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Max winding temp

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lighthunter:
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?
 

Pete:
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

Pete:
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

lighthunter:
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:)

lighthunter:
Does the 3585 nameplate rpm say 15rpm slip at 460V 150HP 166A??? I never thought about it before. Thanks again for your thoughts!

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