Synchronous motors find extensive application for the following classes of service:
Power factor correction
Constant-speed, constant-load drives
Voltage regulation
Power factor correction:
Overexcited SM having leading power factor are widely used for improving power factor of those power systems which employ a large number of IM and other devices having lagging p.f. such as welders and flourescent lights etc.
Constant-speed applications:
Because of their high efficiency and high-speed, SM (above 600 r.p.m.) are well-suited for loads where constant speed is required such as centrifugal pumps, belt-driven reciprocating compressors, blowers, line shafts, rubber and paper mills etc.
Low-speed SM (below \(600 \mathrm{r.p.m}\)) are used for drives such as centrifugal and screw-type pumps, ball and tube mills, vacuum pumps, chippers and metal rolling mills etc.
Voltage regulation:
The voltage at the end of a long TL varies greatly especially when large inductive loads are present.
When an inductive load is disconnected suddenly, voltage tends to rise considerably above its normal value because of the line capacitance.
By installing a SM with a field regulator (for varying its excitation), this voltage rise can be controlled.
When line voltage decreases due to inductive load, motor excitation is increased, thereby raising its p.f. which compensates for the line drop.
If, on the other hand, line voltage rises due to line capacitive effect, motor excitation is decreased, thereby making its p.f. lagging which helps to maintain the line voltage at its normal value.