Timed Semiautomatic Synchronizing

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• Identify terminal markings for a timed semiautomatic controller used with a synchronous motor.

• Describe the operation of a timed semiautomatic controller used to bring a motor up to synchronous speed.

• Connect synchronous motors and timed semiautomatic controllers.

• Recommend ways to troubleshoot these motors and controllers.

A synchronous motor may be brought up to synchronous speed with the use of a definite time delay relay to excite the DC field. This method is shown in the circuit of ill. 1.

The timing relay coil (TR) is energized with the main starter coil (M). The instantaneous normally open interlock (M) in the coil circuit (F) is then closed. Both interlock M and the DC contactor coil F must wait for the closing of the delay-in-closing contact TR. After a preset timing period, the rotor has accelerated to the maximum speed possible at this stage. Contact TR then closes to accelerate the rotor until it reaches the point where it synchronizes.

The timer setting should be adjusted for the maximum time required to accelerate the motor to the point that it can reach the synchronous speed after contact TR closes.

The attempt to synchronize the motor may not be successful. In this case, the stop button is pressed and the starting cycle is repeated. It isn't necessary to bring the rotor to a standstill.

Only the timing cycle must be reactivated.

The equipment operator and the electrician should realize that push-button control and timed semiautomatic control of synchronous motors are not guaranteed to be effective on every attempt to obtain synchronous operation of the motor. The timing cycle can be adjusted, however.

ill. 1 Elementary diagram of a timed, semiautomatic synchronizing installation.


1. What assurance is there that the rotor will lock into step at the synchronous speed with the use of a timing relay?

2. If the motor fails to achieve synchronous operation, what action is necessary?

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