Autotransformers

Objectives:

• Discuss the operation of an autotransformer.

• Connect a control transformer as an autotransformer.

• Calculate the turns-ratio from measured voltage values.

• Calculate primary current using the secondary current and the turns-ratio.

• Connect an autotransformer as a step-down transformer.

• Connect an autotransformer as a step-up transformer.

Ill. 1 Autotransformer used as a step-down transformer.

Ill. 2 Autotransformer used as a step-up transformer.

Ill. 3 Autotransformer with multiple taps.

The word auto means self. An autotransformer is literally a self-transformer. It uses the same winding as both the primary and secondary. Recall that the definition of a primary winding is a winding that's connected to the source of power and the definition of a secondary winding is a winding that's connected to a load. Autotransformers have very high efficiencies, most in the range of 95% to 98%.

In Ill. 1 the entire winding is connected to the power source, and part of the winding is connected to the load. In this illustration all the turns of wire form the primary and part of the turns form the secondary. Since the secondary part of the winding contains fewer turns than the primary section, the secondary will produce less voltage. This autotransformer is a step-down transformer.

In Ill. 2 the primary section is connected across part of a winding and the secondary is connected across the entire winding. In this illustration the secondary section contains more windings than the primary. This autotransformer is a step-up transformer. Notice that autotransformers, like isolation transformers, can be used as step-up or step-down transformers.

Determining Voltage Values

Autotransformers are not limited to a single secondary winding. Many autotransformers have multiple taps to provide different voltages as shown in Ill. 3. In this example there are 40 turns of wire between taps A and B, 80 turns of wire between taps B and C, 100 turns of wire between taps C and D, and 60 turns of wire between taps D and E. The primary section of the windings is connected between taps B and E. It will be assumed that the primary is connected to a source of 120 volts. The voltage across each set of taps will be determined.

There is generally more than one method that can be employed to determine values of a transformer. Since the number of turns between each tap is known, the volts-per-turn method will be used in this example. The volts-per-turn for any transformer is determined by the primary winding. In this illustration the primary winding is connected across taps B and E.

The primary turns are, therefore, the sum of the turns between taps B and E (80 + 100 + 60 = 240 turns). Since 120 volts is connected across 240 turns, this transformer will have a volts-per-turn ratio of 0.5 (240 turns/120 volts = 0.5 volt-per-turn). To determine the amount of voltage between each set of taps, it becomes a simple matter of multiplying the number of turns by the volts-per-turn.

A-B (40 turns _ 0.5 = 20 volts) A-C (120 turns _ 0.5 = 60 volts) A-D (220 turns _ 0.5 = 110 volts) A-E (280 turns _ 0.5 = 140 volts) B-C (80 turns _ 0.5 = 40 volts) B-D (180 turns _ 0.5 = 90 volts) B-E (240 turns _ 0.5 = 120 volts) C-D (100 turns _ 0.5 = 50 volts) C-E (160 turns _ 0.5 = 80 volts) D-E (60 turns _ 0.5 = 30 volts)

Ill. 4 Determining voltage and current values.

Using Transformer Formulas

The values of voltage and current for autotransformers can also be determined by using standard transformer formulas. The primary winding of the transformer shown in Ill. 4 is between points B and N and has a voltage of 120 volts applied to it. If the turns of wire are counted between points B and N, it can be seen there are 120 turns of wire. Now assume that the selector switch is set to point D. The load is now connected between points D and N. The secondary of this transformer contains 40 turns of wire. If the amount of voltage applied to the load is to be computed, the following formula can be used:

Assume that the load connected to the secondary has an impedance of 10 ohm. The amount of current flow in the secondary circuit can be computed using the formula:

The primary current can be computed by using the same formula that was used to compute primary current for an isolation type of transformer.

The amount of power input and output for the autotransformer must also be the same.

Primary 120 _ 1.333 = 160 volt-amp

Secondary 40 _ 4 = 160 volt-amps

Now assume that the rotary switch is connected to point A. The load is now connected to 160 turns of wire. The voltage applied to the load can be computed by:

The amount of secondary current can be computed using the formula:

The primary current can be computed using the formula:

The answers can be checked by determining if the power in and power out are the same.

Primary: 120 _ 21.333 = 2,560 volt-amps

Secondary: 160 _ 16 = 2,560 volt-amps

Ill. 5 Current divides between primary and secondary.

Current Relationships

An autotransformer with a 2:1 turns-ratio is shown in Ill. 5. It is assumed that a voltage of 480 volts is connected across the entire winding. Since the transformer has a turns-ratio of 2:1, a voltage of 240 volts will be supplied to the load. Ammeters connected in series with each winding indicate the current flow in the circuit. It is assumed that the load produces a current flow of 4 amperes on the secondary. Note that a current flow of 2 amperes is supplied to the primary.

I PRIMARY = I SECONDARY/ Ratio

If the rotary switch shown in Ill. 4 were to be removed and replaced with a sliding tap that made contact directly to the transformer winding, the turns-ratio could be adjusted continuously. This type of transformer is commonly referred to as a Variac or Powerstat depending on the manufacturer. The windings are wrapped around a tape-wound torroid core inside a plastic case. The tops of the windings have been milled flat similar to a commutator.

