What is an AC Servo Motor?

An AC Servo motor takes the smooth the three-phase stator control of an AC asynchronous induction motor and the permanent magnet rotor from the brushless DC synchronous motor to create a high-performance motor that is ideal for smooth, accurate and precise positioning with excellent torque and speed characteristics. But how does it actually work? Let’s take a look … On this rotor from an older servo motor the magnets are potted in some kind of plastic. If I hold this magnetic paper up to it, yep we see the magnets and it looks like there are 8 of them! Here’s a disassembled SureServo2 Motor. On this rotor we can actually see the permanent magnets attached to the shaft under this webbing – using webbing instead of all of that plastic helps keep the inertia low which makes the motor more responsive. And of course, we can also see the magnets using the magnetic paper. And there are 10 of them on this shaft. More magnets equal’s smoother operation. Another reason the SureServo2 system performs so well. It’s important to understand that the polarities flip with each magnet. I have a small neodymium magnet on the end of this aluminum rod. It’s attracted to this guy, and then it skips one and goes directly to the next. Why is that? Well, if this is our rectangular magnet on the rotor, the magnetic field radiates out like this and wraps around to the backside of the magnet. We can see that if I put a rectangular magnet under this dish and drop some ferro fluid on it. All the little spikes are showing us the directionality of the magnetic flux coming off the magnet and the number of spikes shows the intensity. If I pull the magnet away the spikes become more distributed or less intense. Bring the magnet close and the spikes get more intense. If I flip the magnet 90 degrees, we can see the flux wrapping around the other side of the magnet. This is big because the rectangular magnets are thick. The magnets on the rotor need to be thin to keep the rotor inertia as low as possible which means this magnetic flux would be real thin too. If I hold a thin magnet up to the ferro fluid, yep we see a much smaller amount of magnetic flux. So, the magnets on the rotor are in this orientation which has flux lines going out in all directions. And that explains why the magnets on the rotor alternate. I have two rectangular magnets with a spacer between them to simulate the space between magnets on the rotor. Watch what happens when I place those under some ferro fluid. Bingo. We get an enormous magnetic field BETWEEN the two magnets. Again, instead of little spikes, we are seeing the spikes connect and go from one magnet to the next. So, it’s really the magnetic flux between the two magnets that the stator will be moving around, not the magnets themselves. If we put the rotor under some ferro fluid – yep we can actually see the magnetic flux between the magnets rotating around! I can’t rotate it smoothly because the magnets are grabbing and holding onto the ferrofluid. How cool is it that we can actually see the magnetic fields rotating with the rotor? By the way this is the ferro fluid I’m using. There are lots of YouTube videos showing you how to make your own ferrofluid using printer toner or powdered ferrous oxide and oil, but I’ve tried all of them and none of them work as well as the real thing. This is NOT an AutomationDirect product so PLEASE don’t call support asking about it – they couldn’t talk about it if they wanted to. It’s really fun to play with especially with big magnets, but it’s incredibly messy, so please be careful. OK, that worked really well on the rotor … can we do the same thing with the stator? Sure! But before we do that, we need to understand how the stator is configured. This is the stator that was around the rotor we were just playing with. It has the three U, V, W wires plus chassis ground coming out. That’s wired like this. The drives whole purpose in life to is adjust the currents in these three windings to create just the right magnetic field in the stator. And of course, this has to be a zero-sum game? That is, if the drive forces three quarters of and amp in here and another quarter amp in here, it better take one amp out of here. The three of these, always have to sum to zero. The magic happens in how those are wound around the stator. Notice that the stator has 12 poles to wrap wire around. It’s hard to see here because it’s all sealed, but that looks like this where the wires get wrapped around these guys. But why are there 12 of these? Seems like we should only need 6 – a north and south pole for each wire. Well, remember on the rotor how it was the side by side magnets that created the big magnetic field for us? The stator uses the same trick. If one pole is wound in a clockwise direction using the U wire, the one next to it is wound counterclockwise with the same wire to create that coupling of the magnetic fields. That gives us that side by side magnetic effect. Let’s label this pole U and U bar and this one little u bar and little u indicating it is reverse wound. Likewise V and W are done the same way. So now we will get a strong magnetic flux between each of these poles on the stator. And by injecting