SOLUTION

1. Synchronous motors are particularly attractive for low-speed drives (<600 r.p.m.) because the p.f. can always be adjusted to 1 (through the change of d.c. field excitation) and the efficiency is high.
2. On the other hand, at low speeds, 3-phase induction motors become heavy, costly and have relatively low p.f. and efficiency.
3. For equal power and speed, the weight and cost of synchronous motors are less than those of 3-phase induction motors. This is particularly true for speeds below 300 r.p.m.
4. The starting torque of a synchronous motor can be made considerably greater than that of a 3-phase induction motor. The reason is that we can raise the resistance of damper winding (i.e. squirrel-cage winding) without affecting the speed and efficiency.
5.  Modern electronic converters can produce very low frequencies, and synchronous motors of 10 MW range can be run at ultra-low speeds desirable in rotary kilns in cement plants and, as in its pioneering application, in ore or rock milling and crushing.