CONSTRUCTION AND OPERATION OF A SYNCHRONOUS MOTOR

The stator winding of a synchronous motor is similar to that of a 3 phase induction motor. The rotor consists of salient poles excited by dc field windings like that of inward-projecting poles of a dc motor. The rotor field windings are energized by direct current passed through slip rings from an external source or from a dc generator, mounted on the same rotor shaft.

When the stator winding is energized from a 3 phase supply, a revolving magnetic field, the speed of which is given by N_s=120f/P is produced. This speed is called synchronous speed. To enable the synchronous motor to run at the above mentioned synchronous speed the rotor field winding is energized and at the same time brought near to the synchronous speed, by some other means. The rotor poles, which are always equal to that of the stator poles, are pulled to synchronous speed and the two set of poles lock with each other and the rotor starts rotating at synchronous speed. Thus, to run a synchronous motor, the rotor has to be brought near to synchronous speed first by some means, say by some external prime mover. This is a big disadvantage of this motor. However, a synchronous motor is made self-starting by providing a squirrel cage winding (like that of an induction motor) along with the dc field winding on the rotor. In such a case when three phase supply is applied across the stator windings the rotor starts rotating as an induction motor and when it reaches near its final speed (near synchronous speed), dc field winding is energized and the rotor thus pulls into synchronism with the revolving field and continues to run at synchronous speed. At synchronous speed there will be no current in the squirrel cage winding since at synchronous speed slip is zero. The squirrel cage winding therefore is designed only for short duty services. During the starting period the dc field winding has to be kept shorted through a discharge resistance. This is done so as to avoid building up of an extremely high voltage in the winding. If field is left open circuited a high voltage will develop in the open field winding as it has large number of turns and the relative speed of stator flux to the windings of the poles is high during starting. But this induced high voltage will gradually decrease as the motor will be picking up speed. The induced emf in the field winding is kept to a safe value by shorting the winding. This would limit the demagnetizing effect on the main flux otherwise caused due to current flowing in the dc field winding as a result of induced emf in it. This demagnetizing effect, if allowed to happen will reduce the starting torque of the motor. If in some special applications a higher starting torque is required the field winding can be left open circuited, but should be sectionalized, to have reduced voltage induced across the separated portions.
From the above, it is seen that the primary purpose of the squirrel cage in this motor, is for starting the motor. As mentioned earlier this winding is designed for low thermal capacity. If, however, the motor picks up speed too slowly under some loading condition, it will run as induction motor for extended period of time and as a result the squirrel cage winding may get over heated and get damaged. To overcome this problem a certain protection must be provided which should disconnect the motor from the supply in the event of its failure to get synchronized properly within a certain prescribed time. A timing relay is used for this purpose to open the control circuit if the motor fails to synchronize within the set time.

Synchronous motors, like the induction motors, can be started by applying line voltage, reduced voltage, or using part winding controllers depending upon the kind of load, frequency of starting, and power service restrictions. The starters for the motor can be manual, semiautomatic, or fully-automatic using a polarizing frequency relay.

From the above it follows that synchronous motor control has two basic functions:

(i) To start the motor as an induction motor (the motor can be started by any schemes such as across the line, auto-transformer, primary resistor or any other method);

(ii) To bring the motor up to synchronous speed by exciting the dc field. Different types of synchronous motor starters are discussed as follows.