more or less current into each winding the drive can very accurately control the amount of magnetic flux between all of these poles. So for example, what we would expect to see is a really strong flux here. Smaller here and smaller here and the inverse of that on the other side to create a super strong magnetic field to position the rotor. Let’s see if the stators magnetic field can be viewed using our ferro fluid trick. I added these lines show where the gap is between each of the stators windings. We’ll drop a spacer in to hold this 3D printed plastic cup at the right level. And we’ll put some ferro fluid in the cup. This rotor, with the encoder attached, is plugged into the encoder port of the SureServo2 drive and this stator plugged into the motor UVW port on the drive. And if you are thinking this is going to confuse the drive … yeah, he drive is NOT going to understand what’s going on. It expects to apply a certain amount of current into the stator and get a specific response from the encoder reacting to that. When it doesn’t see it, it’s not going to be a happy camper. So we’re gonna fake it out. To do that we are going to watch several things using the SureServo2 Pro software that you can download from AutomatioNDirect.com for free. I did a factory reset on the drive, connected to the drive and read the drive’s parameters into the workspace. Here we have the scope monitoring the motor current and I’ve adjusted the scale to around plus and minus 3 amps. We also have the Jog dialog where we can tell the drive to enable the servo and tell it to move the rotor position. I have that set to jog at 2 RPM so things don’t change too rapidly. We also have the alarm dialog in case we need to reset any faults when we confuse the drive. And we’ll want to see the stator and rotor. Here we go … Run the scope … Enable the servo … and … nothing. The ferro fluid didn’t do anything. What happened? Well, when you first power up the drive, it assumes that where ever the rotor is, is where you want to be, so it does nothing. We can see the total motor current is zero right now. If I tell the drive to move the rotor by pressing the JOG button, then look what happens. The total motor current increases and the drive builds up a magnetic field in the stator to try and move the rotor which isn’t actually there. And where is the magnetic field? BETWEEN the two windings – just like we saw with the rotor magnets. And look at the shape of the magnetic field. It looks kind of sinusoidal with the opposite pole mirroring the other one. Exactly what we would expect to see. That magnetic field is staying there because there is no rotor in the stator to react to the magnetic fields and move the encoder. The drive is going to keep pushing until it gets what it wants or faults out. If I press JOG to increase the current even more then eventually the drive faults out. The current drops to zero and the magnetic fields go away. And the drive gave us an over current fault. Ok, lets reset the fault .. and JOG a bit to re-establish the magnetic field. If I gently rotate the rotor - really the encoder - in the direction the drive wants it to go – its really touchy so I’ll finish if off by jogging to zero amps - we see the current reduce and the magnetic field drop. Perfect. If I jog in the other direction, we get the same magnetic fields but opposite polarity to move the rotor in the other direction. It’s a shame we can’t see the direction of the fields with the ferrofluid. And I’ll rotate the rotor in that direction to bring it back to zero current and I’ll finish by jogging to a positive current. Ok, now I’m going to attempt to hold the jog button down and rotate the rotor at the same time to simulate what the drive would normally be doing. As I do that you can actually see the stator magnetic field rotating around. How cool is that? And that’s the beauty of the three phase AC input to the stator – the drive can smoothly move the rotor to any position simply by adjusting the currents in each of the three phases to create whatever magnetic flux it needs here on the stator to move the rotor where ever it wants. We call that synchronous because the rotor is always going to follow the stator’s magnetic flux exactly. Unlike the Asynchronous AC Induction motor where the rotor always lags the stators flux. I say it “simply” adjusts the currents but realize that requires a phenomenal amount of processing – basically all the stuff in the current loop block. And all of that is helped by the high bandwidth and extreme resolution of the SureServo2 system. All of which explains why the SureServo2 system works so well. Hopefully this gives you a little better understanding of how an AC Servo motor works and why AC Synchronous motors are what you need when creating a fast, accurate and precise positioning system. And you’ll have a tough time finding a servo system that can beat the SureServo2 systems price and performance. From … AutomatinDirect.com. Click here to see all the SureServo2 Tutorial videos. Click here to subscribe to our YouTube channel so you will be notified when we publish new videos like this and click here to learn about AutomationDirect’s free award-winning support options.