Thread: Dead 3 phase motor beyond saving or is it?

Greetings brains trust, I suspect that I am only going to have my thoughts confirmed, however you never know until you ask.
I have an older 10Hp 6 pole 3 phase motor that I believe has had a small amount of water enter during a significant storm event. When I switched the motor on, not knowing that it had suffered water ingress it ran for probably a minute until it stopped, having popped the circuit breaker. I reset the breaker and turned the motor on again and it popped immediately. I then tested the winding resistance to ground and discovered that we had leakage, whereupon I stripped the motor in order to dry it out. At this point I also checked winding to winding resistance with a megger and discovered leakage winding to winding (yes, I did separate the terminals so that windings were no longer in Delta). The motor has been left open to the air for about 2 months and while the readings have improved, it still only takes around 100V to get leakage from winding to winding, (it was 5v at one stage) and these readings are no longer increasing, having remained steady for a couple of weeks now.
I am guessing that when the motor got the moisture in it and it tripped the breaker for the first time it probably damaged the insulation on the windings and the motor is now deceased. I have found no visible damage to the windings, but there are a lot of the windings hidden of course. Encyclopedia Google has several references to re dipping motors to "repair" the damaged areas, but I am sceptical, I also see references to connecting a welder on low amps to a winding to dry it out, but given the time of year, the time open to the air and the fact that readings have become stagnant, I'm doubting that I have a moisture issue.
Do we all agree that it is dead? I have grafted another motor into place on the compressor, so I'm not after a quick solution and I would like to recover the motor if realistic. Last time I priced rewinding a motor it was not financially viable, however this is a larger and more expensive motor, so that may be an option down the track if all else fails.
Open to any and all suggestions.

I've just been through a similar wet motor issue, but only with a small single phase motor. After drying out in the sun for a couple of days the winding to earth insulation reading came up from a few hundred ohms to about 20M or so at 500v.

I'm definitely no expert, but think you're probably right about it being dead.
Being a much larger motor than mine though, its possible that you do still have moisture. Along those lines I'd definitely give something like baking it at a suitable temp (maybe 120deg or so??) a try first.

Since its not a dead short - it might be possible to impregnate it somehow with more insulating enamel. I wouldn't expect just dipping it would have any effect - but possibly under vacuum somehow similar to how they do vacuum impregnation of resin into carbon fibre structures?
Just personal speculation though - I've no experience in that area.

Steve

Hi Karl, Guys,

You can buy tins of synthetic varnish like that used on the old insulated wires. Your megger readings suggest that the wires are not actually touching but do have something conductive between them !

Sometimes a good blasting with water from a pressure washer will remove that kind of debris and even dissolve it. But as Steve says it must be dried out completely, and baking for several hours at 100C will get rid of any moisture. While its still warm re-test and varnish the windings if the test is good. You might find that you have your motor back !

Don't use any detergent that could linger, plain hot if possible not boiling water should do the trick ! Remember that there may be paper insulation inside.

Originally Posted by BaronJ

Hi Karl, Guys,

Your megger readings suggest that the wires are not actually touching but do have something conductive between them !

Sometimes a good blasting with water from a pressure washer will remove that kind of debris and even dissolve it. But as Steve says it must be dried out completely, and baking for several hours at 100C will get rid of any moisture.
Remember that there may be paper insulation inside.

That thought had crossed my mind seeing as how it is taking a bit of voltage to get leakage and from memory, my multimeter did not reveal a major short (although I can't fully remember the readings that I got, but can easily recheck).
I did see a reference on google that mentioned hot pressure washing large motor windings, but I hadn't done that as I guess I didn't want to make my situation worse at the time. I might just dig the hot wash out and see how I get on, but that will probably be a few weeks away I think.

