Electrical Engineering ⇒ Topic : Synchronous Motor

The synchronous motor is the one type of 3-phase A.C. motor which operates at a constant speed from no-load to full-load. It is similar in construction to 3-phase A.C. generator in that it has a revolving field which must be separately excited from a D.C. source. By changing the D.C. field excitation, current, the power factor of this type of motor can be varied over a wide range of lagging and leading values.

This motor is used in many individual applications because of its fixed speed from no-load to full-load, its high efficiency and low initial cost. It is also used to improve the power of 3-phase A.C. industrial circuits

SYNCHRONOUS MOTOR

To run the synchronous machine in motoring mode, excitation voltage is given to the field as usual. The supply voltage is given to the armature terminals as shown in Figure  (a). The phasor diagram at lagging pf., unity pf., and leading pf. is shown in Figure (b).

                Figure  (a) Equivalent circuit model of synchronous motor.

                                  Figure (b) Phasor diagram of synchronous motor.

From Figure (a), we have

        ............ (1)

From Figure (b), we get

                            .............. (2)

From Figure (b) (iii), we obtain

    ............ (3)