Synchronous vs Induction Motors | Electrical Machines

SECTION 01

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.
Speed is fixed by supply frequency and pole number while synchronism is maintained; overload beyond pull-out can break synchronism.
- 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.

Demerits:

The cost per kW output is generally higher than that of an IM.
A wound-field synchronous motor requires DC rotor excitation, supplied through brushes or a brushless exciter. Permanent-magnet and synchronous-reluctance motors use other rotor constructions.
SM is inherently not self-starting motor and needs some arrangement for its starting and synchronising.
A motor directly connected to a fixed-frequency supply has fixed synchronous speed. Converter-fed synchronous motors provide variable speed by changing supply frequency.
The basic wound-field motor has zero average starting torque on a fixed-frequency supply without starting assistance. A damper cage or a suitable converter can provide starting torque; starting under load depends on the method and available torque.
It has a tendency to hunt.
It may fall out of synchronism and stop when over-loaded.
Collector rings and brushes are used with some wound-field excitation systems. Brushless excitation and permanent-magnet or reluctance rotors do not require them.
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.

SECTION 02