MM0704, Lesson 3
The more pairs of poles there are for any given frequency, the lower the speed of rotation. A two-pole generator rotates
at twice the speed of a four-pole generator for the same frequency of generated voltage. The frequency is expressed by
the equation
where P is the number of poles and N the speed in rpm. For example, a two-pole, 3,600 rpm generator has a frequency
of
a four-pole, 1,800 rpm generator has the same frequency. A six-pole, 500 rpm generator has a frequency of
and a twelve-pole, 4,000 rpm generator has a frequency of
Motors
AC motors use the same basic principle of operation as DC motors. That principle involves putting electrical energy
into the motor by current flow in the armature (which is located in a magnetic field) and taking mechanical energy from
the rotating armature. The rotation of the armature is caused by the interaction of the fields about the armature
windings and the field winding. The armature's rotation is mechanically transmitted to the motor's load.
AC motors operate from an AC source. There are three general types: induction, series, and synchronous. The
induction motor may be further described according to the type of armature used; squirrel-cage induction motor, or
wound-rotor induction motor.
Induction Motor. An induction motor makes use of the transformer principles explained in the next lesson. A
primary winding is connected to the power source and is mounted on the stator. The secondary winding is mounted on
the rotor. The current in the primary winding causes a magnetic field to surround the winding. This field about the
primary passes through the secondary winding causing a current in the secondary. Recall that this action of a primary
winding current causing a secondary winding current is called induction.
The induction motor is the most widely used AC motor. Its design is simple, and its construction is sturdy. It has no
commutator, so it is much more trouble-free than a DC motor. Although it is usually used where the speed must be
constant, small induction motors (less than one hp) can also operate efficiently at variable speeds. Induction motors
have revolving fields and can be single phase or polyphase (more than one). The operating principles are the same in
either case, except that single-phase motors require special starting windings. Single-phase induction motors vibrate a
great deal because their torque (turning force) is pulsating rather than continuous.
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