STEPPER MOTOR

The most common problem we face when we use normal DC motors is that we don’t have precise control over how much it rotates. To rotate DC motors through a particular number of degrees what we can do is either calibrate it for a delay based operation i.e. if I switch it on for n seconds it moves 360 degrees; or what we can do is attach n encoder to the shaft which gives us a feedback on how much the motor shaft has rotated so that we can stop it when it rotates through the desired angle. Home made encoders give good results but don’t have such a high resolution and high resolution encoders are costly.
In such cases where we need to control the rotary position of the motor we can use stepper motors. Stepper motors have a tendency to make beginners feel uncomfortable about using them. But lets hope this air of discomfort about using stepper motors disappears once we are done through this article.
Stepper motors are motors available in round, square, rectangular shapes with 4 or more wires coming out of them.

In the permanent magnet stepper motor the rotor is a permanent magnet and the stator is a set of coils which are energized one after another. In the unipolar motors the direction of current in the coil doesn’t reverse (so UNI) while in the bipolar the current through the coil flows in both the directions (so BI).In both types of stepper motors there are two coils wound on the stator poles, which gives us 4 wires.

The difference is that in unipolar motors there is a center tap from each of the coil winding. These center taps are either brought out individually (which will give us 4+2 = 6 wires) or are shorted together and brought out (which gives us 4+1 = 5 wires)

Properties:

1) Digitaly controlled brushless motor that rotate a specific number of degrees (a step).
2) Number of degrees (resolution) can be between .72 to 90 degrees. General purpose steppers have a resolution of 15 or 30 degrees a step.
3) High precision, low torque.

The following figure shows a stepper motor deflected by 90 degrees

This occurs only when one coil is excited and other's are not.