A universal motor is a series wound electric motor that is designed to run on either AC or DC current. Advantages of a universal motor over an AC or DC motor are higher start up torque, smaller size, and the ability to operate at very high rpms.
A universal motor is one that operates on either single-phase ac or dc power. These motors are normally made in sizes ranging from 1/ 200 to 1/ 3 horsepower.
You can get them in larger sizes for special conditions. The fractional horsepower sizes are used on vacuum cleaners, sewing machines, food mixers, and power hand tools. The salient-pole type is the most popular type of universal motor. The salient-pole type consists of a stator with two concentrated field windings, a wound rotor, a commutator, and brushes. The stator and rotor windings in this motor are connected in series with the power source. There are two carbon brushes that remain on the commutator at all times. These two brushes are used to connect the rotor windings in series with the field windings and the power source (fig. 7-35). The universal motor does not operate at a constant speed. The motor runs as fast as the load permits; i. e., low speed with a heavy load and high speed with a light load. Universal motors have the highest horsepower-to-weight ratio of all the types of electric motors.
The operation of a universal motor is much like a series dc motor. Since the field winding and armature are connected in series, both the field winding and armature winding are energized when voltage is applied to the motor. Both windings produce magnetic fields which react to each other and cause the armature to rotate. The reaction between magnetic fields is caused by either ac or dc power.