Stepper motors

A stepper motor is an electric motor that rotates in a series of discrete, equal steps rather than spinning continuously. Each electrical pulse sent to its windings advances the shaft by a fixed angle, so the total rotation is simply the number of pulses multiplied by the step angle. Because position is determined by counting steps, a stepper can be driven accurately to a chosen angle without any feedback sensor, which makes it popular in printers, scanners, 3D printers and other equipment needing controlled, repeatable positioning.

Motion comes from energising the stator coils in a definite sequence, called phase sequencing. As the coils are switched, the magnetic field steps around the stator and the rotor follows it one step at a time. The smoothness and resolution depend on the step angle and on how the drive sequences and, optionally, micro-steps the currents. Holding torque keeps the rotor locked in place when the windings are energised but the motor is not commanded to move.

Three main constructions are common. The permanent-magnet stepper has a magnetised rotor and gives relatively large step angles. The variable-reluctance stepper uses a toothed iron rotor with no magnet and is attracted to the energised poles. The hybrid stepper combines features of both to achieve fine step angles with good torque, and is the most widely used in precision machinery. Wiring may be unipolar or bipolar, which affects the driver circuit needed.

Frequently asked questions

How does a stepper motor know its position?

Each pulse moves the shaft by a fixed angle, so the controller tracks position simply by counting the steps it has commanded, often without any feedback sensor.

What is the difference between the main stepper types?

Permanent-magnet steppers have a magnetised rotor, variable-reluctance steppers use a toothed iron rotor with no magnet, and hybrid steppers combine both ideas for fine step angles and good torque.

What is holding torque?

Holding torque is the torque the motor exerts to keep the rotor stationary while its windings are energised but no step is commanded.