- s. Revolutions per minute of the motor;
- p. Number of poles of the motor;
- S. Revolutions per minute of the alternator;
- P. Number of poles of the alternator.
Question. If the field strength of a synchronous motor be altered, what effect does this have on the speed, and why?
Ans. The speed does not change (save for a momentary variation to establish the phase relation corresponding to equilibrium), because the motor has to run at the same frequency as the alternator.
Ques. How does a synchronous motor adjust itself to changes of load and field strength?
Ans. By changing the phase difference between the current and pressure.
If, on connecting a synchronous motor to the mains, the excitation be too weak, so that the voltage is lower than that of the supply, this phase difference will appear resulting in wattless current, since the missing magnetization has, as it were, to be supplied from an external source. A phase difference also appears when the magnetization is too strong.
Ques. State the disadvantages of synchronous motors.
Ans. A synchronous motor requires an auxiliary power for starting, and will stop if, for any reason, the synchronism be destroyed; collector rings and brushes are required. For some purposes synchronous motors are not desirable, as for driving shafts in small workshops having no other power available for starting, and in cases where frequent starting, or a strong torque at starting is necessary. A synchronous motor has a tendency to hunt[1] and requires intelligent attention; also an exciting current which must be supplied from an external source.
[1] NOTE.—See Hunting of synchronous motors, page [1,280].
Ques. State the advantage of synchronous motors.