Author Topic: Max winding temp  (Read 5058 times)

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Offline lighthunter

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Max winding temp
« on: April 17, 2022, 08:50:06 am »
Hi all! Ive had a couple issues lately to do with enamel wire limits and since i enjoy the scenery at the edge it would be nice to know where it is.

Fuse blew last week on my 48v inverter. One leg of split 240 during backcharging at 4-5kw. Has been there for a couple years no problems. Fuse was not blown in middle but melted solder from end. Replaced fuse all good. Days later i grabbed an ir temp meter and checked surface temp of toroid during 4kw of back charging (usage never goes that high). The surface temp of toroid was 180°F or 82C the fuses sit in fan air stream just beyond the toroid. Perhaps bad place for fuses or maybe need to set fan control to run fan more as it only comes on to cool FETS.

Also working with a 150HP air compressor which motor tag states 150HP at 460V 3~.
The manufacturer of course set up locked inverter settings to output 400V3~max. Lord knows what actual hp rating was/is. When you drop motor voltage down slippage increases and V decline is not HP linear.  This is TEFC. It used to work but cooked grease from bearings often. Motor failed once under warranty, twice since.  Recently rewound motor and now monitor winding temp. It hits the 195°F 90°C mark at about 70% capacity. Ive requested access to parameters to scale up voltage but so far theyve not budged. I went to Danfoss the drive mfr and they can give me software to correct no problem but i will loose the IO programming to go that route.
I have asked the guy who rewound what is safe temp but he had nothing to offer, i will inquire again to get specs of wire he used.

Unless i do something crazy like fool the inverter buss volt feedback im out of ideas. How high can i safely go with surface temp of motor winding? Also, anyone know how to calculate HP at a lower than design motor voltage?
 
Health Warning: May contain traces of nut!
LH

Offline Pete

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Re: Max winding temp
« Reply #1 on: April 17, 2022, 09:32:30 pm »
HI LH , the motor should have no trouble running at 90 degrees C.
The problem with dropping the voltage from 460volts to 400 is that the current drawn will increase heaps for the same hp rating.
here is a link to some information on motor temperature ratings.
https://myelectrical.com/notes/entryid/122/understanding-electric-motor-insulation-temperature.
I would start by checking the motor current rating against what it is actually drawing under load.
Pete

Offline Pete

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Re: Max winding temp
« Reply #2 on: April 18, 2022, 03:03:12 am »
LH, a bit more information.
If the winding of the motor is B class then it will have insulation that used to be called elephant hide. It is a blue insulation with mylar on one side. The insulation between the winding phases is usually then a varnished fibreglass cloth.

If it is E or F class it will probably be Nomex slot insuation ( stiff white looking paper stuff). The insulation between phases will be either terylene ( E class) or Nomex for F class.

Of course it is difficult to measure the temperature of the windings when the motor is running, unless thermistors are embedded in the windings. Some motors have them but not many.

As far as the toroid goes that sounds pretty hot but then you are running a lot of power through it.
I have button type thermostats in my inverter that are 40degree C set point.
I hear the fan come on sometimes when I am using loads over a long time but it goes off pretty fast.
Maybe you could put some more vents in the inverter case to get the heat out faster.

you must have some serious battery banks to take 4 to 5 kw back charging.

I just picked up some new panels that came with an SMA 2kw GTI inverter. I am thinking of connecting it up as a mini grid but obviously need to have a voltage sensitive relay on the batteries to prevent overcharging.
So far I have never seen my batteries take more than 2kw in. My panels are way over sized here to take into account the cloudy , misty mountain weather.
Hope you get the compressor sorted
pete

Offline lighthunter

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Re: Max winding temp
« Reply #3 on: April 19, 2022, 12:19:28 am »
Hi Pete, I really appreciate the input, great info there.
"I would start by checking the motor current rating against what it is actually drawing under load"

This was my first clue something wasnt right, the nameplate current was FLA 166 460V 60hz, 110kw, SFA190, 94.5effic,, 680KG, Frame TEFC, SF1.15, PF .88.
Rpm 3585
What i saw was the amperage at 400v climbed to 160A at full load. The catch though is i remember the guy saying that he used smaller wire for some reason on the rebuild. So who knows what it can handle. Nameplate data is questionable. Upon install i squeezed a temp sensor under some windings i could reach through the box. I ran it for 40 hours initially at full load and it survived but had 0°C cooling air of a couple thousand CFM. Im sure it would not survive now.

