# Ac Series Motor - Construction & Principle of Operation

## Principle of Operation of AC Series Motor :

An ordinary dc series motor will run in the same direction regardless of the polarity of the supply. The direction of the torque depends upon the relative directions in space of flux and armature current. If the line terminals are reversed, both the field and armature current are reversed, and the direction of torque remains unchanged. Therefore, the motor continues to rotate in the same direction.

So when normal dc series motor is connected to an ac supply, both field and armature currents reverse simultaneously and unidirectional torque is produced in the motor.

Consider the case of a two-pole motor and let the alternating current be in its positive half, then the polarity of the field poles and the currents flowing through the armature conductors be as indicated in Figure.

The armature conductors carry inward currents +ve under N-pole and outward currents -ve under S-pole. By applying Fleming's left-hand rule it will be seen that the torque developed in the armature will try to rotate in an anti-clockwise direction.

During the next instant, the alternating current goes through the negative half cycle Now the current through the field winding and armature will also change. It will be again seen that the armature will tend to rotate in the same direction because of the uniform torque produced by the two halves of the cycle.

Thus a series motor can run on both the dc supply and ac supply. The performance of dc Series motor works on ac supply is not satisfactory due to the following reasons,

## Constructional Features :

### Characteristics of AC Series Motor :

The characteristics of ac series motor is similar to that of dc series motor. The torque is proportional to the square of the armature current and speed is inversely proportional to the armature current.

The series motors must always be started with some load on them because the starting speed of the motor is very high due to high starting torque i.e., 3 to 4 times the full load torque.

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