A carbon brush makes contact with the windings. When the brush is moved across the windings, the turns-ratio changes, which changes the output voltage. This type of autotransformer provides a very efficient means of controlling AC voltage. Autotransformers are often used by power companies to provide a small increase or decrease to the line voltage. They help provide voltage regulation to large power lines.

The autotransformer does have one disadvantage. Since the load is connected to one side of the power line, there is no line isolation between the incoming power and the load.

This can cause problems with certain types of equipment and must be a consideration when designing a power system.

LABORATORY EXERCISE

Name ______ Date _

Materials Required:

480-240/1volt, 0.5-kVA control transformer AC voltmeter 2 AC ammeter, in-line or clamp-on. (If the clamp-on type is used, a 10:1 scale divider is recommended.) 4 100-watt lamps In this experiment the control transformer will be connected for operation as an autotransformer.

The low-voltage winding won't be used in this experiment. The two high-voltage windings will be connected in series to form one continuous winding. The transformer will be connected as both a step-down and a step-up transformer.

1. Series connect the two high-voltage windings by connecting terminals H2 and H3 together. The H1 and H4 terminals will be connected to a source of 208 VAC. Connect an ammeter in series with one of the power supply lines, as shown in Ill. 6.

2. Turn on the power supply and measure the excitation current. The current will be small, and it may be difficult to determine this current value.

I_(EXC) _________ amp(s)

3. Measure the primary voltage across terminals H1 and H4.

E_(PRIMARY) _______ volts

4. Measure the secondary voltage across terminals H1 and H2. (Note: It is also possible to use terminals H3 and H4 as the secondary winding.) E_(SECONDARY) ___ volts

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

Ratio ____

6. Turn off the power supply.

7. Connect an AC ammeter in series with the H2 terminal and a 100 watt lamp as shown in Ill. 7. The secondary winding of the transformer will be between terminals H2 and H1.

8. Turn on the power supply and measure the amount of current flow in the secondary winding.

I_(SECONDARY) _____ amp(s)

9. Measure the voltage drop across the secondary winding with an AC voltmeter.

E_(SECONDARY) ____________ volts

10. Turn off the power supply.

Ill. 6 Connecting the high-voltage windings as an autotransformer.

Ill. 7 Connecting a load to the autotransformer.

11. Calculate the primary current using the turns-ratio. Since the primary voltage is greater than the secondary voltage, the primary current will be less than the secondary current. To determine the primary current, divide the secondary current by the turns-ratio and add the excitation current.

12. If necessary, reconnect the AC ammeter in series with one of the power supply leads.

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

I_(PRIMARY) ____________ amp(s)

14. Turn off the power supply.

15. Connect another 100 watt lamp in parallel with the existing lamp ( Ill. 8). Ill. 8 Adding load to the autotransformer.

16. If necessary, reconnect the AC ammeter in series with the secondary winding of the transformer.

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

I_(SECONDARY) _______ amp(s)

18. Calculate the primary current using the turns-ratio.

I_(PRIMARY) ______ amp(s)

19. Turn off the power supply.

20. If necessary, reconnect the AC ammeter in series with one of the power supply leads.

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

I_(PRIMARY) ____________ amp(s) 22. Turn off the power supply.

23. Reconnect the circuit as shown in Ill. 9. Terminals H1 and H2 will be connected to a source of 120 VAC. Connect an AC ammeter in series with terminal H2.The entire winding between terminals H1 and H4 will be used as the secondary.

24. Turn on the power and measure the excitation current of this transformer connection. I_(EXC) ______ amp(s)

25. Measure the voltage across terminals H4 and H1.

_____ volts

26. Compute the turns-ratio of this transformer connection. Ratio ______

27. Turn off the power supply.

28. Connect an AC ammeter and four 100 watt lamps in series with terminals H4 and H1, as shown in Ill. 10.

29. Turn on the power and measure the secondary current. I_(SECONDARY) ____________ amp(s)

30. Turn off the power supply.

31. If necessary, connect the AC ammeter in series with one of the primary leads.

Ill. 9 The autotransformer connected for high voltage.

Ill. 10 Adding load to the secondary winding.

32. Compute the value of primary current using the turns-ratio and the measured value of secondary current. I_(PRIMARY) _______ amp(s)

33. Turn on the power and measure the primary current. Compare this value with the computed value. I_(PRIMARY) ____________ amp(s)

34. Measure the voltage across terminals H1 and H4. E_(SECONDARY) _____volts

35. Turn off the power supply.

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

QUIZ:

1. An AC power source is connected across 325 turns of an autotransformer and the load is connected across 260 turns. What is the turns-ratio of this transformer?

2. Is the transformer in question 1 a step-up or step-down transformer?

3. An autotransformer has a turns-ratio of 3.2:1. A voltage of 208 volts is connected across the primary. What is the voltage of the secondary?

4. A load impedance of 52 ohm is connected to the secondary winding of the transformer in question 3. How much current will flow in the secondary?

5. How much current will flow in the primary of the transformer in question 4?

6. The autotransformer shown in Ill. 3 has the following number of turns between windings: A-B (120 turns), B-C (180 turns), C-D (250 turns), and D-E (300 turns).

A voltage of 240 volts is connected across B and E. Find the voltages between each of the following points:

A-B _____ A-C _____ A-D _____ A-E _____ B-C _____ B-D _____ B-E _____ C-D _____ C-E _____ D-E _____