Synchronous-motor-rotor pole

Principle of operation.Basically, the synchronous-motor operation depends on the force produced by a current-carrying conductor lying within a magnetic field. The force, acting at some distance from an axis, produces a torque, which may then produce rotation. In the synchronous motor, the conductor is stationary and the torque causes rotation of the field. In Fig.2 using the conventional right-hand rule, the force on the conductor is to the right. Since the conductor is stationary and the field is on the shaft, motion is produced in a counter-clockwise direction. When the current in the conductor is reversed, the force on it will also reverse, and the rotor will tend to move in a clockwise direction, provided that the conductor is still within the field of the north pole. When starting from the rest, this is substantially true since reversal of the current occurs much more rapidly than movement of the rotor. If, therefore, both rotor and stator are simultaneously energized from rest, there is no starting torque.

On the other hand, if the rotor is already moving at synchronous speed, the rotor will move through 180 electrical degrees during the one half cycle that the current in the conductor takes to reverse. This means that after one half cycle, a pole of opposite polarity is now opposite the same conductor whose current has reversed. With a reversal of both current and magnetic field, force on the conductor remains in the same direction. This is shown in Fig.3.




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