Transformer Polarities

Objectives:

• Discuss buck and boost connections for a transformer.

• Connect a transformer for additive polarity.

• Connect a transformer for subtractive polarity.

• Determine the turns-ratio and calculate current values using measured values.

To understand what is meant by transformer polarity, the voltage produced across a winding must be considered during some point in time. In a 60 Hz AC circuit, the voltage changes polarity 120 times per second. When discussing transformer polarity, it's necessary to consider the relationship between the different windings at the same point in time. It will, therefore, be assumed that this point in time is when the peak positive volt age is being produced across the winding.

Polarity Markings on Schematics

When a transformer is shown on a schematic diagram, it's common practice to indicate the polarity of the transformer windings by placing a dot beside one end of each winding, as shown in Ill. 1. These dots signify that the polarity is the same at that point in time for each winding. E.g., assume the voltage applied to the primary winding is at its peak positive value at the terminal indicated by the dot. The voltage at the dotted lead of the secondary will be at its peak positive value at the same time.

This same type of polarity notation is used for transformers that have more than one primary or secondary winding. An example of a transformer with a multi-secondary is shown in Ill. 2.

Ill. 1 Transformer polarity dots.

Ill. 2 Polarity marks for multiple secondaries.

Additive and Subtractive Polarities

The polarity of transformer windings can be determined by connecting one lead of the primary to one lead of the secondary and testing for an increase or decrease in voltage. This is often referred to as a buck or boost connection ( Ill. 3). The transformer shown in the example has a primary voltage rating of 120 volts and a secondary voltage rating of 24 volts. This same circuit has been redrawn in Ill. 4 to show the connection more clearly. Notice that the secondary winding has been connected in series with the primary winding. When 120 volts is applied to the primary winding, the voltmeter connected across the secondary will indicate either the SUM of the two voltages or the DIFFERENCE between the two voltages. If this voltmeter indicates 144 volts (120 + 24 = 144), the windings are connected additive (boost) and polarity dots can be placed as shown in Ill. 5.

Notice in this connection that the secondary voltage is added to the primary voltage.

If the voltmeter connected to the secondary winding should indicate a voltage of 96 volts (120 - 24 = 96), the windings are connected subtractive (buck) and polarity dots would be placed as shown in Ill. 6.

Ill. 3 Connecting the secondary and primary windings forms an autotransformer.

Ill. 4 Redrawing the connection.

Ill. 5 Placing polarity dots to indicate additive polarity.

Using Arrows to Place Dots

To help in the understanding of additive and subtractive polarity, arrows can be used to indicate a direction of greater-than or less-than values. In Ill. 7, arrows have been added to indicate the direction in which the dot is to be placed. In this example, the transformer is connected additive, or boost, and both of the arrows point in the same direction.

Notice that the arrow points to the dot. In Ill. 8 it's seen that the values of the two arrows add to produce 144 volts.

In Ill. 9, arrows have been added to a subtractive, or buck, connection. In this instance, the arrows point in opposite directions and the voltage of one tries to cancel the voltage of the other. The result is that the smaller value is eliminated and the larger value is reduced as shown in Ill. 10.

Ill. 6 Polarity dots indicate subtractive polarity.

Ill. 7 Arrows help indicate the placement of the polarity dots.

Ill. 8 The values of the arrows add to indicate additive polarity.

Ill. 9 The arrows help indicate subtractive polarity.

Ill. 10 The value of the arrows subtract.

LABORATORY EXERCISE

Name __________ Date _

Materials Required:

480-240/1volt 0.5-kVA control transformer AC voltmeter 2 AC ammeters, in-line or clamp-on. (If a clamp-on type is used, a 10:1 scale divider is recommended.) 4 100-watt lamps In this experiment a control transformer will be connected for both additive (boost) and sub tractive (buck) polarity. Buck and boost connections are made by physically connecting the primary and secondary windings together. If they are connected in such a way that the primary and secondary voltages add, the transformer is connected additive, or boost. If the windings are connected in such a way that the primary and secondary voltages subtract, they are connected subtractive, or buck.

In this exercise only one of the high-voltage windings will be used. The other won't be connected.

1. Connect the circuit shown in Ill. 11.

2. Turn on the power and measure the primary and secondary voltages.

E (PRIMARY) ____________ volts

E(SECONDARY) ____________ volts

3. Turn off the power supply.

4. Determine the turns-ratio of this transformer connection by dividing the higher volt age by the lower voltage. Recall that if the primary winding has the higher voltage, the higher number will be placed on the left and 1 will be placed on the right. If the secondary has the higher voltage, a 1 will be placed on the left and the higher number will be placed on the right. Ratio ____

Ill. 11 Measuring the secondary voltage.

Ill. 12 Connecting X2 and H2.

5. Connect the circuit shown in Ill. 12 by connecting X1 to H1. Connect a voltmeter across terminals X2 and H2.

6. Turn on the power supply and measure the voltage across X2 and H2. _______ volts

7. Turn off the power supply.

8. Determine the turns-ratio of this transformer connection.

Ratio ______

9. If the measured voltage is the difference between the applied voltage and the secondary voltage, the transformer is connected subtractive polarity, or buck. If the measured volt age is the sum of the applied voltage and the secondary voltage, the transformer is connected additive, or boost. Is the transformer connected buck or boost?

