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 isn't 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.
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 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.
Figure 7-36. - Shaded-pole stator.
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.
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 isContinue Reading