AI-3
MULTIPHASE—See polyphase.
POLE PIECES—The shaped magnetic material upon which the stator windings of motors and
generators are mounted or wound.
POLE—The sections of a field magnet where the flux lines are concentrated; also where they enter and
leave the magnet.
POLYPHASE—Term that describes systems or units of a system that are activated by or which generate
separate out-of-phase voltages. Typical polyphase systems are 2-phase and 3-phase whose voltages
are 90- and 120-degrees out of phase, respectively. This term means the same as MULTIPHASE.
PRIME MOVER—The source of the turning force applied to the rotor of a generator. This may be an
electric motor, a gasoline engine, steam turbine, etc.
ROTATING FIELD—The magnetic field in a multiphase ac motor that is the result of field windings
being energized by out-of-phase voltages. In effect, the magnetic field is made to rotate electrically
rather than mechanically.
ROTOR—The revolving part of a rotating electrical machine. The rotor may be either the field or the
armature, depending on the design of the machine.
SELF-EXCITED GENERATORS—Dc generators in which the generator output is fed to the field to
produce field excitation.
SERIES-WOUND MOTORS AND GENERATORS—Machines in which the armature and field
windings are connected in series with each other.
SHUNT-WOUND MOTORS AND GENERATORS—Machines in which the armature and field
windings are connected in parallel (shunt) with each other.
SLIP—The difference between rotor speed and synchronous speed in an ac induction motor. The rotor
will always be slower than the synchronous speed by the amount of slip, otherwise, no voltage would
be induced in the rotor.
SLIP RINGS—Contacts that are mounted on the shaft of a motor or generator to which the rotor
windings are connected, and against which the brushes ride.
SQUIRREL-CAGE WINDINGS—A type of rotor winding in which heavy conductors are imbedded in
the rotor body. The conductors are shorted together at the ends by continuous rings. No insulation is
required between the windings and the core. This type of winding is rugged, easily manufactured,
and practically maintenance free. It is widely applied in ac induction motors. Physically, it appears as
a rotating squirrel-cage, thus the name.
STATOR—The stationary part of a rotating electrical machine. The stator may be either the field or the
armature, depending on the design of the machine.
SYNCHRONOUS MOTOR—An ac motor whose rotor is activated by dc. It is characterized by
constant speed and requires squirrel-cage windings or some other method to be self-starting.
SYNCHRONOUS SPEED—The speed at which the rotating field in an ac motor revolves. This speed is
a function of the number of poles in the field and the frequency of the applied voltage.
