For a rotary electric motor to work, there must be two magnetic fields, one associated with the stator and the other with the rotor. At least one of these fields must be moving so the rotor’s magnetic field constantly pursues the stator’s magnetic field, thus creating rotary mechanical motion and energy output.
One of the magnetic fields can be stationary, created by permanent magnets or dc voltage (in the case of a brushed dc motor) applied directly to the stator windings. Or it can be created through brushes and slip rings to the rotor.
The other alternative is that both magnetic fields rotate, as in an induction motor. For this to work, the magnetic fields must rotate at different speeds. If these fields were exactly in synch, (hypothetically) there would be no interaction of the magnetic fields and no rotary motion.
In an induction motor, because of frictional drag or loading on the shaft, the rotor turns more slowly than the stator’s rotating field. The difference in speed is known as “slip” and it is a necessary condition if the induction motor is to operate.
In the universal motor, the situation is strikingly different. A universal motor contains stator windings and rotor windings. Current in the rotor windings is not strictly the result of inductive coupling as in the induction motor. The defining characteristic of the universal motor with respect to its construction is that the stator and rotor windings are wired in series. This serves to create a voltage difference between them so the rotor can pursue the stator’s rotating magnetic field. Current is channeled into the rotor through brushes.
Universal motors are theoretically able to operate on ac or dc. The unit as described above, however, would thrive on dc but not operate efficiently on ac. To run well off ac mains, the motor needs modifications. A compensating winding is added, and the pole pieces are laminated to prevent wasteful circulating current and attendant heat rise. Compared to the universal motor intended for dc operation, there are more coils and plates in the armature with fewer windings in each coil. The design goal is to reduce the additional inductance that is a consequence of ac operation.
The ac-operated universal motor is common in the home and workplace. It is appropriate for small appliances and tools that require comparatively high power. Examples include portable hand tools, blenders, vacuum cleaners and sewing machines.
The universal motor may be recognized by its distinctive whirring sound when running. If poor performance such as failure to start is noted, the answer is to replace the brushes, which are accessible without motor disassembly. If this is deferred, there will be far more expensive commutator damage.