.

2. Connect the high-voltage winding to a 208 volt AC source with an AC ammeter connected in series with one of the lines, as shown in Ill. 1.

3. Turn on the power source and measure the current. This is the excitation current of the transformer. The excitation current is the amount of current necessary to magnetize the iron in the transformer and will remain constant regardless of the load on the transformer. ______ amp(s)

4. Measure the voltage across the low-voltage winding at terminals X1 - X2. _______ volts

5. Compute the turns-ratio by dividing the primary voltage by the secondary voltage. Since the primary has the higher voltage, the larger number will be placed on the left side of the ratio, such as 3:1 or 4:1. ____________ ratio

6. Turn off the power supply.

7. Connect two 100 watt incandescent lamps in parallel with the low-voltage winding of the transformer. Connect an AC ammeter in series with one of the lines, as shown in Ill. 2.

8. Turn on the power and measure the current flow in the secondary circuit of the transformer.

_____ amp(s)

9. Turn off the power supply.

10. Compute the amount of primary current using the turns-ratio. Since the primary voltage is higher, the amount of primary current will be less. Divide the secondary current by the turns-ratio. Then add the excitation current to this value.

I_(PRIMARY)=I_(SECONDARY)/Turns+ratio + Excitation current

______ I(PRIMARY)

11. Reconnect the AC ammeter in one of the primary lines, as shown in Ill. 3.

12. Turn on the power supply and measure the primary current. Compare this value with the computed value.

____________ I(PRIMARY)

13. Turn off the power supply and reconnect the AC ammeter in the secondary circuit and add two more 100 watt incandescent lamps in parallel with the transformer secondary (Ill. 4).

14. Turn on the power and measure the secondary current.

____ amp(s)

15. Turn off the power supply.

16. Compute the amount of current flow that should be in the primary circuit using the turns-ratio. Be sure to add the excitation current.

_____ I(PRIMARY)

17. Reconnect the AC ammeter in series with one of the lines of the primary winding of the transformer.

Ill. 4 Adding load to the transformer secondary.

Ill. 5 Using the low-voltage winding as the primary.

18. Turn on the power and measure the current flow. Compare the measured value with the computed value.

____ I(PRIMARY)

19. Turn off the power supply.

20. Reconnect the transformer by connecting the low-voltage terminals, X1 - X2, to a 120 volt AC source. Connect an AC ammeter in series with one of the power lines, as shown in Ill. 5.

21. Turn on the power and measure the excitation current of the transformer.

_______ amp(s)

22. Measure the secondary voltage with an AC voltmeter.

_______ volts

23. Determine the turns-ratio by dividing the secondary voltage by the primary voltage.

Since the primary voltage is lower, the higher number will be placed on the right-hand side of the ratio: 1:3 or 1:4.

____________ ratio

24. Turn off the power supply.

25. Connect two 100 watt incandescent lamps in series. Connect these two lamps in parallel with the high-voltage winding. Connect an AC ammeter in series with one of the secondary leads, as shown in Ill. 6.

26. Turn on the power supply and measure the secondary current.

___ amp(s)

27. Compute the primary current using the turns-ratio. Since the primary voltage is less than the secondary voltage, the primary current will be more than the secondary current. To determine the primary current, multiply the secondary current by the turns ratio and add the excitation current.

I_(PRIMARY) = I_(SECONDARY) × Turns+ratio + Excitation-current

____ I(PRIMARY)

Ill. 6 Connecting the load to the secondary.

Ill. 7 Adding load to the secondary.

28. Turn off the power supply.

29. Reconnect the AC ammeter in series with the primary side of the transformer.

30. Turn on the power supply and measure the primary current. Compare this value with the computed value.

______ I(PRIMARY)

31. Turn off the power supply.

32. Reconnect the AC ammeter in series with the secondary winding. Add two more 100 watt lamps that have been connected in series to the secondary circuit. These two lamps should be connected in parallel with the first two lamps, seen in Ill. 7.

33. Turn on the power supply and measure the secondary ______ amp(s)

34. Turn off the power supply.

35. Compute the amount of current that should flow in the primary circuit.

_______ I(PRIMARY)

36. Reconnect the AC ammeter in series with one of the primary lines.

37. Turn on the power supply and measure the primary current. Compare this value with the computed value.

____________ I(PRIMARY) 38. Turn off the power supply and disconnect the transformer and lamps.

39. Reconnect the transformer as shown in Ill. 8 by connecting the two high-voltage windings in series. Connect four 100 watt lamps in series and connect them to terminals H1 and H4 for the transformer. Connect an ammeter in series with the secondary winding.

[Caution: The transformer now has a turns-ratio of 1:4. The output voltage will be approximately 480 volts when 120 volts is applied to terminals X1 and X2. Make sure that the power is turned off before making any adjustments to the circuit.]

40. Turn on the power and measure the secondary current.

_____ amp(s)

41. Make certain the voltmeter is set for a range greater than 480 volts. Measure the voltage across terminals H1 and H4. Use caution when making this measurement.

_____ volts

42. Turn off the power.

Ill. 8 Connect the two high voltage windings in series. The transformer now has a turns ratio of 1:4.

Ill. 9 The ammeter is reconnected in the primary winding.

43. Use the turns-ratio to compute the primary current. Be sure to add the excitation cur rent to the calculation.

____ I(PRIMARY)

44. Reconnect the ammeter in series with the primary as shown in Ill. 9.

45. Turn on the power and measure the amount of primary current. Compare this value with the computed value.

____ amp(s)

46. Turn off the power. Disconnect the circuit and return the components to their proper place.

QUIZ:

1. A transformer has a primary voltage of 277 volts and a secondary voltage of 120 volts. What is the turns-ratio of this transformer?

2. A transformer has a turns-ratio of 1:6. Is this a step-up or a step-down transformer?

3. A transformer with a turns-ratio of 3.5:1 has a secondary current of 16 amperes. What is the primary current?

4. A transformer has a primary current of 18 amperes and a secondary current of 6 amperes. What is the turns-ratio of the transformer?

5. A transformer has a primary voltage of 240 volts and a secondary voltage of 60 volts.

It has a power rating of 7.5 VA. What is the rated current of the secondary?

6. A 75 kVA transformer has a secondary voltage of 480 volts and a current of 183 amperes.

Is this transformer being operated within its power rating?

7. A transformer has a primary voltage of 120 volts and a secondary voltage of 18 volts.

The primary excitation current is 0.25 amp. The total primary current is 6.5 amperes.

What is the secondary current?

8. Would a 1 kVA transformer be large enough to supply the load of the transformer in question 7?

9. A transformer has a primary voltage of 12,470 volts and a secondary voltage of 2,400 volts. If the secondary current is 22.6 amperes, what is the primary current (disregard excitation current)?

10. Would a 75 kVA transformer supply the power needed by the load in question 7?

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