Autotransformer Starter

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Objectives:

• Discuss the operation of an autotransformer starter.

• Explain the operation of an autotransformer starter.

• Connect an autotransformer starter in the laboratory.

Autotransformer starters are used to reduce the amount of inrush current when starting a large motor. The autotransformer starter accomplishes this by reducing the voltage applied to the motor during the starting period. If the voltage is reduced by one-half, the current will be reduced by one-half, and the torque will be reduced to one-fourth of normal.

There are several different ways to construct an autotransformer starter. Some use three transformers, and others use two transformers. In this experiment two transformers connected as an open delta will be used. Two 0.5 kVA control transformers will be employed.

Since these transformers are to be used as autotransformers, only the high-voltage windings will be connected. The low-voltage windings (X1 and X2) won't be used in this experiment.

The high-voltage windings can be identified by the markings on the terminal leads of H1 through H4. These high-voltage windings are to be connected in series by connecting a jumper between terminals H2 and H3. This jumpered point provides a center tap for the entire winding.

Ill. 1 Autotransformer starter.

Obtaining Enough Contacts

A schematic diagram of this connection is shown in Ill. 1. Notice that there are a total of five starting contactor (SC) load contacts needed during the starting period. Contactors that contain five load contacts can be purchased, but they are difficult to obtain and they are expensive. For this reason, two three-phase contactors will be used to provide the needed load contacts. This can be accomplished by connecting the coil of SC1 and SC2 contactors in parallel with each other.

Circuit Operation

When the start button is pressed, coils CR, TR, SC1, and SC2 energize. When the SC1 and SC2 load contacts close, the motor is connected to the center tap of the open delta auto transformer. Since the transformers have been center tapped, the motor is connected to half of the line voltage. A basic schematic diagram of this connection is shown in Ill. 2.

The normally closed SC1 and SC2 auxiliary contacts connected in series with the R coil open to provide interlock and prevent the R contactor from energizing as long as SC1 or SC2 is energized.

After some time, TR timer reaches the end of its timing sequence and the two timed TR contacts change position. The normally closed TR contact connected in series with coils SC1 and SC2 opens and de-energizes these contactors. This causes all SC1 and SC2 load contacts to open and disconnect the autotransformer from the line. The normally closed SC1 and SC2 auxiliary contacts connected in series with R coil reclose.

When the normally open TR contact connected in series with R coil closes, the R contactor energizes and closes all R load contacts. This connects the motor directly to the power line.

The normally closed R auxiliary contact connected in series with coils SC1 and SC2 opens to provide interlock. The motor will continue to run until the stop button is pressed or an overload occurs.

Ill. 2 Schematic diagram of a basic autotransformer connection.

LABORATORY EXERCISE

Materials Required

Three-phase power supply Control transformer 3 three-phase contactors with at least one normally open and one normally closed auxiliary contact

2 0.5-kVA control transformers (480/2120) Three-phase motor or equivalent motor load On-delay timer (Dayton model 6A855 or equivalent) and 11-pin tube socket 8-pin control relay and 8-pin tube socket 2 double-acting push buttons (N.O./N.C. on each button) Three-phase overload relay or three single-phase overload relays with the overload contacts connected in series (In this circuit it's possible to replace the two SC contactors with a single contactor that contains five load contacts, if one is available. Also, if true contactors are not available, it's permissible to use motor starters for the two SC contactors.)

1. Assuming that relay CR is an 8-pin control relay, and that timer TR is a Dayton model 6A855, place pin numbers beside the components of CR and TR in Ill. 1.

Circle the pin numbers to distinguish them from wire numbers.

2. Place wire numbers beside all circuit components in Ill. 1.

3. Place corresponding wire numbers beside the components shown in Ill. 3.

Make certain to make the connection between H2 and H3 on the high-voltage side of the control transformers.

4. Connect the control section of the circuit shown in Ill. 1.

5. Set the timing relay for a delay of 5 seconds.

6. Turn on the power and test the control section of the circuit for proper operation.

7. Turn off the power.

8. Connect the load section of the circuit.

9. Turn on the power and test the circuit for proper operation. (Note: Connect a voltmeter across the motor or equivalent motor load terminals and monitor the voltage. When the circuit's first energized, the voltage applied to the motor should be one-half the full line value. After a delay of 5 seconds, the voltage should increase to full value.)

10. Turn off the power and disconnect the circuit. Return the components to their proper places.

Ill. 3 Developing a wiring diagram.

QUIZ:

1. How does the autotransformer reduce the amount of starting current to a motor?

2. Is the autotransformer used in this experiment connected as a wye, delta, or open delta?

3. What is the advantage, if any, of using an open delta connection as opposed to a closed delta or wye?

4. Assume that the line-to-line voltage in Ill. 1 is 480 volts. Also assume that when the start button is pressed, the motor starts with 240 volts applied to the motor.

When the start button is released, however, the motor stops running. Which of the following could cause this problem?

a. SC1 coil is open.

b. CR coil is open.

c. TR coil is open.

d. The stop push button is open.

5. Refer to the circuit shown in Ill. 1. When the start button is pressed, nothing hap pens for 5 seconds. After 5 seconds, the motor suddenly starts with full voltage connected to it. Which of the following could cause this problem?

a. CR coil is open.

b. TR coil is open.

c. R coil is open.

d. R normally closed auxiliary contact is open.

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