Episode #46: Types of Thermal Protection For Motors

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The Motion Control Show

The next step to understanding and solving performance problems that you may be having, like not getting enough torque or speed or burning up motors, is to understand the various thermal protection models that your drive and motor may have.  So, let's take a look at a few of those.  I'm Corey Foster at Valin Corporation.  If you need any help, or have any questions, reach out to us here at this email address and website.  I hope this helps.

The three that we're going to talk about are the thermal switch, the thermistor and the thermal model.  There may be some others out there, but these are the three most common that I have seen.  Here's the motor on the left and the drive on the right.  The windings are the pink here.  This is the copper.  This is where the heat comes from because that's where the current is going through those windings.  There will be a thermal switch here, right about here maybe.  There's not enough room to put it into the windings, but you could put it kind of right on the outside of them.  It sends a signal back to the drive.   When it gets hot, it goes from off to on at a set temperature; it goes from a low temperature to high temperature.  Then the drive knows that it’s gotten too hot.  The downside of this is that this thermal switch can't be inside the winding.  It takes some time for those windings to heat up, and for that heat to move out here to this switch.  So, the switch is going to lag a little bit behind the windings’ actually getting to too high of a temperature.  You just kind of have to be aware of that.  That's one of the gotchas.  There's usually a little bit of a difference between what the windings can get to and what the switch is going to trip at.

The thermistor is very similar and how it sits against the windings right there.  But instead of being a digital on/off, it’s an analog value.  The resistance changes over time and the drive can interpret that for the actual temperature.  As it goes from low to high, the drive picks some particular temperature for it to suddenly throw the error that it's too hot.  The downside here, still like it was with the switch, is the thermistor is still not being inside the windings.  So, there is some time lag for the heat to transfer from the windings to the thermistor itself.

Another way that the motor can be protected is a thermal model inside the drive.  The drive is putting the power into the motor.  It can keep track of that and over time it knows how much power it's putting into the motor.  So, it can calculate.  It can keep track of that power going in and knows about how much that's going to increase the temperature based on the specs of the motor.  It will get to a certain point where it's going to calculate it and say, oh hey, it's too hot now.  The downside here is that the drive has to start at a known temperature.  For drives like this, you have to put in what the ambient temperature is.  What's the temperature that the motor is starting at?  Maybe it's 20C, which is room temperature, but what if you put it into a hot environment, you know, 40C?  Well, you have to tell the drive that.  Otherwise it's making a false assumption.  Also, the problem with this is that, let's say the motor gets hot because it's working hard and it's only at 50C, if you cycle the power on the drive, it's lost track that it's calculated that the motor is at 50C.  So, it starts back over at the ambient temperature, maybe 20.  If you do that a couple of times, the motor temperature is going to increase, but the drive doesn't know it because each time you reset the drive it starts back at that known value.

How can you check your motor’s temperature?  You can use an infrared gun, for example, or a thermocouple externally.  Now you're going to be looking at the casing and, as I talked about before, there are these thermal resistances, the time lag for the heat to transfer from the windings inside to the outside.  If you're only looking at the outside of the motor, you have to realize that the inside of the motor is hotter.  And if the motor is working hard, the temperature hasn't increased very much yet on the outside of the motor.  So, you kind of have to keep track of it over time to see how it's increasing.  Through a lot of experience, I found that if I can put my hand on a motor and hold it there, the motor hasn't gotten too hot yet, at least not externally.  If I can feel it getting hotter and hotter, clearly there's a heat increase.  But if I put my hand on a motor and pull it away really quickly, and know I'm getting burned, that means the motor is way too hot inside, that's for sure, and the motor is probably in danger of being burned up.  But, if I can put my hand on the motor and hold it, even though it may be uncomfortable, but it's not burning me and I can hold it there, then I can actually know that the motor inside is probably okay over time as long as its not getting hotter and hotter as I do that.

I hope that helps.  This is going to play into our next episode and talking about performance problems.  If you have any questions or have any problems, reach out to us here at this email address and website.  I am Corey Foster at Valin Corporation.  I hope this 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.