Out in Front: December 7, 1995

The ML4421 controller IC from Micro Linear helps solve the speed-control problem in ac induction motors. Because such low-cost motors are reliable, they are ubiquitous, finding use in appliances, avionics, HVAC, and factories. The motors’ shortcoming, however, is that you often have to use belts and pulleys to control the output speed. This problem results from the motors’ physics, which require you to increase the drive-waveform amplitude as you increase the drive frequency. Otherwise, torque can fall, leading to inefficient operation and motor stalling.

The analog ML4421 can run induction motors in forward or reverse and maintain a desired speed to within 5%. To achieve this feature, the device digitally creates a drive waveform with the required frequency and provides a matching amplitude for this waveform to produce optimum motor torque. It maintains this target speed by using a sophisticated feedback circuit and algorithm that monitors motor speed, based on the phase difference between motor voltage and current.

The IC can drive single- and two-phase motors over a 10-to-1 variable speed range. It provides a constant voltage/frequency factor with a programmable ratio. You can optionally use the device to sense the motor-slip angle for more precise speed control. The controller has internal low-side drivers and interfaces to industry-standard high-side drivers. The 28-lead device comes in DIP or SOIC packages, requires a 12V supply, and costs $5.95 (1000). -- by Bill Schweber

Micro Linear Corp, San Jose, CA. (408) 433-5200.