Episode #40: Types of Servo Motors

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The Motion Control Show

I'm continuing on our topic of different types of electric motors.  Now we're moving on to brushless servo motors.  I'm Corey Foster at Valin Corporation.  Let's see what we can learn.

I looked at this graphic last time and it shows the breakdown of what the motor construction is like.  You can see here the feedback.  A servo motor is pretty much always going to have some sort of feedback.  That is a given.  It's going to have a housing.  There's going to be a motor shaft.  There's going to be some bearings and a permanent magnet rotor here.  It's going to have the stator around here.  This might be what a typical rotor looks like with the different magnets around it in North/South configurations.

Now let's talk about three different typical types and designs of servo motors: slotted, slotless and bridged.  It's not going to be all of them, but is going to cover the different philosophies and design decisions that go into different types of servo motors.

The slotted in this case has the most detent torque and has a high amount of torque.  It has a design with the stators like this, where the teeth are sticking in and the copper is spit in.  Around it, sometimes, that gap is going to be on the outside here with the lamination that keeps it in around it and then this is closed.

Here there's the slotless design, which has absolutely no detent torque.  It has the highest amount of torque.  But, with this stator around here, it's going to have a large rotor on the inside.

Then there's the bridged which has a low amount of detent torque and it has a pretty high amount of torque capability.  Now if you look here, it's a little difficult to see, but if you're looking down this hole right in here, you can see that the teeth of the stator are actually skewed, so they don't go straight down along the barrel of that outside ring.  It goes at an angle.  This is so that the North and South poles never line up perfectly with the rotor, so it always keeps it off balance a little bit.  That reduces the amount of detent torque and kind of creates an offset in the balance there.  But you can see how there's this outer ring with a gap right here and then the tooth lamination on the inside. 

The slotted here is great for low inertial loads.  The slotless is great for high inertia loads because of the inertia matching.  And the bridged is also pretty good for the higher inertial loads…particularly the larger motors, whereas the slotless is better for the smaller motors.

Take a closer look at that slotted motor.  You can see the housing on the outside.  Here's the stator.  Here's the coil.  The magnet would be something like this with these blue shapes on the outside of the rotor with the shaft in the middle.  The slotless motor has what's called a basket winding.  Here's the housing.  Here's that round stator, and then it has what is called a basket winding here, and that's the coil around right here.  This basket winding literally is rolled out with a rolling pin to smash it so it can be formed into place and pushed inside that stator.  Then it has magnets here and will have a rotor and shaft there.  Again, these are very good for very smooth applications.  No cogging.  Good for high inertia.  They’re low inductance though, so the switching frequency of the drive comes into play and then they have a pretty difficult stator construction from a manufacturing standpoint.  It's really why they are really limited to certain sizes of motors.

Then there's the bridge gap, which is good for low cogging and has a high slot fill where the amount of copper that fits in there is really pretty high comparatively.  It's a pretty inexpensive stator.  It has the housing and has this outside ring lamination that keeps the wire inside the slot.  Has the coils here.  Again, the magnet, the rotor and has the shaft.  And then here is the slot opening and that's where that copper is put in there.

These large tooth interfaces of the servo motors allow for the high speeds, accelerations and torques that we know servo motors for.  They do have to be commutated.  See my previous episode for what commutation is using Hall Effect sensors, encoders or resolvers.  And they always have this peak torque versus a continuous torque, where this continuous torque shown here in the green is the amount of torque it can do all day, every day at 100% duty cycle.  But roughly 3 times that, there's this peak torque, and it can only do that intermittently, maybe a few seconds at a time.  Now, that can be adjusted a little bit based upon the heating and cooling properties of the application.  But we could do this for accelerations and then this is maybe the constant velocity or high duty cycle application whereas other motors will have just a 100% duty speed/torque curve.  Remember the brushless servo motors, they have this continuous and peak so we have to size knowing the difference between those.

Here's a kit motor which gives you an idea of what the innards of a servo motor might look like in real life.  These are the magnets that'll be arrayed around an alternating North/South configuration.  Here's the copper winding that is put in through the stator.  And then here's a commutation board, which has Hall Effect signals coming out of it.  This has to be aligned just right to the windings in order to give us the right timing table.  As I was just talking about the Hall Effects, this Hall Effect board here, this Hall Effect is the on/off signals here in the green on this one, and this red is the Back-EMF of a winding.  So there's three windings, U, V and W, and there's three Hall Effects.  This gives an example of how the Hall Effects are to be aligned with those phases in the Back-EMF and if you have this Hall Effect off by 120 degrees, and the drive is expecting them to be aligned, you're going to mess up the commutation of the electronics.  Now if we are using a resolver, just as an example, you don't usually see a timing diagram with the resolver like this, but for example, you can see a resolver has unique positions all the way through it.  Here's the cosine and sine waves of a resolver being the blue and the red, and you can see how that could be lined up with those phases just as well.  If you rotate that resolver you're going to throw that timing signal off.  Likewise, with that commutation board, if you throw that off just a little bit, that timing diagram is off.

So, I hope that helps.  Reach out to us here if you have applications or questions or need some help.  I'm Corey Foster at Valin Corporation.  I hope that helped.

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.