A dc series motor specifically designed for ac operation is usually referred to as a universal motor. A universal motor is wound and connected just like a dc series motor. That is, the field winding is connected in series with the armature winding.
The construction of universal motor is very similar to dc series motor except that whole of its magnetic path is laminated. The laminated stator is necessary because the flux is alternating when motor is operated from ac supply. The armature is of wound type and similar to that of a small dc motor. It essentially consists of a laminated core having skewed or straight slots and a commutator to which the leads of armature windings are connected. The compensating winding is used to reduce the reactance voltage present in the armature when motor runs on a.c. supply.
When a series motor is connected to a dc source, the current is unidirectional in both the field and the armature windings. Therefore, the flux produced by each pole and the direction of the current in the armature conductors under that pole remain in the same direction at all times. Hence, the torque developed by the motor is constant.
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 both 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; the principle of operation is the same as that of a d.c. series motor.
The operating characteristics of an a.c. series motor are similar to those of a d.c. series motor.
- The speed increases to a high value with a decrease in load. In very small series motors, the losses are usually large enough at no load that limit the speed to a definite value (1500 – 15,000 r.p.m.).
- The motor torque is high for large armature currents, thus giving a high starting torque.
- At full-load, the power factor is about 90%. However, at starting or when carrying an overload, the power factor is lower.
Universal Motors are used in vacuum cleaners, mixer grinders, portable drills and domestic sewing machines, etc.