Synchronous vs Induction Motors: Weighing Pros & Cons

The ease with which the power factor can be controlled.
- An over-excited synchronous motor having a leading power factor can be operated in parallel with induction motors and other power apparatus operating at lagging power factors, thereby improving the power factor of the supply system.
The speed is constant and independent of load.
- This characteristic is mainly of use when the motor is required to drive another alternator to generate a supply at a different frequency as in frequency changers.
Electro-magnetic power varies linearly with the voltage.
These motors can be constructed with wider air gaps than induction motors, which make them better mechanically.
These motors usually operate at higher efficiencies, especially in the low-speed unity pf ranges.

The cost per kW output is generally higher than that of an IM.
It requires DC excitation which must be supplied from external source.
SM is inherently not self-starting motor and needs some arrangement for its starting and synchronising.
It cannot be used for variable speed jobs as there is no possibility of speed adjustment.
It cannot be started under load and Its starting torque is zero.
It has a tendency to hunt.
It may fall out of synchronism and stop when over-loaded.
Collector rings and brushes are required.
For some purposes synchronous motors are not desirable as for driving shafts in small work-shops having no other power available for starting and in cases where frequent starting or strong starting torque is required.