Everything You Need to Know to Drive a Stepper Motor (1)

Published:2011/7/26 3:31:00 Author:Amy From:SeekIC

By Horst Hubert

First steps

Stepper motors have found their way into Elekfor Electronics many times, most recently in the November and December 2003 issues. In contrast to previous articles, well keep it practical and won’t go into the theory. If you want to know more about the background, have a look at some previous articles (listed in the reference section at the end of this article).

To get an unknown motor to work you really need to do two things: find out what properties your motor has and choose the right driver electronics. With the help of this article you will find that it’s not that difficult. The fol­lowing three steps will help you dis­cover the most important properties of a motor. The first step helps you determine whether you have a bipolar or unipolar motor.

I. Type of motor

A bipolar motor (Figure 1) has wind­ings whereby the current alternately flows in one direction, then in the opposite direction. This creates a mag­netic field that also changes direction, causing the motor to turn. The winding of this type of motor has two connec­tions. The driver electronics therefore has to continuously reverse the voltage to the winding. This requires a bridge circuit using four transistors.



A unipolar motor has a winding with a centre tap. This winding therefore has three connections. The centre tap has to be connected to the positive supply voltage. If we now connect the start connection to ground (and leave the end connection open), there will be a current flow through the first half of the winding. If we connect the end to ground (and the start is left open), there will be a current flow through the second half of the winding. Since the current in the second half of the winding flows in the opposite direc­tion, the magnetic field will also be reversed. The driver electronics can therefore be simpler, since we only need to connect either the start or the end of the winding to ground. Two transistors are sufficient for this.

A stepper motor has at least two pairs of windings, so 2_2 or 2_3 connections. When a motor has four leads it is almost certainly a bipolar type. And when it has five or six leads it is almost certainly a unipolar type. It’s as simple as that. Should you find a motor with more leads, then you have a problem. There is nothing left but to open up the motor and hope that you can see what the internal wiring is. This is usually not successful. In that case you’ll just have to use a different motor!

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