AC SERIES MOTOR
AC series motors are also known as the modified DC series motor as their construction is very similar to that of the DC series motor. Before we discuss these modifications, here it is essential to discuss what is the need and where do we need to do modifications. In order to understand this, consider this question. What will happen when we give an AC supply to DC series motor? Answer to this question is written below:
- An AC supply will produce an unidirectional torque because the direction of both the current (i.e. armature current and field current) reverses at the same time.
- Due to presence of alternating current, eddy current are induced in the yoke and field cores which results in excessive heating of the yoke and field cores.
- Due to the high inductance of the field and the armature circuit, the power factor would become very low.
- There is sparking at the brushes of the DC series motor.
So considering above points we can say that we don’t have good performance of DC series motor on the application of AC supply. Now in order to reduce the eddy currents there is need to laminate the yoke and field core. This is our first modification to DC series motor.
What about power factor how we can improve power factor? Now the power factor is directly related to reactance of the field and armature circuit and we can reduce the field winding reactance by reducing the number of turns in the field winding. But there is one problem: on reducing the number of turns, field mmf will decrease and due to this the air gap flux decrease. The overall result of this is that there is an increase in the speed of the motor but decrease in the motor torque which is not desired. Now how to overcome this problem? The solution to this problem is the use of compensating winding. On the basis of the usage of compensating winding we have two types of motor and they are written below:
- Conductively compensated type of motors.
- Inductively compensated type of motors.