Short note: Universal motor

A universal motor is a series-wound commutator motor that runs on both a.c. and d.c. supplies.

• Construction: the field winding is in series with the armature. The stator core and armature are laminated, to reduce eddy-current losses on a.c. The motor has a commutator with carbon brushes.
• Principle: the torque ∝ φ · I_a. As the field and armature currents are the same current, both reverse together on a.c., so the direction of the torque stays the same. The torque pulsates at twice the supply frequency, but rotor inertia keeps the speed steady.
• Characteristics: high starting torque (T ∝ I²), and a speed that varies widely with load. Under no load, the speed can be dangerously high. The speed on a.c. is lower than on d.c. because of the reactive voltage drop, and the power factor is lower on a.c.
• Speed control: changing the applied voltage with a thyristor or a tapped field winding.
• Uses: vacuum cleaners, mixers, food blenders, portable drills, sewing machines and hair dryers.
• Why "universal": because it operates on both a.c. and d.c.