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06-14-2008, 12:10 AM #1
Motor running hot on VFD - normal?
I'm running a Reliance Electric 3/4HP 1725rpm motor, 3ph, 230v, 3.44A. Supply is 1ph230v through an IMO iDrive VFD.
Problem is, if I run the motor at 20hz or less for any amount of time (3+ minutes or so), it gets hot and the VFD's thermal overload trips.
Also, if I run up in the 50-60Hz range, the motor still gets really warm, but isn't tripping.
It's a continuous duty SF1.0 motor in an unvented casing with external fan, operating temp 40 degrees on the tag.
Should this motor be running toasty or is something amiss?
I'm running the VFD in sensorless vector mode with the motor parameters inputted into the drive. 5sec start and brake. Max freq. set to 60Hz and min set to 15Hz.
Any help would be appreciated.
06-14-2008, 12:19 AM #2
Running a fan cooled motor slow also runs the fan slow... makes for a hot motor. How hot is it when it runs at normal 50/60 hz? If you plan on running alot at the slower speeds, adding a large pancake fan and a shroud whould help.
06-14-2008, 12:41 AM #3
Well, the VFD shouldn't over temp!
Do you have a temp sensor installed on the motor?? If not, the VFD would have NO IDEA the motor was getting hot, and the over temp error is the VFD itself overheating! That's not good, sounds like you've got too much load on the motor at low speed, and as the motor gets hot, it draws more juice causing the VFD to overheat too. Try a BIG 120v computer fan on the end of the bell to work in conjunction with the motor's fan. If that still causes the VFD to give an overtemp error, you're working the motor too hard @ low RPMs.
IF you have a temp sensor on the motor hooked up to the VFD, there's a good chance the fan will fix all your problems.
06-14-2008, 01:02 AM #4
Sounds like you might have the base frequency set to 50 Hz instead of 60 Hz. As others have said add a self powered fan if you intend working the motor at low Hz's.
What amps is the motor drawing at 20, 50 and 60Hz? most VFD's will display this if requested. It may just be a case of motor too small for the job.
06-14-2008, 09:45 AM #5
Do you have the V/Hz set right? The VFD sounds like it is supplying too much current at low freq.
Are you certain it is an overtemp trip?
If it were an overcurrent trip that might make more sense. Most VFDs , unless they are put in a small closed-up box, won't overheat at any current etc that they will run at.
At low freq, the motor is not cooled well which explains the heat there. And if it is drawing excess current due to the volts not being properly reduced (V/Hz setting), that could cause an overcurrent trip, as well as heating the motor more than normal.
06-14-2008, 10:39 AM #6
The VFD is set to 60Hz, not 50. There is ZERO load on the motor - at this point I'm just testing the setup before installing it on a lathe. The error I get is OL1. The manual indicates this means "overload protection" caused by either:
1) too much load applied to motor (not applicable in this case obviously)
2) current rating of motor is set incorrectly (for sensorless vector mode) (it is set correctly at 3.4A).
The iDrive itself is not overloading or overheating, that would be code OL2 and also the drive remains nice and cool throughout.
One other thing I should have mentioned, the drive is only tripping when the motor is running in reverse as well.
So these are the factors:
- running in reverse
- below 20Hz
- running for around 5 minutes or so.
And yes, it is a reversible squirrel cage motor.
I don't have a temp sensor to measure the exact temp, but it does heat up much more and much faster running below 30Hz. I've never gotten it to trip to the OL1 code over 20 Hz or so, only at the really low frequencies.
I have the load current set to 3.4 (motor plate rates the motor at 3.44) and the no-load current at 1.7 (50% of load value).
The drive is rated for 1HP but I'm only using a 3/4HP motor, so I doubt the drive is undersized. it has more current and power capacity tha nI am drawing.
Helicalcut: I have the drive set up for a straight curve current increase (e.g. at 60Hz, 100% current, at 30Hz, 50% current) at load. At no load, I imagine it defaults to the no-load current setting (1.7A).
I'm wondering if maybe the drive is supplying a steady 1.7A at all frequencies right now because no load is applied to the motor - could that be it?
06-14-2008, 04:04 PM #7
Hitachi drives log the values when they trip.
The log values of whatever tripped it. I forget how to access it, but other VFDs have this feature too. See if yours has that and if it will tell you the current consumption when it trips. I'd also put the fan on it, and up the max current to 3.5 amps. I'd imagine the reason it's tripping when it's running in reverse, the fan is blowing air the wrong way through the motor. Maybe remove the fan from the shaft of the motor all together, and put a fan on it that blows air through the bell. I dunno if the settings you have for the current/voltage/rpm curve are correct or not, hitachis don't have those options.
06-14-2008, 09:30 PM #8
Well, the fan thing can't be an issue as enclosed 3ph motors don't have fans that blow through the bell housing - the fan is external and just blows air over the enclosed housing. The log says all parameters were normal, just that the motor drew current to overload state.
In any event, I have now decided that the motor is the problem, not the drive. I just swapped in another motor with the same parameters, but this one is new, not a rebuild, and everything immediately began working again. I suspect the other one must have a short in one of the phase windings or something causing all the issues. Also, something was causing friction inside the motor - not sure what. The new motor is running just fine though, so this older Reliance motor is heading to the scrap heap - thanks for all the help guys!
06-14-2008, 09:41 PM #9
[I'm wondering if maybe the drive is supplying a steady 1.7A at all frequencies right now because no load is applied to the motor - could that be it?[/quote]
Good to see you fixed it as for the 1.7A that is the magnetizing current of the motor, it will be pretty much constant up to 60Hz then fall off at higher frequencies. High magnetizing current at low revs was what I was looking for but as your drive was set up for linear F/V that wouldn't have been an issue.
06-15-2008, 01:24 PM #10
"...If not, the VFD would have NO IDEA the motor was getting hot..."
Not necessarily true. Many VFDs take the motor speed into account in their overload algorithms to account for the decrease in fan cooling at low speeds. The AC Tech SMV does this as a default, but it can be disabled when using a motor that is rated for full load at low speed.
06-15-2008, 03:07 PM #11
It could be a bad bearing or even assembled wrong (which is a common cause of overheating and excessive current draw). A motor may look like it can be assembled with either end bell at one end, but that is no guarantee. Check the stator and rotor alignment and see if it isn't off. I have seen many a "rebuilt" motor assembled wrong. Flip the endbells and armature around and all is good.