Do you know about Synchronous Motor?

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A Synchronous Motor has the same relationship to an alternate as a DC motor as to a DC generator i.e. if an alternator is supplied AC power it is capable of rotating as a motor and doing mechanical work. If the mechanical power supplied to rotating alternator is removed while the DC field remains energized, and an supply is then connected across the armature terminals, torque will be developed and the alternator field will continue to rotate at a speed determined by the ac supply frequency and numbers of poles on the Synchronous machine.



Principle of operation:-

 The action of the motor may be followed by referring to figure (A), which illustrate a 3-phase, 2-pole Synchronous machine. The rotor has 2-poles and stator 2-poles per phase. The armature winding is housed in slots in periphery of the stator, in practice it is usual to have salient poles on the rotor.




As  the rotor is excited from DC supply, so the poles of the rotor has permanent in nature but the polarity of the stator poles has alternating in nature as it is connected to AC supply. Let us consider the rotor has stationary and in position shown in figure (a).



At this instant the rotor S-pole is attracted to the stator N-pole and, therefore the rotor tends to rotate in clockwise direction after half a period the polarity of the stator pole is reversed but the polarity of the rotor poles remains the same as shown in figure (b). At this instant, the rotor S-pole is repelled by the stator S-pole beings similar in nature and therefore rotor tends to rotate in counter-clockwise direction. Thus the torque acting on the rotor of a synchronous motor is not unidirectional but pulsating one and due to inertia of rotor, it will not move in any direction. So the synchronous motor has got no self starting torque.

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