04 - Physics of a real stepper motor

Submitted by Webbot on June 10, 2008 - 10:47pm.


So lets now leave our imaginary world and look at how this extends to real stepper motors.

Our rotating ‘Bar A’ only had two poles – one at each end of the bar – and so we only got two steps per revolution. A real stepper motor may have, for example, 48 steps per revolution and each step would therefore move through 360/48 or 7.5 degrees per step. How does this happen? Well, rather than it being a bar, then think of it as a cylinder where every 7.5 degrees the polarity is either North or South but it always alternates from one to the other. That’s why stepper motors always have an ‘even’ number of steps. Here’s what it may look like if we look down on it and there are only 8 steps:-



Now instead of one Bar B we actually have two. Each of these two electro-magnets will have a coil around them. To complicate things slightly there are two different ways that these coils are configured: bi-polar, and ‘uni-polar’.

We will look at Bi-Polar first because it is the simplest to understand whereas a Uni-Polar motor has other advantages and disadvantages but can, when required, be used as if it was a bi-polar motor. We will find out why later.