Admittedly im being cautious but its obvious its days would be numbered if i just increased temp setting to 250F and pushed the start button and walked away. I dont need 100% capacity so id be ok with 70% but i need to get the voltage up somehow or it cant even do that. I could boost the 80V with 3 toroids with 200A 80V secondaries. Ive in fact had to do this in the past to level shift voltage and it worked well still in use to my knowledge.  but doing this would cause current on the ground wire (common for the Y feeder) and i think the distribution has GFCI so that would cause a fault.  I may have to get the software from  the vfd mfr and somehow configure the pressure control loop another way. I just hate to alter an oem machine but when they violate such basic things as rated voltage on a motor... The motor failure under warranty proves it was a bad idea. Its possible they will still come through with the codes to change it but im not counting on it.
I just had a crazy thought, maybe nameplate volts is for wye and mfr. Is using as delta, that would explain?? Nope, just looked, nameplate says 460 with triangle which means delta which is how we are connected.
No blue color on windings. I will try to find out.
What do you think the temperature difference between core and surface of windings is? Thermal transfer of copper is good but the varnish has to slow that down some.

How can i go about calculating heat generated in the motor? For example if i know volts, amps, rpm and frequency, i can calculate slippage at a known input power. This might give me an idea of the red zone of loading. Im guessing all motors exhibit a normal range of slip and anything beyond a certain point results in an overheat motor.
Looks like ive only got two options, reduce load, or increase voltage. I guess replacing motor or scrapping compressor might be options as well but would not make those happy who've just spent $14k on it. 

On my inverter, I think the toroid is ok with the temp, it just as pretty as the day i finished winding it. There is a monster 243 cfm fan that I will set to run more often though. Maybe having the fuses in the hot air stream is a good backstop for temp limits due to an improper fan setting   ::)
 
" SMA 2kw GTI inverter. I am thinking of connecting it up as a mini grid but obviously need to have a voltage sensitive relay on the batteries to prevent overcharging."

Hook it up, you wont ever regret that. They are so efficient you would be amazed. I had a group of 12 panels direct connected through relays, 29 volt panels two in series, 6 sets in parallel. Never got  even close to 2kw. Connected in series, on a grid tie, I saw those same panels run near 3kw whenever sun is shining. I think they are 240W panels. I will admit my wiring was not perfect but it was night and day difference in power. You can control output in a number of ways. I use high volt relays just on/off with 30 min time delay but a common relay can be used with a dump resistor to parallel across the dc inputs and cut output in half.  Or a relay on AC side with a 1 to 2 kw water heater element. You can never have too much power:)

Health Warning: May contain traces of nut!
LH

Offline lighthunter

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Re: Max winding temp
« Reply #4 on: April 19, 2022, 01:18:50 am »
Does the 3585 nameplate rpm say 15rpm slip at 460V 150HP 166A??? I never thought about it before. Thanks again for your thoughts!
Health Warning: May contain traces of nut!
LH

Offline Pete

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Re: Max winding temp
« Reply #5 on: April 19, 2022, 02:45:34 am »
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

Offline lighthunter

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Re: Max winding temp
« Reply #6 on: April 21, 2022, 07:12:39 am »
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.




Health Warning: May contain traces of nut!
LH

Offline Pete

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Re: Max winding temp
« Reply #7 on: April 22, 2022, 02:14:26 am »
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

Offline rossw

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Re: Max winding temp
« Reply #8 on: April 23, 2022, 05:23:39 am »
Star connected, that would explain the max 400 volts on the VFD. Long shot but worth a check.

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"?

Offline Pete

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Re: Max winding temp
« Reply #9 on: April 23, 2022, 05:45:53 pm »
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

Offline lighthunter

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Re: Max winding temp
« Reply #10 on: April 23, 2022, 05:57:46 pm »
Pete, youve already been way more help than the mfr reps. Seems like youre onto something. Wierd thing is, I never got to remove...disconnect this motor, others did and I was really scratching to understand how to hook it up.  I do know there are 6 leads, always were and 3 terminals, (two each and not 1 each, with 3 bound together.) Since this is VFD the wye/delta start is not needed. Im willing to listen to your experience on this cuz ive tried all i know and it works well, just generates too much heat. Makes total sense downsizing wire if he couldnt get wire in slot.

