Universal motor and why so called

A universal motor is a series-wound commutator motor that can be operated on either a d.c. or a single-phase a.c. supply with nearly the same speed and torque.

Construction

        Laminated stator poles (field)
          +--------[ field coil ]--------+
 Supply --+                              +--- brushes
          +--- armature (commutator) ----+
       Series connection: field in series with armature

• Stator: a laminated field core, either with salient poles and concentrated field coils (non-compensated type, small ratings) or with a distributed field winding and a compensating winding (compensated type, larger ratings). The field winding is in series with the armature. The core is laminated because the flux alternates on a.c.
• Armature: a wound rotor on a laminated steel core, with a commutator and carbon brushes, as in a d.c. series motor.
• A small air gap is used, and larger units have a compensating winding.

Operation

• On d.c., it works exactly like a d.c. series motor: T ∝ φ I_a ∝ I_a².
• On a.c., the current and flux reverse together, so the torque (∝ φ I_a) always acts in the same direction. The torque pulsates at twice the supply frequency (average T ∝ I²), but because of the rotor inertia the speed remains steady.
• On d.c., E_b = k φ N and V = E_b + I(R_a + R_se). On a.c. there is an additional reactive drop (IX), which lowers the power factor and the speed compared with d.c.

Characteristics

 N (speed)
  |\
  | \
  |  \___           Torque - speed:
  |      ----        very high speed at light load,
  +-----------> T    speed falls sharply with load

• Very high starting torque (T ∝ I²).
• The speed varies greatly with load (as in a series motor). At no load it can run up to 20 000 rpm or more, so it is always connected directly to the load or through gearing.
• For the same voltage and load, the speed on a.c. is lower than on d.c., since part of the voltage is dropped across the reactance.
• Speed is controlled by changing the voltage (using a thyristor or a tapped field).
• Sparking at the brushes is greater on a.c. and radio interference may occur; a compensating winding reduces the reactance and improves commutation.

Why it is called "universal"

Because the same motor works on both a.c. and d.c. supplies. The torque direction does not depend on the polarity of the supply, because the field and armature currents reverse together.

Uses: vacuum cleaners, mixer grinders, drills, sewing machines, hair dryers, portable tools.