AC Series Motor

Key learnings:
  • AC Series Motor Defined: An AC series motor is a modified version of the DC series motor adapted to run on alternating current (AC).
  • Modification Needs: Necessary modifications include reducing eddy currents and improving the power factor to enhance performance with AC supply.
  • Compensating Windings: These are crucial for maintaining torque and motor speed, available in conductively and inductively compensated forms.
  • Circuit Diagrams: Detailed circuit diagrams illustrate the different types of compensating mechanisms in series motors, explaining their operational advantages.
  • Practical Applications: AC series motors are widely used in household appliances, showcasing their versatility and the practical benefits of their design enhancements.

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:

  1. An AC supply will produce an unidirectional torque because the direction of both the currents (i.e. armature current and field current) reverses at the same time.
  2. The alternating current induces eddy currents in the yoke and field cores, causing them to heat excessively.
  3. Due to the high inductance of the field and the armature circuit, the power factor would become very low.
  4. 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.
How can we improve the power factor? The power factor relates to the reactance in the field and armature circuits, which we can lower by reducing the turns in the field winding.

However, reducing the number of turns decreases the field mmf and air gap flux. This change increases the motor’s speed but undesirably lowers its torque. 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:

  1. Conductively compensated type of motors.
  2. Inductively compensated type of motors.

Conductively Compensated Type of Motors

Conductively compensated motors feature a compensating winding in series with the armature, positioned in the stator slots. Its axis is electrically 90 degrees from the main field axis.
ac series motor

Inductively Compensated Type of Motors

Given below is the circuit diagram of the inductively compensated type of motors. In this type of motor, the compensating winding has no interconnection with the armature circuit of the motor. In this case, a transformer action will take place as the armature winding will act as primary winding of the transformer and the compensation winding will acts as a secondary winding. The current in the compensating winding will be in phase opposition to the current in the armature winding.
series motor
Given below is the complete schematic diagram of the single phase AC series motor with all the modifications (i.e. compensating winding and inter pole).
series motor with interpoles
Speed control of this type of motor is best obtained by solid state device. The motor has numerous applications such as portable drills, hair dryers, table fans, kitchen appliances, etc. We have already discussed the advantage of having compensating winding. Let us discuss what is the use of the inter pole? The main function of the inter poles is to improve the performance of the motor in terms of higher efficiency and a greater output from the given size of the armature core.

To reduce the series field inductance, we use a higher reactive voltage drop across the series field than the armature or compensating field. The inter pole winding connects in parallel with a non-inductive shunt, as illustrated above.

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