Hi Karl,

You have nothing to loose by giving the motor a blast with hot water ! As I mentioned the paper insulation, once its wet it will deteriorate very quickly. So the trick is to be quick and get it dry as quickly as possible, certainly before the paper has chance to turn into mush. Then baking it for long enough to dry it out properly. You should find that any leakage has now dropped to a very low value, ideally zero or several megohms.

Good luck, I believe its worth the effort to save it !

Best to get motor rewound + bearings replaced
(if water can cause leakages, some insulation of the coil must have cracked or peeled off).
415V is high voltage.
In Tasmania try
https://electrorewinds.com.au/

Originally Posted by reuelt

In Tasmania try
https://electrorewinds.com.au/

I know electro rewinds fairly well, I worked next door to them for nearly 10 years.
When I get a proper chance, I will look at this motor more closely and see just what I find and may even get a quote to rewind it.

It would be interesting to know how much the quote is, 7.5Kw motors seem to go for around $600-$800. As you mention in your first post it's not usually financially viable to rewind, I found this back a few years ago with my Arboga mill which prompted me to rewind the motor myself, turned out not such a hard thing to do, all up cost just under $100 including bearings, and I still have a few Kg's of wire left - the motor is still running fine, maybe something to consider if you have the time.

Give it a try with baking. If your wife wont let you use the oven, look on gumtree for a free one. A good long bake at just above 100 degrees will probably clear the fault.
I wouldnt use high pressure water to clean it, a degreaser gun with non residual solvent is best, sun dry for a day then bake. If meggers out ok out of oven then spray with varnish

Originally Posted by Karl Robbers

I know electro rewinds fairly well, I worked next door to them for nearly 10 years.
When I get a proper chance, I will look at this motor more closely and see just what I find and may even get a quote to rewind it.

Did you do a post on rewinding the motor. I have a dead motor and would be very interested how you went about it.

Thanks Steve

Not sure about Karl but I did a post about a diy 3ph motor rewind 5 yrs back link here https://metalworkforums.com/f65/t200...ghlight=arboga

one thing that puts me off getting the motor rewound is that, unless there has been a change of policy, the previously mentioned rewinder did not offer any warranty on rewinds.
I've had a few more pressing jobs crop up, so my investigations will have to go on the backburner for a little while, but I thank all those who have offered suggestions and the benefit of their knowledge.

Hello Karl. How old is "older?" Also a picture of the motor and nameplate may help as it should show operating temperatures and insulation class. What is it being used for, and how often? A six pole 1000 (theoretical) RPM 10HP motor. Do you really need one that big or would a smaller four pole (1440RPM) motor do the trick??

Originally Posted by A J in WA

Hello Karl. How old is "older?" Also a picture of the motor and nameplate may help as it should show operating temperatures and insulation class. What is it being used for, and how often? A six pole 1000 (theoretical) RPM 10HP motor. Do you really need one that big or would a smaller four pole (1440RPM) motor do the trick??

The short answer is that I don't believe that a 6 pole motor is strictly required as I have replaced this one with a 2 pole motor (it was free ) with appropriately downsized pulley of course so as to maintain the required speed of the compressor pump. When I acquired this air compressor, that's just the way it was set up and I guess whoever set it up originally did so with the intention of everything being slow revving for long life.
Photo of motor plate, although it's not too clear. I probably need to give it a hit with a cloth and some polish.
20221025_191533.jpg

Thanks Karl. Not much information on that nameplate! Why should the manufacturer give anything away! Plainly an older motor - as you said, and I would expect some lowly temperature rating.

I have a three phase drill press with an English motor. Temperature rating of 40c max. This means on a warm day in Perth - like tomorrow - 39c - I could not turn the drill press on as it is already too hot!

You were wise to replace it. Having said that, modern motor wire has insulation ratings of 90c, 110c, and 130c or was it 140c- all this from memory, so a re-wind with modern wire is a distinct possiility for a motor that cannot be replaced. Not cheap, but when you are out of production there is no such thing as expensive - as compared to the "penalty rates" in the contract!