10. Connect an AC ammeter in series with one of the power supply lines.

11. Turn on the power supply and measure the excitation current of the transformer. I(EXC.) ______ amp(s)

12. Turn off the power supply.

13. Reconnect the transformer as shown in Ill. 13 by connecting X2 to H1. Connect an AC voltmeter across terminals X1 and H2.

14. Turn on the power supply and measure the voltage across terminals X1 and H2. _____ volts

15. Turn off the power supply.

16. Is the transformer connected buck or boost?

Ill. 13 Connecting X1 and H2.

Ill. 14 Placing polarity dots on the transformer windings.

Ill. 15 Determining the placement of polarity dots.

Ill. 16 Connecting load to a subtractive polarity transformer.

17. Determine the turns-ratio of this transformer connection.

Ratio ____________

18. Connect an AC ammeter in series with one of the primary leads.

19. Turn on the power supply and measure the excitation current of this connection. I(EXC) _____amp(s)

20. Compare the value of excitation current for the buck and boost connections. Is there any difference between these two values?

21. Turn off the power supply.

22. Ill. 14 shows the proper location for the placement of polarity dots. Recall that polarity dots are used to indicate which windings of a transformer have the same polar ity at the same time. To better understand how the dots are placed, redraw the two transformer windings in a series connection as shown in Ill. 15. Place a dot beside one of the high-voltage terminals. In this example a dot has been placed beside the H1 terminal. Next, draw an arrow pointing to the dot. To place the second dot, draw an arrow in the same direction as the first arrow. This arrow should point to the dot that's to be placed beside the secondary terminal. Since terminal X2 is connected to H1, the arrow must point to terminal X1.

23. Reconnect the transformer for subtractive polarity. If two ammeters are available, place one ammeter in series with one of the primary leads and the second ammeter in series with the secondary lead that isn't connected to the H1 terminal. Connect a 100 watt lamp in the secondary circuit, and connect a voltmeter in parallel with the lamp, as shown in Ill. 16.

24. Turn on the power supply and measure the secondary current. I_(SECONDARY) ______ amp(s)

25. Measure the secondary voltage. Since the lamp is the only load connected to the secondary, the voltage drop across the lamp will be the secondary voltage.

E_(SECONDARY) ________ volts

26. Calculate the amount of primary current using the measured value of secondary cur rent and the turns-ratio. Be sure to use the turns-ratio for this connection as determined in step 8. Since the primary voltage is greater than the secondary voltage, the primary current should be less. Therefore, divide the secondary current by the turns-ratio and then add the excitation current measured in step 11.

I(PRIMARY) ____________ amp(s)

27. If necessary, turn off the power supply and connect an AC ammeter in series with one of the primary leads.

28. Turn on the power supply and measure the primary current. Compare this value with the calculated value.

I (PRIMARY) ______ amp(s)

29. Turn off the power supply.

30. Connect another 100 watt lamp in parallel with the first as shown in Ill. 17. Reconnect the AC ammeter in series with the secondary winding if necessary.

31. Turn on the power supply and measure the amount of secondary current. I (SECONDARY) ______ amp(s)

32. Calculate the primary current. I (PRIMARY) ______ amp(s)

33. If necessary, turn off the power supply and connect the AC ammeter in series with one of the primary leads.

Ill. 17 Adding load to the transformer.

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

I (PRIMARY) ________ amp(s)

35. Turn off the power supply.

36. Reconnect the transformer for the boost connection by connecting terminal X2 to H1.

If two ammeters are available, connect one AC ammeter in series with one of the power supply leads and the second AC ammeter in series with the secondary. Connect four 100 watt lamps in series with terminals X1 and H2 as shown in Ill. 18. Connect an AC voltmeter across terminals X2 and H1.

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

I (SECONDARY) ____________ amp(s) 38. Turn off the power supply.

39. Compute the primary current using the turns-ratio. Be sure to use the turns-ratio for this connection as determined in step 17. Since the primary voltage in this connection is less than the secondary voltage, the primary current will be greater. To calculate the primary current, multiply the secondary current by the turns-ratio and then add the excitation current.

______ amp(s) (PRIMARY) ( (SECONDARY) ) × (EXC) + =

I(PRIMARY) Ill. 18 Connecting load to the boost connection.

40. If necessary, connect the AC ammeter in series with one of the power supply leads.

41. Turn on the power supply.

42. Measure the primary current. Compare this value with the calculated value.

I(PRIMARY) ____________ amp(s)

43. Turn off the power supply.

44. Disconnect the circuit and return the components to their proper place.

QUIZ:

1. What do the dots shown beside the terminal leads of a transformer represent on a schematic?

2. A transformer has a primary voltage rating of 240 volts and a secondary voltage rat ing of 80 volts. If the windings are connected subtractive, what voltage would appear across the entire connection?

3. If the windings of the transformer in question 2 were to be connected additive, what voltage would appear across the entire winding?

4. The primary leads of a transformer are labeled 1 and 2. The secondary leads are labeled 3 and 4. If polarity dots are placed beside leads 1 and 4, which secondary lead would be connected to terminal 2 to make the connection additive?