each stator pole (fig. 7-36). This copper band delays the
magnetic field through that portion of the pole. When
ac power is applied, the main pole reaches its polarity
before the shaded portion of the pole. This action
causes the shaded poles to be out of phase with the main
poles and a weak rotating magnetic field is produced.
Because of the low-starting torque, it isnt feasible to
build motors of this type larger than 1/20 horsepower.
They are used with small fans, timers, and various light-
load control devices.
Remember, all single-phase induction motors have
some auxiliary means to provide the motor with starting
torque. The method used for this starting torque
depends on the application of the motor.
A wide variety of motors are used for fans and
blowers. Here we will discuss the different methods of
varying the speed of common fan motors.
Different manufacturers use different methods for
varying the speed. On some motors only the running-
winding voltage is varied while the voltage in the
Figure 7-36.Shaded-pole stator.
starting winding is constant On others the running
winding consists of two sections connected in series
across 230 volts for high speed. If low speed is required,
the two sections are connected to 155 volts through an
auto-transformer. Usually, these motors are connected
for three speeds.
SPEED CONTROL OF SHADED POLE
Many fans have a shaded-pole type motor. The
speed of these motors is varied by inserting a choke coil
in series with the main winding. Taps on the choke coil
provide the different speeds.
SPEED CONTROL OF SPLIT-PHASE AND
Split-phase and capacitor motors are commonly
used in floor and wall fans. Two-speed, split-phase,
motors are normally made with two run windings and
either one or two start windings, depending on the
manufacturer. In a three-speed, split-phase motor, the
speeds are obtained with only three windings: one
running, one auxiliary, and one starting winding. For
high speed, the running winding is connected across the
line, and the starting winding is connected in series with
the auxiliary winding across the line. For medium
speed, the running winding is connected in series with
half the auxiliary winding, and the starting winding is
connected in series with the other half of the auxiliary
winding. For low speed, the running and auxiliary
windings are in series across the line, and the starting
winding is connected across the line. Actually, a tap at
the inside point of the auxiliary is brought out for
medium speed. A centrifugal switch is connected in
series with the starting winding.
The capacitor motor used for two-speed floor fans
is a permanent-split capacitor motor. This motor does
not use a centrifugal switch. For three speeds, the
auxiliary winding is tapped at the center point, and a
lead is brought out for medium speed. This motor is
similar to the three-speed, split-phase motor, except that
the centrifugal switch is removed and a capacitor
substituted. This motor is used extensively for blowers
in air-conditioning systems.
Split-phase motors used on wall fans are wound like
the ordinary split-phase motor, but many do not have a
centrifugal switch. A special type of autotransformer,
located in the base of the fan, is used to change the speed
and also to produce an out-of-phase current in the
starting winding. The primary of the transformer is