AC Motors: Principle of Operation

Universal Motors

A universal motor is a single-phase series motor, which is able to run on either alternating current (ac) or direct current (dc) and the characteristics are similar for both ac and dc. The field windings of a series motors are connected in series with the armature windings

Basic principles of Universal Motors

The electrical design areas of a universal motor are the magnetic circuit, the field and armature windings, the commutator and brushes, the insulation and the cooling system.

Commutation Process of Universal Motors

Performance Characteristics of Universal Motors

Shaded Pole Motors

A Shaded Pole motor is an AC single phase induction motor. The auxiliary winding, which is composed of a copper ring, is called a shading coil. The current in this coil delay the phase of magnetic flux in that part of the pole in order to provide a rotating magnetic field. The direction of rotation is from the unshaded side to the shaded ring.

Basic principles of Shaded Pole Motor

  • This shading-coil (ring) arrangement displaces the axis of the shaded poles from the axis of the main poles
  • When power is applied to the stator, the flux in the main part of the pole induces a voltage in the shading coil, which acts as a transformer secondary winding.
  • Since the current in the secondary winding of a transformer is out of phase with the current in the primary winding.
  • The current in the shading coil is out of phase with the current in the main field winding.
  • Thus, the flux of the shading pole is out of phase with the flux of the main pole.

Rotating field of a Shaded Pole Motor

Synchronous Motors

Synchronous ac motors are constant-speed electric motors and they operate in synchronism with line frequency. The speed of a synchronous motor is determined by the number of pairs of poles and is always a ratio of the line frequency.

  • The stator is provided with two simple coils, which can be directly connected to the mains.
  • The rotor consists of a cylindrical permanent two-pole magnet, which is diametrically magnetized.

Basic principles of Synchronous Motors
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