Every day engineers design products that employ single-phase induction motors. Speed control of single-phase induction motors is desirable in most motor control applications since it not only provides variable speed but also reduces energy consumption and audible noise.

Most single-phase induction motors are unidirectional, which means they are designed to rotate in one direction. Either by adding extra windings, external relays and switches, or by adding gear mechanisms, the direction of rotation can be changed. Using microcontroller-based control systems, one can add speed variation to the system. In addition to the option of speed variation, the direction of rotation can also be changed, depending upon the motor control algorithms used.

Permanent Split Capacitor (PSC) motors are the most popular type of single-phase induction motors. This article will discuss different techniques and drive topologies to control the speed of a PSC motor in one and two directions.

Microcontroller Interface

A microcontroller is the brain of the system. Often, the controllers used for motor control applications have specialized peripherals like motor-control PWMs, high-speed analog-to-digital converters (ADCs), and diagnostic pins. The PIC18F2431 and dsPIC30F2010 from Microchip both have these features built in.

Having access to the microcontroller's specialized, on-chip peripherals makes the implementation of control algorithms easier.

ADC channels are used to measure motor current, motor temperature and heat sink temperature (connected to the power switches). A third ADC channel is used to read potentiometer levels, which is then used to set the speed of the motor. Additional ADC channels can be used in the final application to read different sensors, such as the proximity switch, turbidity sensors, water level, freezer temperature, etc.

General-purpose inputs and outputs (I/Os) can be used for interfacing switches and displays in an application. For example, in a refrigerator application, these general-purpose I/Os can be used to control an LCD display, seven-segment LED display, push-button interface, etc....