Dynamic performance of a synchronous motor fed from a variable frequency supply

The d.c motor is widely used in variable speed drivesowing to the ease by which its speed can be varied, either.byarmature or field voltage control. However, due to thepresence of the commutator and the brushgear, it has thefollowing disadvantages :It requires periodic maintenance due to the presence ofmoving contacts subjected to wear.The commutator increases the size and cost of the machine.The commutator limits the output power for a given framesize because of the limitations on current, voltage andspeed.It can not be used in hazardous areas or working conditionswhich requires sparkless operation.A convential a.c mechine (either of the synchronous orinduction type) does not require a commutator. As a result ofthis, it is considerably less expensive than the dv c machineof the same rating, as well as being more robust mechanically,therefore, less maintenance is required. The only disadvantageof these types of machines is that the speed is dependent onthe frequency of t.hesupply, which is n’:)r:mally fixed. The