Step motors explained


CONVENTIONAL ac and dc motors operate on continuously applied input voltage and most often produce a continuous (steady state) rotary motion.

Most commercial type motors are single phase, equipped with two lead wires, or three lead wires, the third lead as a grounding lead. Current flows from one lead wire into the motor and returns through the other lead wire.

Unlike these motors, a step motor (also called a stepping motor or a stepper motor) will not produce continuous motion from a continuous input voltage. It will stay at a particular position as long as the power is "on". An electrical phase change is necessary to make a step motor move. This means that no single phase step motor can exist.

A typical two-phase step motor has four lead wires. Four of them represent the two phase coils, normally coloured black and green, red and blue.

Turning one phase on will hold the rotor in one position (called a detent position). Turning off this phase and turning on the second phase will move the rotor to rest at another detent position.

This "on and off" current flow is called a pulse. One pulse into the motor causes the rotor to increment (move) one precise angle. This movement is repeated with each input pulse.

When properly applied and controlled, the number of output steps is always equal to the number of input pulses.

The widespread acceptance of the digital approach for overall control of machine and process functions has generated a high demand for mechanical motion devices capable of delivering a discrete incremental motion of known accuracy.

The replacement of mechanical devices, such as clutches and brakes, with step motors, provides considerably greater reliability and consistency.

Norman G. Clark 03 9450 8200.