Minimum Drive Frequency
Minimum Drive Frequency
(OP)
I've just been told that motors run through a VFD shouldn't be run under 20 Hz... No reason provided, just the advice.
I can't think of any good reason except that the cooling drops with the speed. In my case, I'm typically drawing less than 50% of FLA (though in a continuous-duty application with a moderately variable load) and the motor casing doesn't get too warm.
Can anybody validate or debunk that advice, along with justification either way?
At this point, I'm not at liberty to start fiddling with the drive components to change shaft-to-output ratio - I've just got to go with the VFD.
I just don't want to do something stupid and destroy my drive or motor.
I can't think of any good reason except that the cooling drops with the speed. In my case, I'm typically drawing less than 50% of FLA (though in a continuous-duty application with a moderately variable load) and the motor casing doesn't get too warm.
Can anybody validate or debunk that advice, along with justification either way?
At this point, I'm not at liberty to start fiddling with the drive components to change shaft-to-output ratio - I've just got to go with the VFD.
I just don't want to do something stupid and destroy my drive or motor.
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RE: Minimum Drive Frequency
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com - kcress@<solve this puzzle>
RE: Minimum Drive Frequency
With some loads the torque requirement reduces along with speed, and you can sometimes get away with it. If full rated torque is still required at extremely low speed, external independent forced air cooling will probably be required.
RE: Minimum Drive Frequency
There used to be a problem - and still is with some drives - when running at low speeds. The first/is torque. It simply vanished at low speeds in the earlier (non-vector) drives. So, the recommendation may be from those days. With a rather thick margin added.
The second problem at low speeds was vibrations due to voltage being applied in chunks rather that PWM. That made the motor rather jerky.
I think that you can forget about that today. But keep an eyay on the temperature. Modern drives usually go below 5 Hz with ease.
Gunnar Englund
www.gke.org
RE: Minimum Drive Frequency
RE: Minimum Drive Frequency
The comments above about resonance are interesting for one of the system characteristics that has "chased" me down to this speed is trying to cope with feedback vibration at a slightly higher speed (~23 Hz with a 50 Hz motor).
Perhaps the root cause of the whole vibration problem is just what you're talking about - trying to run the motor too slow in an application with a highly variable load (variable in both frequency and magnitude).
I know this is taking the thread in a new direction but would you expect the vibration to diminish at a higher motor speed under the same loading conditions?
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RE: Minimum Drive Frequency
If force has a magnetic source then the magnitude will be dependant on the machine flux which is probably constant with speed.
RE: Minimum Drive Frequency
yours
RE: Minimum Drive Frequency
RE: Minimum Drive Frequency
One problem is that if you maintain the volts/hertz ratio the voltage becomes very low at low hertz. With a 240V/60hz motor, 4:1 ratio, the voltage would be 20 volts at 5 hertz. The drives boost voltage and do other black magic stuff at low hertz to counter this. The better the black magic the slower you can run at load.
Barry1961
RE: Minimum Drive Frequency
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Minimum Drive Frequency
I have had bearing failures du to high rotor temperature. An effective way to detect such problems is to check the shaft temperature. Try to keep it below 90 C. The grease starts to drip out of the bearings above that - if you do not have HT grease.
Gunnar Englund
www.gke.org
RE: Minimum Drive Frequency
Your point about rotor heating raises my curiousity a bit. The motor manufacturer (Marathon) doesn't offer much in a way of caution about rotor heating. Am I missing something here?
RE: Minimum Drive Frequency
A totally enclosed fan cooled motor is essentially sealed, and there is no obvious direct path available for rotor cooling. Internal rotor windage, and internal radiation are the only cooling modes that I can see. If approximately half the motor losses occur due to rotor slip, how does the heat normally escape from the rotor?
It must surely mostly be internal windage and subsequent transfer to the outer casing, or am I missing something ?
RE: Minimum Drive Frequency
Q = rhoT4
Q = heat transfer
rho = Stephan-Boltzman constant
T = Absolute temp Rankin (F + 460)
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>
RE: Minimum Drive Frequency
But if windage is a significant cooling contributor too, any large speed reduction could spell trouble. Fascinating.
RE: Minimum Drive Frequency
DickDV, does Marathon say that you can run that motor like that? Just because you did and got away with it doesn't mean it was designed for that. In any application, the load is what determines heating anyway. If, as Warpspeed said early on, there is little load at that low speed, then cooling may not be an issue at all.
Anyway, go ahead and run your TENV motors at zero speed and no fan cooling. If it works, great. If not, it will end up being a lot more expensive than that small fan. I just recommend erring on the side of caution if it's important to operate at low speeds like that for prolonged times.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376