For voltage regulation of the transmission lines, synchronous motors are used as:

Synchronous Motors

Over-excited synchronous motors draw leading currents and hence act as synchronous condensers. So, they are used to improve the power factor and to improve the voltage regulation.

• ​​Synchronous motors are inherently not self-starting. They require some external means to bring their speed close to synchronous speed before synchronizing. The speed of operation is in synchronism with the supply frequency and hence for constant supply frequency, they behave as constant speed motors irrespective of load condition.
• A synchronous motor can be used to compensate for voltage drop in a constant voltage transmission line by acting as a synchronous condenser. When the system voltage drops, the motor can be over-excited to generate reactive power, which helps to maintain the system voltage at the desired level.
• Synchronous motor finds applications where operating speed is less (around 500 rpm) and high power is required. For low power output below 40 kW in the medium speed range, the size, weight, and cost of the corresponding three-phase induction motor are very high. ​