Ross, your right also, i'm guessing where this came from, chicago pneumatic, 400v might have been the likely voltage fed to vfd so wouldnt have been any more available. I did talk with rep of mfr. Of drive, danfoss, and he said it was capable of outputting 480 no problem but then encountered the locked software.  Soon i should know if they will send a guy out to change it. If not, im tempted to reload danfoss software, my hesitancy on that involves liabilities of a blown compressor. Even though i know zero chance of a voltage change causing that its just the idea of a modification. Changing a volt setting in my opinion is not changing functional design. Reloading software deleting I/O and rewiring/configuring the pressure control loop is changing design. At the time of failure a new motor did not exist even though this unit is little more than 10 yrs old. Now i really wish id have insisted on a "new motor". Yet this issue existed when it was delivered new. What Pete describes with need for insulated bearings etc fits this description exactly. Any thoughts if bearing arcing is a function of heat or not related? Ive heard of this before, even seen it but ive never understood it. Is it always a VFD that causes it? Or is it motor design? Maybe we could remove the vfd, put wye/delta contactors for startup and run it direct. Ha, here i go again wanting to change design. I better stop thinking about this  :)

Thanks a million guys! Your input is appreciated!
Health Warning: May contain traces of nut!
LH

Offline Pete

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Re: Max winding temp
« Reply #11 on: April 23, 2022, 07:27:01 pm »
Hi LH , from what I have read the bearing sparking is caused by the spikes on the waveform fed from the VFD.
I am guessing that the sharp peaks are inducing a voltage into the rotor shaft and the only place to earth is through the bearings.
The shaft must be operating as one turn of a transformer and as such generating a current flow through the bearings.
I may be wrong but then it makes sense to me.
With a sinewave the drop off of the waveform is much more uniform so an induced voltage would be lower and less damaging to bearings.
It doesn't make much sense to me to have a VFD on a compressor. Maybe they are trying to limit starting current under load by using the VFD on startup but a Star/Delta starter or a compensator ( transformer) starter would be much simpler.
LH on the motor leads, it is very important that they are connected the right way.
If a phase were reversed then problems will happen fast.
Hopefully the leads are marked so that no one can stuff up and get the starts and finishes mixed up.
Usually European motors are marked U, V , W for the start of each winding
and U1, V1, W1 for the ends.
So delta is U and V1 connected together
                 V and W1 connected together
                 W and U1 connected together.

Star would be
Line connected to U, V, W
U1, V1, W1 connected together.

I am not sure what you mean by the terminals when you say two each and not 1 each it sounds like the leads are in parallel. Often if the lead cables are too big one would parallel them up. But then you say that 3 are bound together, that sounds like a Star point.
Can you put a photo of the terminal block and leads up.
Pete

Offline Pete

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Re: Max winding temp
« Reply #12 on: April 23, 2022, 07:39:52 pm »
Hi LH , I have attached some information on terminal markings of motors and connections.
Some of them are  complicated as they are for Dual voltage, or dual speed motors.
It may come in handy though in the future
good luck
pete

Offline lighthunter

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Re: Max winding temp
« Reply #13 on: April 23, 2022, 08:30:54 pm »
Thanks Pete! Looks like some valuable  info to hang on to.

 This is how i hooked it up.

(1,6)--(2,4)--(3,5) doesnt the nameplate info say the 460V is for delta connection? Or am i reading that wrong?
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LH

Offline Pete

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Re: Max winding temp
« Reply #14 on: April 24, 2022, 02:01:08 am »
Hi LH, looks right. The nameplate does say 460 volt for delta.
Your connection looks right for Delta.
With Delta connection each phase winding has 460 volts across it.
Talk to the manufacturers and see what they say about trying it in Star.

In a Star connection you would have with the 400 volts your controller can supply a phase voltage of 230 volts.
The current would be down too so you may not get enough power but at least it would not run as hot.
See what they say, as it seems the VFD is not suited to the motor running in Delta.
good luck
Pete