04 - Physics of a real stepper motor

 

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.

 

 

4.1 - Bi-polar stepper motors

A bi-polar stepper motor looks like this:

 

 

 

As you can see it has two coils and will normally therefore have 4 leads. If you have no documentation for your motor then you can just use an ohmmeter to work out each pair of pins. I.e. 1a and 1b will have a low resistance between them, and 2a and 2b will also have a low resistance. If you measure across two pins and have an infinite resistance then they are from different coils. So once you know which two pins are coil 1 and which are coil 2 then you will still need to find out which are ‘a’ and which are ‘b’ – but more on that later.

 

Bi-polar stepper motor driver

 

To drive the motor then let us consider one of the coils – say coil 1. In order to become an electromagnet we need to be able to change the direction of the current through the coil. So each side of the coil needs to be either or low.

 

If  ‘1a’ is high and ‘1b’ is low then we will have a current through the coil.

If  ‘1a’ is low and ‘1b’ is high then we will have a current through the coil in the opposite direction.

In the other two cases: where they are both high, or both low, then no current flows and so it is no longer a magnet.

 

A suitable circuit to do this as an H-Bridge. As you will see from that link – you need four switches in the H-Bridge to be able to give the four control states we have mentioned above.

 

 

4.2 - Uni-polar stepper motors

A uni-polar stepper motor looks like this:

 

 

 

The only difference is that each of the coils now has 3 wires and will normally there have 6 leads. The new wire on each coil is called a ‘centre tap’ and is connected to the middle of the coil. If you have no documentation on your motor then use your ohmmeter to check the connections. By checking for infinite resistance then you should be able to identify the leads that are for one coil, and the other leads for the other coil. Given a group of 3 leads you can tell which is the centre tap because, for coil 1, the resistance between ‘1a’ and the centre tap will be same as that between the centre tap and ‘1b’. The resistance between ‘1a’ and ‘1b’ will be double this value. NB the resistance tend to be very low, a few ohms, so you will need to select the appropriate resistance scale on your meter.

Uni-polar stepper motor driver

As mentioned earlier: you can drive a uni-polar motor ‘as if’ it was a bi-ploar motor. To do this you just ignore the centre tap and then use the other two leads per coil as if it was a bi-polar.

Otherwise: you want to use the centre tap and, assuming you connect it to ground, then you will need two switches to dictate which direction the current flows through the coil.

 

 

 

Note that this mode of operation means that you are only using half of the coil in each direction. This will mean the ‘half coil’ only has half of the total resistance. Using Volts = Amps x Resistance (and assuming your supply voltage is the same) then if the resistance is halved then the current drain has doubled.

 

So why would you choose uni-polar over bi-polar if it requires twice as much current?

Compared to the bi-polar H-Bridge driver, which requires ‘4 switches; per coil then the uni-polar circuit only needs ‘2 switches’ per coil. So less electronics!

The price you pay is that you may be using twice as much current and because you are using half of the coil at a time then you may only get half the torque. Despite its name (uni versus bi) it sounds as if it is less capable but remember you can always use a uni-polar motor as if it was a bi-polar by ignoring the centre tap. So a uni-polar can be thought of as a bi-polar with extra choices.