View Full Version : Inverter problem

26-09-2011, 07:06 PM
Hi guys,
I`ve got a problem with a 150kw motor that`s been retrofitted with an inverter. beofre the inverter was fitted it ran ok (compressor has run for 50,000 hours) but on start up, after it was fitted, the motor burnt out. The company that carried out the work has done dozens of these installations all with no problem and carried out full comissoning eg power factor, speed, kws etc. I`ve heard from our motor rewinder that some inverters can draw higher current when running at low speed so the cooling fan will not be able to cool the windings properly. Given that the motor has high hours could it be that the windings were "on the edge" and the inverter just pushed it over. The last thing we want is it to go tits up again.

Cheers guys,

26-09-2011, 07:26 PM
But did they check with the motor manufacturer to ensure it was inverter compatable?Frequency modulation can result in higher current draw but the inverter set up should sense this and cut the motor out. A properly tuned inverter should protect the motor.Im sure opinions will vary on this one.CheersStu

Yuri B.
26-09-2011, 07:30 PM
On low speed it will draw less power, less of it will have the need to be dissipated.
If you said the motor ran for a while, I would more suspected the bearings. But, shortly after the start ....
Probably, still, an improper parametrisation.

install monkey
26-09-2011, 09:20 PM
maybe the ramp time was too slow so the motor almost stalls on start up causing the windings to cook,at low freq the inverter may not detect locked rotor as its not at full frequency thus continues trying to ramp up, is there an isolator between inverter and motor?? check the poles have continuity when on.

27-09-2011, 01:24 AM
The motor thermistors if fitted and connected should have protected motor from slowly cooking itself, they would not protect a short to earth failure.
Had similar problem on new 100 Kw motor installed with new VSD. Kept tripping on over current, and the motor was getting really hot at low speed. I resolved by changing motor input settings and telling VSD motor voltage was 460vac., originally set at 420vac. The VSD fires more voltage at motor at low speeds, and drops current draw

27-09-2011, 04:45 PM
I go with magoo ,had a same problem .
low speed will indeed cause this problem ,most manuals that people dont usually read ,warn users from this problem and how to avoid it !!
good luck

27-09-2011, 07:20 PM
Cheers guys for the rapid responses, i`ll pass on the suggestions to the inverter installer when he`s on site when we start the motor up. I`ll let you know the outcome.


28-09-2011, 03:52 AM
Hi Glenn.
can you describe the original motor failure, as in. Were all coils cooked, was one set of coils cooked, or was there a sudden failure in the coil slots, was the motor drip proof or totally enclosed, did the motor have thermistors.

17-11-2011, 10:13 PM
Hi guys,
A follow up to the motor problem. I wasn`t involved in the refitting of the motor (my boss has a habit of sending different guys back to finnish a job) so I`m only going by what I`ve heard. The rewinders used a heavier insulation on the windings and the inverter installation engineer went in deep on the settings and made a few adjustments. The motor`s been running for two weeks now and all`s well.
Thanks for all the help guys.

17-11-2011, 11:04 PM
Thanks for the update Glen, always useful.

17-11-2011, 11:24 PM
Hi Glenn,
thanks for the follow up. See if you find the settings the tech changed on the drive.