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Autotransformer starters reduce the amount of in-rush current by reducing the voltage applied to the motor during the starting period. Many autotransformer starters contain taps that can be set for 50%, 65%, or 80% of the line voltage. Reducing the voltage applied to the motor not only reduces the amount of in-rush current, but also reduces the motor torque. If 50% of the normal voltage is connected to the motor, the in rush current will drop to 50% also. This will produce a torque that is 25% of the value when full voltage is connected to the motor. If the motor torque is in sufficient to start the load when the 50% tap is used, the 65% or 80% taps are available.
Autotransformer starters are generally employed to start squirrel cage type motors. Wound rotor type motors and synchronous motors do not generally use this type of starter. Autotransformer starters are inductive type loads and will affect the power factor during the starting period.
Most autotransformer starters use two transformers connected in open-delta to reduce the voltage applied to the motor during the period of acceleration (FIG. 1). During the starting period, the motor is connected to the reduced voltage taps on the trans formers. After the motor has accelerated to about 75% of normal speed, the motor is connected to full voltage. A time delay starter of this type is shown in FIG. 2. To understand the operation of the auto transformer starter more clearly, refer to the schematic diagram shown in FIG. 2. When the Start button is pressed, a circuit is completed to the coil of control relay CR, causing all CR contacts to close. One contact is employed to hold CR coil in the circuit when the Start button is released. Another completes a circuit to the coil of TR timer, which permits the timing sequence to begin. The CR contact connected in series with the normally closed TR contact supplies power to the coil of S (start) contactor. The fourth CR contact permits power to be connected to R (run) contactor when the normally open timed TR contact closes.
When the coil of S contactor energizes, all S contacts change position. The normally closed S contact connected in series with R coil opens to prevent both S and R contactors from ever being energized at the same time. This is the same interlocking method used with reversing starters. When the S load contacts close, the motor is connected to the power line through the autotransformers. The autotransformers supply 65% of the line voltage to the motor. This reduced voltage produces less in-rush current during starting and also reduces the starting torque of the motor.
When the time sequence for TR timer is completed, both TR contacts change position. The normally closed TR contact opens and disconnects contactor S from the line, causing all S contacts to return to their normal position. The normally open TR contact closes and supplies power through the now closed S contact to coil R. When contactor R energizes, all R contacts change position. The normally closed R con tact connected in series with S coil opens to provide interlocking for the circuit. The R load contacts close and connect the motor to full voltage.
When the Stop button is pressed, control relay CR de-energizes and opens all CR contacts. This disconnects all other control components from the power line, and the circuit returns to its normal position. A wiring diagram for this circuit is shown in FIG. 3.
Open transition starting is generally used on starters of size 5 and smaller. Open transition simply means that there is a brief period of time when the motor is disconnected from power when the start contactor opens and the run contactor closes. The circuit shown in FIG. 2 and 3 are examples of an open transition starter.
Closed transition starting is generally used on starters size 6 and larger. For closed transition starting, two separate start contactors are used (FIG. 4).
When the motor is started, both S1 and S2 contactors close their contacts. The S1 contacts open first and separately from the S2 contacts. At this point, part of the autotransformer windings are connected in series with the motor and act as series inductors. This permits the motor to accelerate to a greater speed before the R contacts close and the S2 contacts open. Although the R and S2 contacts are closed at the same time, the interval of time between the R contacts closing and the S2 contacts opening is so short that it does not damage the autotransformer winding.
Notice that the circuit in FIG. 4 contains three current transformers (CTs). This is typical in circuits that control large horsepower motors. The CTs re duce the current to a level that common overload heaters can be used to protect the motor. A schematic diagram of a timed circuit for closed transition starting is shown in FIG. 5. When the motor reaches the run stage, it is connected directly to the power line and the autotransformer is completely disconnected from the circuit. This is done to conserve energy and extend the life of the transformers. A typical autotransformer starter is shown in FIG. 6.
1. Why is it desirable to disconnect the autotransformer from the circuit when the motor reaches the run stage?
2. Explain the differences between open and closed transition starting.
3. Autotransformers often contain taps that permit different percentages of line voltages to be connected to the motor during starting. What are three common percentages?
4. Refer to the circuit shown in FIG. 2. Assume that timer TR1 is set for a time delay of 10 seconds. When the Start button is pressed, the motor does not start. After a period of 10 seconds, the motor starts with full line voltage applied to it.
Which of the following could cause this condition?
a. Timer TR coil is open.
b. CR coil is open.
c. Contactor S coil is open.
d. Contactor R coil is open.
5. Refer to the circuit shown in FIG. 2.
Assume that timer TR is set for a delay of 10 seconds. Assume that contactor coil R is open. Explain the operation of the circuit if the Start button is pressed.
6. Refer to the circuit shown in FIG. 5. Assume that timer TR1 is set for a delay of 10 seconds and timer TR2 is set for a delay of 5 seconds. After the Start button is pressed, how long is the time delay before the S1 contacts open?
7. Refer to the circuit shown in FIG. 5.
Assume that timer TR1 is set for a delay of 10 seconds and timer TR2 is set for a delay of 5 seconds. From the time the Start button is pressed, how long will it take the motor to be connected to full line voltage?
8. Refer to the circuit shown in FIG. 5. Ex plain the steps necessary for coil S2 to energize.
9. Refer to the circuit shown in FIG. 5. What causes contactor coil S2 to de-energize after the motor reaches the full run stage?
10. Refer to the circuit shown in FIG. 5. Assume that timer TR1 is set for a delay of 10 seconds and timer TR2 is set for a delay of 5 seconds. When the Start button is pressed, the motor starts. After 10 seconds, the S1 contacts open and the motor continues to accelerate but never reaches full speed. After a delay of about 30 seconds, the motor trips out on overload. Which of the following could cause this problem?
a. TR1 coil is open.
b. S2 coil is open.
c. S1 coil is open.
d. R coil is open.