Electrical Machines - Universal Motor

The motors that can be used with a single-phase AC source as well as a DC source of supply voltages are called as universal motors. A universal motor is a commutator type motor. If the polarity of the line terminals of a DC series motor is reversed, the motor will continue to run in the same direction. Therefore, it might be expected that a DC series motor would operate on AC supply also, therefore, it is also known as AC series motor. However, some changes must be made in a DC series motor so that it may operate satisfactorily on AC supply. The changes are given as follows −

• The entire magnetic circuit is constructed of a material having low hysteresis loss. It is laminated to reduce the eddy current loss.
• The series field winding must have small number of turns. The area of field pole is increased so that the magnetic flux density is decreased. This decreases the core loss and the reactive voltage drop.
• In order to obtain the required torque with the low magnetic flux, the number of armature conductors is increased.
• A compensating winding is used to reduce the effect of armature reaction, thereby improving the commutation and reducing the armature reaction.

Construction of Universal Motor

The construction of a universal motor or AC series motor is shown in the figure below.

It consists of a laminated stator and a rotor or armature. The field poles are bolted to the inner periphery of the stator. The field winding and the armature windings are connected series and fed by a single-phase AC source. A commutator is mounted on the rotor shaft of the motor and brushes ride on the commutator.

Operation of Universal Motor

When the motor is connected to a single-phase AC supply, the same alternating current flows through the field and the armature winding. The series field winding of the motor produces an alternating flux that reacts with the magnetic field produced by the currents in the armature winding to produce a torque. Since the currents through the series field winding and the armature winding reverse simultaneously, the produced torque is unidirectional.

Characteristics of Universal Motor

The operating characteristics of a universal motor or AC series motor are given as follows −

• Universal motors have high starting torque.
• The speed of the universal motors increases to a high value with a decrease in the load.
• In very small universal motors, the no-load losses are usually large enough that limit the speed of the motor to a definite value ranging from 1500 to 20000 RPM.
• The power factor of universal motor at full load is about 90%. However, the power factor at starting or overloading is low.

Applications of Universal Motor

The universal motors or AC series motors have high speed, relatively small size and large starting torque. Therefore, these motors are used to drive −

• Drills
• Electric shavers
• Sewing machines
• High speed vacuum cleaners
• Machine tools, etc.