UNIVERSAL MOTORS A universal motor is one that operates on either single-phase ac or dc power. These motors are normally made in sizes ranging from 1/ 200 to 1/ 3 horsepower.
You can get them in larger sizes for special conditions. The fractional horsepower sizes are used on vacuum cleaners, sewing machines, food mixers, and power hand tools. The salient-pole type is the most popular type of universal motor. The salient-pole type consists of a stator with two concentrated field windings, a wound rotor, a commutator, and brushes. The stator and rotor windings in this motor are connected in series with the power source. There are two carbon brushes that remain on the commutator at all times. These two brushes are used to connect the rotor windings in series with the field windings and the power source (fig. 7-35). The universal motor does not operate at a constant speed. The motor runs as fast as the load permits; i. e., low speed with a heavy load and high speed with a light load. Universal motors have the highest horsepower-to-weight ratio of all the types of electric motors.
The operation of a universal motor is much like a series dc motor. Since the field winding and armature are connected in series, both the field winding and armature winding are energized when voltage is applied to the motor. Both windings produce magnetic fields which react to each other and cause the armature to rotate. The reaction between magnetic fields is caused by either ac or dc power.
SHADED-POLE MOTORS The shaded-pole motor is a single-phase induction motor that uses its own method to produce starting torque. Instead of a separate winding like the split-phase and capacitor motors, the shaded-pole motor's start winding consists of a copper band across one tip of
Figure 7-35.- Universal motor schematic.
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. 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.
FAN MOTORS 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 MOTORS 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.