Universal motor operation and applications

Universal motor

A universal motor is a series-wound commutator motor designed to run on both ac and dc supplies, with similar performance on both. It is a small motor, up to about 750 W, and runs at high speed (3000 to 20000 rpm).

Construction

• Stator: a laminated (to reduce eddy currents on ac) salient-pole field, with the field winding connected in series with the armature.
• Armature: a laminated, slotted rotor with its winding connected to a commutator, and carbon brushes.
• Larger motors may have a compensating winding to reduce armature reaction and commutation problems on ac.

 Supply ---[Field]---[Armature]--- Supply
              (series connection)

Operation

1. On dc: the field flux $\phi \propto I$, and the torque $T \propto \phi I_a \propto I^2$, as in a dc series motor.
2. On ac: the current and the flux reverse at the same time, so the product $\phi I_a$ remains unidirectional. The torque ($\propto I^2$) always acts in the same direction but pulsates at twice the supply frequency, so the motor rotates continuously.
3. The speed varies strongly with load: high speed at light load (it can reach a dangerous speed at no load) and low speed at heavy load. The speed can be controlled by changing the voltage, using a regulator or a tapped field.
4. Performance on ac is poorer than on dc: the reactance of the windings causes a voltage drop, the power factor is lower and the losses are higher. At the same voltage, the speed on ac is lower than on dc.

Applications

Because of its high speed, high starting torque and compact size, it is used in vacuum cleaners, mixer-grinders, hair dryers, portable drills, sewing machines, food mixers, electric shavers, blenders and other power tools.