Stepper Motors are a type of brushless DC motors, they're different from typical DC motors as they don't simply spin when voltage is applied. They're made from multiple phased coils which can be turned on/off quickly using a driver to create precise step movements in both directions.
These phased coils break up a full rotation into much smaller precise part rotations often referred to as steps. Unlike standard DC motors, stepper motors will stay secure in their current position if they lose a control signal until power is removed.
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Instead of spinning coil-like other motors, a stepper motor uses magnets in the center with wire coils surrounding it. These coils are grouped into phases which determines the step size of the motor. Energizing these coil phases in sequence will cause the motor to rotate.
The motor converts pulses in current from a driver into preceding step movements by turning coils on/off almost instantly. Each driver pulse equates to one step of a full motor turn. As stepper motors use wire coils around a central magnet they can make part rotations (steps) extremely accurately and stop almost instantly.
Stepper motors come in a wide range of different coil configurations and phase types. The more phases a motor has the smaller the step size is meaning the resolution is higher. The most notable types of stepper motors are Unipolar and Bipolar.
Unipolar vs Bipolar
Unipolar: This type of motor energizes the coils in the same direction every time, meaning two coils are needed per phase in order to give unidirectional. This means the internal circuitry is simple but more space and more connections are required. Typically these motors are easier to use and lower in cost.
Bipolar: These motors use a single coil winding per phase, the current is just reversed to give motion in the opposite direction. This means sone more complex circuitry is required but the motors are smaller. These motors are generally considered better than unipolar ones as they have more torque and are more efficient.