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- Feb 25, 2021
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Measuring heat, or letting the VFD do that
Ok, so lots of (useless?) info on heat, heat, heat, heat. Yeah, a lot of this translates to "makes things hot. Or at least warm".
A very reasonable approach to this is try it and see if your motor is getting hot. If you can keep your hand on the motor indefinitely, it's not too warm. If you can't, you might try a thermometer. The IR thermometers are great for this type of thing. How hot is too hot? Well, how long do you want the thing to last? "Forever" isn't an option. Motors are generally designed to work in an environment up to 40 degrees C (104 F). That's the air temperature coming in to the motor to cool it. If you're below that you haven't even started to warm it up
To test heating, you need to really give it a chance to heat up. The temperature rise in the windings and laminations need to conduct outward, so one quick test cut and checking heat isn't going to work. You need to run the same test load for 5-10 minutes a least, longer would be better. If that test load is threading, that may mean in and out of a cut, but you need to repeat the in and out for a while. Or have a long heavy cut at a slow feed, or ....
I'd worry if my motor was over about 150 degrees on the surface. The internal windings and poles will be a lot hotter. NEMA insulation specifies anywhere from 60 to 125 degrees C allowable temperature rise. But that in the windings, not the motor surface. You can look into NEMA motor testing standards if you want to get precise, you just need a very accurate ohmmeter, and disconnect the wiring to do the tests.
Classic industrial motor configurations involved using a larger contactor (or two to enable reversing), plus a "heater", sort of a slow trip circuit breaker, that opens up the contactor if the motor is overloaded (drawing too much current). This was an attempt to guess how much load the motor could handle, and cut it off before it self-destructed from overheating.
Good VFD's replace the need for the a forward/reverse contactor and the "heater". They attempt to *guess" how much heat a motor is experiencing, based on load, and operating frequency, and some factors you enter when you set up the VFD. One parameter you might find is whether the motor is inverter rated. Again, this allows the VFD to guess when the motor is getting overloaded. Just like the heater was a guess.
This is also a good place to bring up "service factor". There is an implied service factor of 1.0 for a motor that doesn't have a service factor specification. 1.0 means the motor manufacturer says it is safe to operate the motor at 1.0 times its rated horsepower, no more. A service factor of 1.15 means the motor can be operated at up to 1.15 times its rated horsepower (i.e., 11.5 HP for a 10 HP motor), for a short period of time, as long as the average is still at or below 10HP. NEMA specs for service factor warn that operating at over rated HP, even on one approved for a service factor greater than 1, may reduce the life of the motor. Your VFD may have a parameter for service factor, or may just specify an allowable overload factor. Common wisdom is to set that overload factor to around 1.5, which probably works fine in most cases. But setting it more than the service factor "may void your warrantee", lol.
Another thing that is notable. A lot of inverter duty motors have the option of coming with a installed thermocouple that will open (or close) if the motor is getting too hot. Basically a built in thermometer calibrated to what the motor manufacturer says is a reasonable max temp. If I had that on a motor, I'd definitely run it to a panel light that was pretty obvious. You could also run it to the VFD, as a lot of VFD's have the capability to react to motor overheat. But you may not want your VFD shutting your machine off in the middle of a finishing cut.