A.C. Series Motor OR Universal Motor

§  A universal motor is built like a series d.c. motor with the difference that both its stator and armature are laminated. A universal motor is defined as a motor which can be operated on both a.c. supply and d.c. supply at approximately the same speed and output.

§  A d.c. series motor (both armature and field windings are in series) can be operated on a single phase a.c. supply. It is then called an a.c. series motor. Following changes are made in d.c. series motor to operate satisfactory on a.c. supply.

1.    The entire magnetic circuit is laminated in order to reduce the eddy current loss. Hence an a.c. series motor requires a more expensive construction than a d.c. series motor.

2.    The series field winding uses a few turns to reduce the reactance of the field winding. This reduces the voltage drop across the field winding.

3.    A high field flux is obtained by using a low – reluctance magnetic circuit.

4.    When the motor is operated on a.c. supply, there will be possible sparking between the brushes and the commutator. This can be eliminated by using high-resistance leads to connect the coils to the commutator segments.

• Operation: When the motor is connected to a.c. supply, the same alternating current flows in the field and armature windings. The field winding produces an alternating flux that reacts with the current flowing in the armature to produce a torque. The flux produced is only alternating not the rotating.

Characteristics of Universal Motor:

1. It is self starting motor, starting torque is twice or thrice, therefore no starting device is required.
2. The speed increases to a high value with a decrease in load. In small motors, the losses are enough at no-load to limit the speed to a definite value. However, in a d.c. series motor, the speed at no-load may rise to a dangerously high value.
3. The motor torque is high for large armature currents, thus giving a high starting torque.
4. At full load, the power factor is about 90%.
5. They operate at high speeds (1500 – 15000 r.p.m)

Application:

These motors are used to drive

(1) high speed vacuum cleaners   

(2) Sewing machines 

(3) Electric shavers 

(4) Drills 

(5) Machine tools etc.