How universal motor works?

Imagine a motor which can work both in continous current and alternate. This motor exists, it is called universal motor and is this post’s subject.

Winding

The universal motor is monophasic, the bigger motors have two windings, one to generate the main magnetic field (field winding) and other as compensating winding. The smaller motors don’t need compensating winding. This type of motor uses high resistance brushes.

This is the model circuit of conductively compensated universal motor.

Exists a type of universal motor whose compesating winding is short-circuited. This type of motor is inductively compesated and can only be used in alternate current. Because the compesating winding depends on electromagnetic induction.

This is the field winding with 2 poles.

The compensating winding serves to reduce the effect of armature reaction.

Operation and characteristics

When operated by continous voltage CC, behave exactly like a DC series motor. When operated by alternate voltage, the direction of magnetic field changes in the stator and the current direction changes in the rotor. Due to Flemming’s rule, or left hand rule, the torque remains in the same direction.

To change rotation direction, have to change the winding connections with the armature.

Why not use a DC series motor in alternate voltage? This motor in AC does not work well, one of the reasons is the high loss by edge currents, since the series motor was made to operate in direct current. For that, both stator and rotor of universal motor have laminated cores to minimize Foucault currents, which work as I explained in the post about transformers, click in the button below to read the post.

TransformersClick here

Another reason is the high reactance in the winding and armature which produce high voltage drop, reducing torque. These are the universal motor’s characteristic curves operating in DC and AC. You can see that this motor reaches high speed when it isn`t connected with any load.

Why some universal motors have compensating winding? To compensate the reactance voltage in the armature and improve the speed when operate with load in AC.