Universal motors seldom exceed one horsepower, and do not run at constant speeds. The speed of a universal motor varies with its load. Among the applications using these motors are vacuum cleaners, food mixers, portable drills, portable power saws, and sewing machines.
In most cases, little more than a few hundred rpm is reached with heavy loads. When the motor operates with no load, the speed may attain 15,000 rpm.
The universal series motor differs in design from a true induction motor. The rotor of a universal motor is made of laminated iron wound with wire coils. The ends of the coils, or loops, connect to a commutator. Electric current in the motor flows through a complete circuit formed by the stator winding and rotor winding. Brushes ride on the commutator and conduct the current through the rotor from one stator coil to the other. Directed by these brushes the rotor current interacts with the magnetic field of the stator causing the rotor to turn. When the direction of current flow changes in the stator, it changes in the rotor. Since the magnetic field is reversed, the rotor continues to turn.
Universal motors have series wound rotor circuitry similar to that of DC motors. They have high starting torque and high starting current. The name universal derives from the motor's capability of operating on either AC or DC power sources.
Universal, variable speed motors slow down with increased loads. High horsepower-to-size ratio is characteristic of their design. Due to the brush/commutator setup, universal motors require more maintenance than other motor designs.
Top of Motors | Index | FAQs
