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Single-Phase Basics

Because it has only a single alternating current source, a single-phase motor can only produce an alternating field: one that pulls first in one direction, then in the opposite as the polarity of the field switches. A squirrel-cage rotor placed in this field would only twitch, since there would be no movment upon it. If pushed in one direction, however, it would spin.

The big distinction between different types of single-phase AC motors is how they start the rotor in a particular direction such that the alternating field will produce rotary motion in the desired direction. This is usually done by a device that introduces a phase-shifted magnetic field on one side of the rotor.

The figure at right shows the torque curves of the four major types of single-phase AC motors.

Split-Phase Motors

The split phase motor achieves its starting capability by having two separate windings wound in the stator. The two windings are separated from each other. One winding is used only for starting and it is wound with a smaller wire size having higher electrical resistance than the main windings. From the rotor's point of view, this time delay coupled with the physical location of the starting winding produces a field that appears to rotate. The apparent rotation causes the motor to start. A centrifugal switch is used to disconnect the starting winding when the motor reaches approximately 75% of rated speed. The motor then continues to run on the basis of normal induction motor principles.

Capacitor-Start Motors

Capacitor start motors form the largest single grouping of general purpose single phase motors. These motors are available in a range of sizes from fractional through 3HP. The winding and centrifugal switch arrangement is very similar to that used in a split phase motor. The main difference being that the starting winding does not have to have high resistance. In the case of a capacitor start motor, a specialized capacitor is utilized in a series with the starting winding. The addition of this capacitor produces a slight time delay between the magnetization of starting poles and the running poles. Thus the appearance of a rotating field exists. When the motor approaches running speed, the starting switch opens and the motor continues to run in the normal induction motor mode.

This motor produces relatively high starting torque, 225 to 400% of full load torque. The capacitor start motor is ideally suited for hard to start loads such as conveyors, air compressors and refrigeration compressors. It is also used for many applications where high starting torque may not be required. The capacitor start motor can usually be recognized by the protrusion on the frame where the starting capacitor is located.

Permanent-Split Capacitor Motors

The capacitor of this motor is left in series with the starting winding during normal operation. The starting torque is quite low, roughly 40% of full-load, so low-inertia loads such as fans and blowers make common applications. Running performance and speed regulation can be tailored by selecting an appropriate capacitor value. No centrifugal switch is required.

Shaded-Pole Motors

The shaded pole motor is the simplest of all single phase starting methods. In the shaded pole motor, the stator poles are notched and a copper short circuiting ring is installed around a small section of the poles.

As a result of the alteration of the filed pole configuration, the build-up of the magnetic field is delayed in the portion of the pole surrounded by the copper shorting ring. From the rotor's point of view, this makes the magnetic field seem to rotate from the main pole toward the shaded pole. This slight appearance of field rotation is adequate to start the rotor moving and, once started, it will accelerate up to full speed.

The shaded pole motor is simple and inexpensive, but has low efficiency and a very low starting torque. Speed regulation is poor, and it must be fan-cooled during normal operation. Shaded-pole motors are thus used in shaft-mounted fans and blowers, and also small pumps, toys, and intermittently used household items.

In the pictures on the right, you can see the heavy coils around the stator to the left of the rotor.