AC Series Motor or Universal Motor

AC SERIES MOTOR OR UNIVERSAL MOTOR

A dc series motor will rotate in the same direction regardless of the polarity of the supply. One can expect that a dc series motor would also operate on a single‒phase supply. It will then be called as ac series motor. However, some changes must be made in a dc motor that is to operate satisfactorily on ac supply. The changes effected are:

(i) The entire magnetic circuit is laminated in order to reduce the eddy current loss. Hence an ac series motor requires a more expensive construction than a dc series motor.

(ii) The series field winding uses few turns as possible to reduce the reactance of the field winding to a minimum. This reduces the voltage drop across the field winding.

(iii) A high field flux is obtained by using a low‒reluctance magnetic circuit.

(iv) There is considerable sparking between the brushes and the commutator when the motor is used on ac supply. It is because the alternating flux establishes high currents in the coils short‒circuited by the brushes. When the short‒circuited coils break contact from the commutator, excessive sparking is produced. This can be eliminated by using high‒resistance leads to connect the coils to the commutator segments.

Construction

The construction of an ac series motor is very similar to a dc series motor except the above modifications (Ref Fig. 5.33). Such a motor can be operated either on ac or dc supply and the resulting torque‒speed curve is the same in each case. For this reason, it is some‒times called, a universal motor.

Operation

When the motor is connected to an a.c. supply, the same alternating current flows through the field and armature windings. The field winding produces an alternating flux ϕ that reacts with the current flowing in the armature to produce a torque. Since with armature current and flux reverse simultaneously, the torque always acts in the same direction. It may be noted that no rotating flux is produced in this type of machines and the principle of operation is the same as that of a dc series motor.

Characteristics

The operating characteristics of an ac series motor are similar to those of a dc series motor.

(1) The speed increases to a high value with a decrease in load. In very small series motors the losses are usually large enough no load that limit the speed to a definite value (1500‒ 15,000 rpm).

(ii) The motor torque is high for large armature current, thus giving a high starting torque.

(iii) At full‒load, the power factor is about 90%. However, at starting or when carrying an overload, the power factor is lower.

Applications

The fractional horsepower ac series motors have high‒speed (and corresponding small size) and large torque. They can, therefore, be used to drive

(a) High‒speed vacuum cleaners

(b) Sewing machines

(c) Electric shavers

(d) Drills

(e) Machine tools etc.