When any one-phase of a 3-phase synchronous motor is short-circuited, the motor
A) Will overheat in the spot
B) Will fail to pull into step
C) Will refuse to start
D) Will not come upto speed
Correct Answer is C) Will refuse to start
Explanation
- Failure of a synchronous motor to start is often due to faulty connections in the auxiliary apparatus. This should be carefully inspected for open circuits or poor connections.
- An open circuit in one phase of the motor itself or a short circuit will prevent the motor from starting.
- Most synchronous motors are provided with an ammeter in each phase so that the last two causes can be determined from their indications: no current in one phase in case of an open circuit and excessive current in case of a short circuit.
- Either condition will usually be accompanied by a decided buzzing noise, and a short-circuited coil will often be quickly burned out.
- The effect of a short circuit is sometimes caused by two grounds on the machine.
Difficulties in starting synchronous motors:-
- A synchronous motor starts as an induction motor. The starting torque, as in an induction motor, is proportional to the square of the applied voltage.
- For example, if the voltage is halved, the starting effort is quartered. When a synchronous motor will not start, the cause may be that the voltage on the line has been pulled below the value necessary for starting.
- In general, at least half voltage is required to start a synchronous motor.
- Difficulty in starting may also be caused by an open circuit in one of the lines to the motor. Assume the motor to be three-phase. If one of the lines is open, the motor becomes single-phase, and no single-phase synchronous motor, as such, is self-starting.
- The motor, therefore, will not start and will soon get hot. The same condition is true of a two-phase motor if one of the phases is open-circuited.
- Difficulty in starting may be due to a rather slight increase in static friction. It may be that the bearings are too tight, perhaps from cutting during the previous run. Excessive belt tension, if the synchronous motor is belted to its load or any cause which increases starting friction will probably give trouble.
- Difficulty in starting may be due to field excitation on the motor. After excitation exceeds one-quarter of normal value, the starting torque is influenced.
- With full field on, most synchronous motors will not start at all.
- The field should be short-circuited through a proper resistance during the starting period.