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ST offers state-of-art, rugged devices such as TRIACs, thyristors and AC switches as well as microcontrollers allowing motor speeds to be regulated in various ways, depending on the cost and performance requirements of the final application.
Single Phase AC Motors basics
A single-phase motor with a coil winding is the simplest type of AC motor but needs a starting mechanism. This leads to the three main types of single-phase induction motor: shaded-pole, split-phased, and capacitor motors.
Starting the motor can be achieved by designing the stator with two windings, a main and auxiliary coil. Connecting a capacitor in series with the auxiliary winding means the current flows through both coils are out of phase. It is this phase difference that generates a torque to start the rotation.
An electrolytic start capacitor is used to achieve the best phase angles between start and main windings and is disconnected from the start circuit when the motor reaches about 75% of full-load speed. It is designed for short-time duty, and using it for longer than necessary can cause problems, so the accurate control is essential.
Main types
Shaded-pole induction motors
A shaded-pole motor uses a squirrel cage rotor and usually range from 1/20th to 1/6th of a horsepower for small motors. This has additional windings in each corner of the stator, called shade windings. These are not connected but generate a current from the induced field. This inhibits the field, creating a low torque to get the motor moving.
Split-phase induction motors
A split-phase induction motor has two windings, a run winding and a secondary start winding, and usually operate up to 1/3 horsepower to drive blades on a ceiling fan, washing machines tubs, blower motors for oil furnaces, and small pumps.
The higher power start winding gets the motor moving up to 75 to 80% of its speed, and then a centrifugal switch is used to switch over to the less powerful run coil to save energy.
Other Considerations
Single-phase AC motors are extremely common in all walks of life. The vast majority of motors powered by the household or light industrial mains supply are single phase. One of the keys is to size the motor correctly for the application. If the motor doesn’t generate enough torque for the design it will always be running at maximum, putting more strain on the components and generating too much heat. Similarly, if the motor is too big it will not operate efficiently and will waste energy.
However, single-phase power sources can be used to generate a three-phase variable frequency supply to drive a 3-phase induction motor.
A single-phase induction motor drive can also see torque ripple, which is a regular change in the output torque, and the difference between the maximum and minimum figure is often expressed as a percentage to highlight the controllability of the motor.
Fault detection in single-phase induction motors usually requires sensors as there is not sufficient information to implement more complex sensorless algorithms.
