what is stepper motor

A stepper motor, also known as step motor or stepping motor, is a Brushless DC electric motor that rotates in a series of small and discrete angular steps. 

Stepper motors can be set to any given step position without needing a position sensor for feedback. The step position can be rapidly increased or decreased to create continuous rotation, or the motor can be ordered to actively hold its position at one given step. Motors vary in size, speed, step resolution, and torque.

Switched reluctance motors are very large stepping motors with a reduced pole count. They generally employ closed-loop commutators.

There are three main types of stepper motors: permanent magnet, variable reluctance, and hybrid synchronous. 

Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the attraction or repulsion between the rotor magnet and the stator electromagnets. Pulses move the rotor clockwise or anticlockwise in discrete steps. If left powered at a final step, a strong detent remains at that shaft location. This detent has a predictable spring rate and specified torque limit; slippage occurs if the limit is exceeded. If current is removed, a lesser detent still remains, holding shaft position against spring or other torque influences. Stepping can then be resumed while reliably being synchronized with control electronics.

Permanent magnet stepper motors have simple DC switching electronics, a power-off detent, and no position readout. These qualities are ideal for applications such as paper printers, 3D printers, and robotics. Such applications track position simply by counting the number of steps that each motor has been instructed to take.

Variable reluctance (VR) motors have a soft iron rotor and operate based on the principle that minimum reluctance occurs with minimum gap, so the rotor points are attracted toward the stator's magnetic poles. Variable reluctance motors have detents when powered on, but not when powered off.

Hybrid synchronous motors are a combination of the permanent magnet and variable reluctance types, to maximize power in a small size.