Episode #41: Types of Stepper Motors

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The Motion Control Show

Let's continue on in talking about different types of electric motors.  In industrial automation, stepper motors are very common, so let's talk about those.  I'm Corey Foster of Valin Corporation.  Let's see what we can learn.

First, let's start with a couple of different qualities of stepper motors.  But I'm going to start by differentiating between the motor and drive.  What we're talking about today is the motor.  Some of the things you may think of as stepper motor qualities are actually because of the drive because of the electronics driving and using those stepper motors.  Quick one: I'm going to be talking about stepper motors being open loop.  Well, you can use them in a closed loop application, but that's because of the drive.  Right now, we're talking about the motors.  They are highly repeatable.  They're naturally stable in position.  They don't dither like servo motors.  They have constant current.  Again, some drives can cut that current back, but the motors are designed to be for constant current.  And they do have natural resonant frequencies.  Like I said a moment ago, they're open loop.  They do not inherently need feedback to close their loop.  And, typically, they're pretty darn inexpensive.

The type of stepper motor that we are talking about is actually a hybrid motor, but that term really started way back in the ‘60s and that has become so common that we don't even talk about their being hybrid anymore.  There are a few different types of stepper motors that are older than that, which is where the hybrid came from.  We're not even going to bother talking about those because these stepper motors are so common and have been for 60 years now, so we're going to continue on with what this is.  Here's the geometry and a cross section.  You have the stator here, where the copper goes through these gaps here.  This is a lamination, very thin steel laminations that are glued together in stacks.  And then here's the rotor with the magnets in the middle and you have the teeth and the air gap separating them.  So here are multiple stacks.  Here are the teeth of the rotor.  The stator goes around here and then, of course, we've got the shaft and bearing.  So that's the geometry and physical construction of a motor, but that motor can be used in a couple different ways.  There are a couple of variations on it.

The first one and the oldest, as far as I know, isn't really used anymore, is the unipolar.  Here are the electronics driving it where the power comes in from the top with the ground at the bottom and if one of these transistors is turned on, this current is going to go through the winding of the motor here, down and out.  There's a center tap right here so that when this transistor is turned off and this one is turned on here, the current is going to go the other way.  This is one winding.  This is the other set of windings.  That's unipolar.  It's only using half of the windings at a time.

These days the bipolar, or two-phase, is far more common.  If this is one winding of a motor, and this transistor here is turned on, and this transistor here is turned on, the current is going to go down through the winding here to the end.  But then when those are turned off, and the opposite transistors are turned on, the current goes the other way in order to change directions.  So that's bipolar.

Then there's one other common type, which is 5-phase.  Here are the five windings in a motor, the five phases.  The current may be controlled up through this phase and then out through the other side and out and then maybe inside here and out.  So, it's controlled in different combinations.  Each of them has their pluses or minuses but it all really depends upon the system of the drives and motors.  Those are topics for later.  Right now, we're just focusing on the stepper motor constructions and designs themselves and how they're controlled a little bit.

Here is a simplified stepper, two phase version, where if the current is put into Phase 1 here and here, and you can see that they are wound in the opposite directions, this is North, this is South.   Because this is the North pole, this South pole of the rotor is going to be attracted to it.   If you multiply that by 50 times, you get 50 poles around the outside.  And the way that it is controlled is that the drive injects current into the winding.  Again, this is the North Pole.  It attracts the South Pole and it lines up.  As the current is moved over into another winding, so now we have two windings with current going through them, this North pole attracts the South, but this South pole also attracts the North, so now it puts it in between a little bit.  Going a little bit further, if we turn off the current into this phase, now this pole is simply attracting this North pole and it's moved a little bit further.  This is relatively a small amount, but it's even smaller when you have that 50 poles instead of just the couple that are shown here. 

Full stepping: a 50-pole motor takes 4 steps in order to get through one whole pole so that 50 is actually multiplied by 4, so it's called a 200-step motor.  You divide the 360 degrees of a rotation by 200 steps and you get 1.8 degrees.  That's why two-phase motors are often times referred to as 1.8-degree motors.  Let's take a look at this.  If you have two phases and you turn on both phases, you’re at one step here.  You turn off one phase, but leave the other one on, that's a second step.  Now you turn off the first phase, so now both phases are off, that's a third.  And then you turn this one back on, that's a fourth.  The next step here is going to be this phase on, so that's four steps just to go through that first little electrical cycle.  So that's full stepping.

Half stepping takes it a little bit further, where a phase is turned halfway on, and then all the way on, and then halfway off, and then off.  And if you alternate that between the two phases, you're going to get eight steps.  So now half-stepping gives us 400 steps per revolution.  Take that a step further to mini-stepping.  Let's say instead of 400 steps we divided into 2000 and each one of those steps is just a little bit smaller.  We divide that even further and we get into micro-stepping, and that's been around for decades now as well.  Both phases are partially energized, and they line up in various intermediate positions between the poles and that helps to manage some of the resonance points and helps you to get in between to the smaller steps.  But you can't necessarily position to each tiny little step because of the magnets in the polls that are fighting each other, so you don't necessarily get the repeatability and accuracy of the full resolution.  But that's a topic for another day.

That is stepper motors, in their basics, as far as talking about what types of electric motors there are.  If you have any questions, reach out to us here at this website and this email address.  I'm Corey Foster at Valin Corporation.  I hope that helps.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.