Episode #38: What is Motor Commutation?

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The Motion Control Show

Now that we've talked about what an electric motor is and what a drive is, we're going to go into the more sophisticated topics of feedback and the different types of motors and how to control them.  But before we do that, I want to talk about the more obscure topic of what commutation is.  It's pretty abstract, and a lot of people will tend to have difficulties with it.  So, if you have questions, or if you have any applications you want to discuss, or are having problems, reach out to us at this website or reach out to us at this email address.  I'm Corey Foster at Valin Corporation.  Let's see what we can learn.

The engineering definition is that the goal is to maintain a 90-degree angle between the rotor field and the stator field of the motor.  This yields the optimum torque output.  The further we are away from that 90 degrees, the more unused torque there is.  The more layman's explanation that I prefer to use is like getting on a bike and knowing where to put your force; on the right foot or the left foot, or a combination of the two.  And how do you change that force in your feet as the pedals go around?  Do you push down on your right?  Do you push down on your left, or do you pull up on your left?  That's the basics of commutation.  The definition is really the switching sequences of the drive voltage into the motor phase windings necessary to assure continuous motor operation.  If you look here at these brushes, and there's a break in the wire here, as the winding rotates, the brushes are going to change from one side to the other.  Therefore, the current is going to change directions and it's going to move smoothly with the North and South.  If we look at a brushless motor, there's a commutation sensor here.  You can see where it would be off here and on here.  Therefore, it knows which way to change this current to match where the North is.  As it goes around, it is going to change current so it can attract the right pole of the magnet.

Traditionally, there are two typical methods.  There's the resolver, which if it's used for the feedback, the commutation can be accomplished via that resolver because the resolver has two phases, a sine and a cosine wave.  Through one electrical cycle, it's absolute.  Any slice along here is unique from all the others, so when you turn it on, you look at these phases.  You know exactly where in the rotation you are.  Another one is a Hall effect sensor which goes along with an encoder or a tachometer, if they're used for feedback.

What is the Hall Effect?  It is simply that if you pass current through a thin metal strip and subject it to a magnetic field, a voltage is produced.  So, if you put a sensor by the magnet of the rotor, you know whether it's a North or South pole and how close those are.  Here are some graphics that show that.  However, these days, encoders use Hall Effect signals, but they aren't actually Hall Effect sensors.  What they do is they embed the Hall Effect signals into them, and then they align the encoder to the back of the motor, so the Hall Effect signals are produced at the right rotation of the motor.  That's why you can't take an incremental encoder off and put it right back on if it has the Hall Effects in it, because if you take it off and rotate it and put it back on, those Hall effect signals are no longer aligned properly.  Again, they're not actually Hall Effect sensors, but they create the same signals for the same purpose. 

These days, digital drives can sinusoidally commutate.  They use the Hall Effect signals at the start up to get the right smooth motion and then the drive knows where it's at and so now it's going to go through the phases based upon the feedback because the resolution of the encoder is high enough at that point.  And, of course, absolute encoders have unique positions, so that can be used.  And even if it's only a single-turn absolute encoder, it knows where in the rotation of the motor it is.  So again, they could just turn on and know which phase to put the current into, A or B or C, and move forward properly.  But if you take off that absolute encoder and put it on a different location, now you have to re-zero it out.  That all has to be aligned properly.

It is possible to commutate a motor without any feedback or Hall Effects.  It's called Wake ‘n’ Wiggle and there are other names for it from different manufacturers, but the idea is the same.  If you get onto a bike and you push the pedal forwards and backwards, you can figure out where in the rotation of those pedals, or the rotor, you are based upon that motion.  So, imagine getting on a bike, moving the pedal back and forth a little bit like that.  Now I know where it is, so now I'm going to move forward from there.  You have to be OK with a little bit of motion and taking the time to do that.  But it works fine.  There are some catches with that as far as having too big of a load and all that, but it can work pretty well in a lot of applications. 

So, this raises the obvious question, if you're familiar with steppers that don't typically have feedback, and definitely do not have Hall Effects: do steppers commutate?  Well, they don't have a sensor.  So, what the drive does is it locks the motor into one phase.  It turns on either phase A or phase B and locks it into place.  Because a stepper motor has fifty poles, as opposed to two or four or even eight poles of servo motors, it doesn't have to move very far in order to be locked in.  It might only be a degree or so.  It locks into place and then the drive knows where it's at and it moves forward.  It moves forward basically on a sequence table a lot like if you step forward with your left foot, you know the next thing you need to do is step forward with your right foot.  Left foot, right foot.  If you step forward with your right foot, you know you need to go with your left foot next.  That's what I mean by a sequence table, and that's how a stepper operates, too.  If it puts current into the phase A, now it knows it needs to transition over to phase B.  It's a set sequence. 

I hope that helps.  If you have questions or want to reach out to us.  Here's the website.  Here's the 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.