View Full Version : Current derating on VFD

02-10-2010, 01:51 PM
I am looking for a VFD for a 1.5KW/2HP 220V phase 3 motor. I am at an altitude of about 1400 metres. Max current draw by my math, 1,492 watts / 220 = 6.78amps.

I've seen a derating factor for output current for 1000 to 1500 metres at 0.97 which would make the derated output current for my motor 6.78 / 0.97 = 6.98 amps.

So, as long as my VFD rated output (at 1000 metres or less) is 7 amps or better I should be alright? Am I way off base here?

Seems like the margin for error is a bit tight here - would it be better to go to a 3HP VFD which would put the derated current output closer to 9.9amps?

02-10-2010, 02:19 PM
The reason for the derating is the ability for the heatsink to be cooled efficiently, if the temperature does not go that high where you are, it may not be an issue.
Also depends of whether you expect to push it to the current limits on a frequent basis for long periods.
Otherwise I would not consider it an issue maybe.

02-10-2010, 09:22 PM
Thanks Al - Summer temps July and August are typically mid to upper 30's with occasional +40 days - machine is going in my garage which does not have air conditioning.

Will definitely be looking at some long and large sized bores - I fully expect to be drawing max current quite regularly.

May need to add a fan based cooling solution as well?

02-10-2010, 11:10 PM
Anything that keeps the heat sink cool will help.
Most have a thermal sensor on the heat sink that will shut it down in the event of O.H.

02-11-2010, 01:56 PM
Alright, good to know about the thermal sensor.

What about HP? Think it makes sense to increase to 3 HP anyways?


02-11-2010, 04:26 PM
I have a couple of 2hp spindles that are run by a 2hp VFD on 1 phase with no problem, but I have a gear reduction when using low rpm, 1:1 otherwise, but I do not really push it to the max.
If doing heavy work then it maybe a good idea.

02-11-2010, 05:11 PM
Interesting solution with gear reduction - did you do this for torque reasons? What gear reducer are you using and what are the final ratios? Can you run full torque at the lowest RPM with it? Pardon my questions but now my curiosity is getting the best of me, LOL!

Which brings another question to mind whether constant torque VFD's really are constant torque at low frequency/rpm.

02-11-2010, 05:18 PM
They happen to be on a couple of Excello knee mills and the gear box is built in ( 6:1 on low).
There are constant torque down to low rpm, many use a P.G. or encoder on the motor for this.
WEG has come out with a VFD Vector motor combo they guarantee 100% rated torque down to 1 rpm, no fan on the motor.