Ac Series Motor - Construction & Principle of Operation

Principle of Operation of A.C. Series Motor :

      An ordinary d.c.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 be reversed, both the field and armature current are reversed, the direction of torque remains unchanged. Therefore, the motor continuous to rotate in the same direction. 

     So when normal d.c. series motor is connected to an a.c. 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 anti-clock wise direction.

Ac Series Motor

     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 uniform torque produce the two halves of the cycle.

Ac Series Motor

      Thus a series motor has the ability to run on both the d.c. supply and a.c. supply

     The performance of dc Series motor works on A.C. Supply is not satisfactory due to the following reasons  

1. The efficiency is low. This is because of the increase in core losses due to alternating flux.

2. Armature and field winding offer high reactance to a.c. due to which operating power factor is very poor.

3. Considerable sparking at brushes will occur. This is due to poor commutation. The voltage induced by transformer action in the coil undergoing commutation further intensifies commutation difficulties.

Constructional Features :

Modification in Design of A.C. Series Motor :

     Some modifications are required to have the satisfactory performance of d.c. series motor on a.c supply, when it is called as a.c. series motor. The modifications are : 

1. To reduce the eddy current losses, yoke and poles should be fully laminated.

2. The power factor can be improved by reducing field and armature reactances. 

     In order to reduce field reactance, the field winding is designed with less number of turns. Lower pole flux also reduces the transformer emf in the commutating coil.

3. The motor should be provided with a large number of poles each supplying less flux per pole.

4. Reduction in number of turns on the field winding would also reduces field flux. To keep the torque constant on the shaft, the armature turns should be increased proportionately. This increases the armature reaction and armature reactance. 

5. Compensating winding should be employed to lower the armature reactance as far as possible. Compensation also improves commutation.

   The flux produced by compensating winding is opposite to that produced by armature and effectively neutralizes the armature reaction.

6. The armature coils are single turn coils and brushes of less width are used not to short circuit more than two coils at a time. 

7. The air gap is made very small so that less field turns can be used per pole.

8. The frequency of supply used is reduced. The transformer emf is proportional to frequency and hence good commutation is easy at lower frequencies.

Characteristic of A.C. Series Motor :

Ac Series Motor

     The characteristics of a.c. series motor are similar to that of d.c. series motor. The torque varies as square of the armature current and speed varies inversely as the armature current. The series motors must always started with some load on it because at 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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