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Episode #44: Servo Motor Speed & Torque Curves Explained

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The Motion Control Show

Now that we've talked about different types of electric motors, we're going to dive deeper into servo motors specifically and how to understand their speed/torque curves.  Over the next few episodes, we’re going to go into this in depth, and we're going to be talking about specific performance problems that people may have that helps to understand how they operate.  I'm Corey Foster at Valin Corporation.  I hope this helps.  Reach out to us to this email address and website here if you want some help and want to talk about an application.

First, the performance questions that we eventually are going to answer are:
• Why is my motor overheating?
• Why don't I get as much speed as I should?
• Why don't I get as much torque as I should?
• Why did my motor burn up when it's only operating in the continuous region of the curve?
• How can I check my motor’s temperature?

But, we need to understand the basics, so let's look at a few speed/torque curves, for example.  Here is one.  You look here and this curve here is what's called the continuous region.  And this is called a peak region.  If we look at another one, this one has two sets of regions.  That's because it operates at different voltages, let's say at 110 VAC it might get here.  At 240 VAC, it's going to get twice as much speed here.  This one shows a lot more detail.  You can see different voltages here.  You get different speed capabilities based on the bus voltage.  These different thermal curves are all based upon the thermal assumptions, the heat sinks and the air flow.  We're going to be talking more about this and why that is.  Occasionally, you see this one is flipped around and backwards, occasionally you see ones where the speed is up here on this side, and the torque is across the bottom.  I don't see those very often.  Every once in a while, it kind of throws me for a loop just because the shape is a little bit different, but just keep an eye out for that.

Let's go back to this one.  Here's the continuous, like I said, at this voltage.  This is the continuous; this is what it can do all day, every day without having to worry about duty cycle.  The further we go above this line here we get into this peak region and our duty cycle is limited.  Down here we might be able to do it for minutes at a time, but up here we can only do it for seconds at a time lest the motor gets too hot.  But if we're running with a higher bus voltage, we have even more continuous here and more peak here.  The current that is available affects how much torque capability we have here, how much current the drive can put out.  And how much bus voltage the drive has, that's the voltage potential that is inside the drive in the PWM switching frequency.  Go back to a previous episode to understand that.  That bus voltage capability is what defines this here.  Again, at this higher voltage we get more continuous and more peak there.  If we take a look at this speed torque curve here that is far more detailed, you can see the different voltage curves that we get, so different voltages give us different speed capabilities.  This is maybe 200 VAC.  Maybe this is 208, 230, 240, something like that.  If this is 240 here, then there's going to be a 120 curve here.  Now remember the way the drive works, 120 VAC is rectified to 170 VDC.  So 240 VAC gets rectified to 340 VDC.  These curves don't necessarily equate the AC power that you're looking at.  You need to rectify it up to the DC voltage.

Then we look at these curves here.  I made a comment a moment ago about the current affecting them.  Yes, the current does limit how much your torque can be, but it also is thermal limitations.  How much torque you can get continuously is all about making sure the motor doesn't overheat.  A motor with a good heat sink can run continuously longer than a motor can that doesn't have a good heat sink.  A motor that is air cooled can run longer than one that is not air cooled.  One that's water cooled could even run longer than one that is air cooled, so it's all about heat dissipation, getting that heat out of the motor.  And so that is why the thermal characteristics affect here.  This particular curve looks at the different rise in temperature in the different heat sinks, and the different assumptions that are made.  This one right here at this assumption is a pretty limited continuous duty cycle whereas you get a longer, bigger duty cycle up here.  But, then above this one, it's all peak.  We really have to keep in mind what the application is with the motor.  If the motor’s in a box where there's no airflow at all, it’s not going to get nearly as much heat dissipation as one that is out in the open with the fan blowing air across it.

I'm Corey Foster at Valin Corporation.  I hope that helps.  Reach out to us here at this website and email address and see if we can help you out.

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.
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