Renewable Energy Questions/Discussion > Renewable Energy Q&A
Max winding temp
Pete:
Hi LH, I am used to using 50 hz here so a 2 pole motor would have a synchronous speed of 3000 rpm.
In your case it would have a synchronous speed of 3600 rpm. There has to be some slip in an induction motor to produce torque so yep 3585 would be about right.
Most motors I worked on had about 50 rpm slippage. So a 2 pole motor would run about 2950 on 50 hz, a 4 pole motor 1450rpm.
Formula is N ( speed) = 120 x Frequency/Poles.
If you are using a variable frequency drive and slowing the motor down it will not cool as well as it should.
What revolutions is it running at when on 400 volts?
It may be worth adding an external fan to increase cooling if it is running too hot.
The only reason I can think that the motor winder would put thinner wire in is if he did not have the guage he needed on hand.
Big motors like that have many wires in parallel. Usually one measures the guage of each wire, calculates how many circular mils that adds up to and replaces it with the same area of wire. Different guages can be used in parallel as long as the area of the winding is the same.
Some motors ( Siemens) is one example are very tight to rewind. They pack a massive amount of wire into the slots are are a mongrel to rewind.
Pete
lighthunter:
Hi Pete :)
Ive not made much progress as yet but am hopeful the mfr. of compressor will come through soon. I emailed them about coming onsite to make the voltage change hoping they will go for that rather than giving out access to their software.
What revolutions is it running at when on 400 volts?"
At 400V 60Hz is the output frequency, as for the RPM , i dont know yet. I will try with a tach to get that number. Hopefully i can estimate load or how underpowered it is by the slip.
"may be worth adding an external fan to increase cooling"
I wish i could... fact is i dont think i could improve on it. It uses the entire enclosure for the aftercooler radiator airflow which is ducted from outside and has a centrifugal fan of something like 10HP. There is so much air flowing around that motor its crazy, i dont think i could improve on that. It literally takes effort to shut a cabinet door while on there is so much air pressure from fan.
"Siemens" you nailed it, thats the brand of this motor.
I will get an rpm measurement at various frequencies including A and V.
Thank You so much for your thoughts! I hate to give up on it seems so close. My hope is that if voltage were bumped from 400 to 480 the motor would not run high temp.
Pete:
Hi LH.
Siemens motors eh, the bane of my motor rewinding days. Most other motors have plenty of spare room in the slots where the windings go. So some more room for cooling, not Siemens, they fill the slots totally.
They also seem to have small frames for the HP ratings they squeeze in.VFDs create. There is mention of sparking and heating in the motor bearings and the need for insulated bearings in some cases. Sounds like it may relate to your compressor.
Also compressors being required to start under load can be a problem, if the drive is set at too low a speed or voltage the motor will not have the torque to overcome the load and will run hot.
I wonder, I helped a fellow sparky once who was having trouble with a motor on a VFD that was drawing way over the rated current. It was only a small motor but it was connected wrongly. The motor was only 1kw from memory and it was delta connected. When the drive was turned on it ran very hot, the current was way up and the VFD just shut down.
I suggested that we change the terminal connections to Star and that solved the problem.
Seems that someone changed the motor or changed the connections and it all went pear shaped from there.
Check the nameplate on the motor to make sure it is not supposed to be Star connected, that would explain the max 400 volts on the VFD. Long shot but worth a check.
Pete
Have a look at this article, it is short but mentions some of the problems
https://new.abb.com/drives/segments/motors-and-drives-in-potentially-explosive-atmospheres/effects-of-variable-speed-drives-to-motors
rossw:
--- Quote from: Pete on April 22, 2022, 02:14:26 am ---Star connected, that would explain the max 400 volts on the VFD. Long shot but worth a check.
--- End quote ---
For a given motor, I'd expect that a delta-connected motor would run on a lower voltage than the same motor star-connected.
But they're not so close as 400/480V.
I think a 480V 3phase supply would only see 277V across each coil in a star connected motor.
Isn't 400V common in Europe? Is it possible the motor and VFD were not a "pair"?
Pete:
HI Ross, I was mainly thinking of the current the motor is drawing. For it to be running so hot and to have burnt out twice already something is definitely wrong. In Star the motor would only be drawing 1.73 times less current. Whether it would have the power needed for the compressor is something that would only be found out on experimentation.
You may be right the controller may be the wrong one for that motor.
I guess the manufacturer needs to come and look.
Pete
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