It's just that their speed varies least from no load to full load of all ac-induction motors. Other induction-motor types don't even come close to matching the speed regulation of these motors. Yet, ac-induction motors are widely used mechanical power sources because of their inexpensive yet rugged construction. When coupled with variable-frequency drives, they create a useful source of variable-speed power.
Many situations don't need exact control of motor speed. The difference between the frequency of the variablespeed drive and the motor rpm, called slip, is ignored. However, some situations require more-precise speed regulation. At those times a special sensor called an encoder is attached to the motor shaft to monitor rpm.
Encoders generate one or more series of pulse trains that feed to the motor-drive electronics. From these pulses, the drive determines motor rpm, direction of rotation, and even the amount of rotation far more accurately than old-style tachometer-generators.
The simplest encoder is the incremental style. Typically, it uses a slotted wheel placed between an LED light source and a phototransistor. As the wheel rotates, it alternately passes then blocks light shining on the phototransistor. The phototransistor turns on and off with the flashing light, creating the pulse output.
Encoders are rated by their pulses per revolution (PPR) — the number of pulses they generate per complete revolution of their shaft. A typical encoder generates 2,000 PPR. However, some can reach as high as 36,000 PPR. Motor rpm is found by measuring the time between pulses. Divide 60 by the product of the encoder PPR and the period of time between output pulses in seconds to determine rpm: rpm = 60/(PPR t).
Some encoders have a second phototransistor positioned so that it generates pulses 90° out-of-phase with the first. The two channels, typically labeled A and B, form a quadrature encoder because they generate four distinct quadrants for every pulse: A and B both low, A high with B low, A and B both high, and A low with B high. This boosts encoder resolution by a factor of four.
Quadrature encoding is also used to determine direction of rotation. For example, if channel A goes high, then channel B goes high, then the motor is turning clockwise. Alternately, if channel B goes high first followed by A, the motor is turning counterclockwise.
Turck Inc. (turck.com) provided information for this column.