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.
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. Communication channels like I2C (TM) or SPI (TM) are used to connect the motor control board with another board to exchange data.
Fault and diagnostics interfaces include input lines with special features like the ability to shut down the PWMs in case of catastrophic faults in the system. For example, in a dish-washer, if the drive is blocked due to accumulated waste, it could prevent the motor from rotating. This blockage can be detected in the form of over current in the motor control system. Using the diagnostics features, these types of faults can be logged and/or displayed, or transferred to the trouble-shooting PC of a service person. Often, this will prevent hard failures and reduce the downtime of the product, resulting in reduced service costs.
|The hardware interface for the PIC 18F2431 or dsPIC30F2010.|
PWMs are the main peripherals used to control the motor. Using the above inputs, the microcontroller's motor control algorithm determines the PWM duty cycle and pattern of output. The PWM's most valuable features include complementary channels with programmable dead time. PWMs can be edge-aligned or center-aligned. Center-aligned PWMs have the advantage of reduced electromagnetic noise (EMI) being emitted by the product.
Option #1: Unidirectional Control
VF control in