Basics of stepper motors

There are several varieties of stepper motors, having somewhat different types of commutation and operating characteristics, but they all have in common the ability to spin continuously in either direction or to turn a part rotation and then stop, with or without holding torque. In this, they resemble the servomotor. But the servo typically operates with closed-loop control — a sensor usually feeds back information about he position of the motor shaft or load. In contrast, a step motor typically has no positional feedback from the motor shaft or driven load to the controller. If for any reason the stepper falls out of synch, it must halt and resynchronize. In applications where loss of sync is an issue (as might be the case with large inertial loads), the more expensive servomotor is the better choice.

The stepper motor (also correctly called the step or stepping motor) resembles the brushless dc motor in that it is externally commutated. Therefore, it needs a controller.

Steppers in small sizes are durable because they contain no brushes and no moving parts except for the rotor, supported by permanently lubricated sealed bearings. They are also inexpensive. Experimenters and robotics enthusiasts can easily harvest a good inventory of stepper motors from discarded ink-jet printers, which also contain low-powered dc motors.

Small steppers are exempt from the National Electrical Code (NEC) requirement for nameplates, but they are readily identified and extensive information can be obtained from data sheets. DC motors have two leads while steppers have a separate pair for each winding, or fewer pairs if one side of each pair joins internally to make a common lead. There is no consistent industry-wide color code, but the five leads can be identified by ringing them out with an ohmmeter.

The simplest stepper is known as a variable reluctance motor. To the outside of the armature attaches a soft iron ring with notched teeth. This structure turns with the shaft and is sized so the notched teeth pass in close proximity, with a small air gap, to the stator. Because the protruding teeth are relatively much closer to the stator, the rotor turns, conforming to the physical principle that a free-moving body of high magnetic permeability, in this case soft iron, will move so as to acquire the magnetic flux path with the least reluctance. (Reluctance in a magnetic circuit is analogous to resistance in an electrical circuit.)

Because of the varied separation between the teeth and the notches with respect to the stator, the motor will turn in small incremental steps in synch with the variable waveform supplied by the external controller. You could hook a music synthesizer to the controller, play middle C and make the stepper spin.

Step motors can have various numbers of notched teeth and field coils. These metrics determine the smallest possible step, expressed in degrees. A 3.6° stepper has a maximum of 100 steps in a single rotation. From this parameter comes the stepper’s resolution.

Instead of notched teeth, permanent magnets may be affixed to the armature. These motors have greater dynamic and holding torque compared to a variable reluctance step motor. The hybrid stepper is a little more expensive. It combines variable reluctance and permanent magnet technologies to make a small, relatively powerful step motor.

Noted for its small size, the Lavet stepping motor, invented in 1936, is used in quartz clocks and automotive dashboard instrumentation. Requiring very little power, it is perfect for wrist watches. A small battery will last for years.