Service manual

- Some basics.

DC Motor AC Motor

- DC motors consist of rotor-mounted windings (armature) and

stationary windings (field poles). In all DC motors, except permanent

magnet motors, current must be conducted to the armature windings

by passing current through carbon brushes that slide over a set of

copper surfaces called a commutator, which is mounted on the rotor.

The commutator bars are soldered to armature coils. The

brush/commutator combination makes a sliding switch that energizes

particular portions of the armature, based on the position of the rotor.

This process creates north and south magnetic poles on the rotor that

are attracted to or repelled by north and south poles on the stator,

which are formed by passing direct current through the field

windings. It's this magnetic attraction and repulsion that causes the

rotor to rotate.

A magnetic field is produced in an AC motor through the action of

the three- phase voltage that is applied. Each of the three phases

is 120° from the other phases. From one instant to the next, the

magnetic fields combine to produce a magnetic field whose

position shifts through a certain angle. At the end of one cycle of

alternating current, the magnetic field will have shifted through

360°, or one revolution.

Torque in an AC motor is developed through interactions with the

rotor and the rotating magnetic field. The rotating magnetic field

cuts the bars of the rotor and induces a current in them due to

generator action. This induced current will produce a magnetic

field around the conductors of the rotor, which will try to line up

with the magnetic field of the stator.