Universal motor

[4][note 1] If an ordinary series-wound DC motor were connected to an AC supply, it would run very poorly.

Therefore the need can arise for an additional resistance in series with the armature to limit the current until the motor rotation can build up the counter-EMF.

[5] By way of contrast, AC synchronous and squirrel-cage induction motors cannot turn a shaft faster than allowed by the power line frequency.

[7] Motor damage may occur from over-speeding (running at a rotational speed in excess of design limits) if the unit is operated with no significant mechanical load.

In some smaller applications, a fan blade attached to the shaft often acts as an artificial load to limit the motor speed to a safe level, as well as a means to circulate cooling airflow over the armature and field windings.

[3] A negative aspect is the maintenance and short life problems caused by the commutator, as well as electromagnetic interference (EMI) issues due to any sparking.

Due to the dramatically increased risk of overheating, totally-enclosed fan-cooled universal motors would be impractical, though some have been made.

In the past, repulsion-start wound-rotor motors provided high starting torque, but with added complexity.

A centrifugal mechanism, when close to running speed, connected all commutator bars together to create the equivalent of a squirrel-cage rotor.

[9] Operating at normal power line frequencies, universal motors are not often found in a range less than 1000 watts.

They are also commonly used in portable power tools, such as drills, sanders, circular saws, and jigsaws, where the motor's characteristics work well.

The DC power is fine for typical jobsite (outmoded) incandescent lighting and the universal motors in some drills and grinders.

Universal motors also lend themselves to electronic speed control and, as such, were an ideal choice for domestic washing machines.

The motor can be used to agitate the drum (both forward and in reverse) by switching the field winding with respect to the armature.

Starters of combustion engines are usually universal motors, with the advantage of being small and having high torque at low speed.

Modern low-cost universal motor, from a vacuum cleaner . Field windings are of copper wire, toward the back on both sides. The rotor's laminated metallic core is gray, with darker slots for winding the coils having high efficiency. The similarly shaped metallic commutator (partly hidden toward the front) has become dark from use. The large brown molded-plastic piece in the foreground supports the brush guides and brushes (both sides), as well as the front motor bearing.
Universal motors' field coils are series wound with the rotor coils and commutator.
Equivalent circuit