Testing AC Motors
Testing AC Motors
(OP)
We have several drilling machines that utilize .5 and .75 hp motors running drill heads. After some time we get a stall condition. Our maintenance people pull the motor and say it is smoked and needs to be rewound. We send the motor to our local rebuilder. He puts it on his Hipot tester and finds nothing with the motor. This has happened so much as of late we are considering a hipot tester of our own or a megger. Any thoughts on additional testing we could do short of those two tests. We periodicly check leg to ground on each of the poles. But we would like to develop a better method for evaluating these motors for rebuild.
Thanks in advance.
Thanks in advance.





RE: Testing AC Motors
Further more what happens at the rebuilder's??? He hipots them sez, "nothing wrong", and hands it back? You reinstall and continue using it?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Testing AC Motors
Thanks
RE: Testing AC Motors
Keith Cress
kcress - http://www.flaminsystems.com
RE: Testing AC Motors
Are these on VFDs by any chance? Sometimes people use VFDs on spindle drives because they want 3 phase motors so they can do simple reversing, braking and speed control and they only have 1 phase power available. Unfortunately, if they didn't know what they are doing and used cheap VFDs without Vector Control, they can easily stall at low speeds. That happens quite a lot.
If you are not using VFDs, you may have a bearing problem in the shaft of the drill head and as it heats up, it starts to seize. Changing the motor would not solve the problem and it may not show up as a damaged motor, especially if your overload relay is working properly. By the way, are the overloads not tripping off? Something else you didn't say. They should trip off long before the motor burns up!
If they are single phase motors, is the motor shop checking the start caps and the speed switch to make sure the starting capacitors are not still in the circuit?
There are plenty of other possibilities if you tell us more. We don't charge you by the word you know.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Testing AC Motors
Build a test bench to energize the motors in the shop.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Testing AC Motors
1. Ground Insulation Test - Hi-pot or Megger - In your case, megger is good enough since hi-pot requires special testing skills.
2. Turn Insulation Test - Your low voltage type motors fail more often due to turn insulation failures than due to ground insulation failure. To do this test, you require a specialized tester called surge comparison tester (google surge tester, you will get names like Baker). This equipment would require test skill. In your case, 2 KV surge tester would suffice.
More importantly, when you rebuild a motor, ask the service company to fit 3 RTD's (one per phase) and use a matching digital temperature scanner/controller for each motor. Plenty of companies make these scanner/controllers.
RE: Testing AC Motors
Thanks,
Jason
RE: Testing AC Motors
VFD Parameters should be set so that the motor can never be hurt.
DOL motors should should have properly rated CBs in the AC Line for each motor.
RE: Testing AC Motors
Why did your maintenance folks say the motor was smoked?
RE: Testing AC Motors
I'm real curious why your Main't people say that the motor is bad. Like "I Ohmed it out and it's shorted between the motor leads." Or something like that.
A conventional Ohmmeter won't measure lead to lead resistance with any kind of meaningful results except for any open.
Ed
RE: Testing AC Motors
RE: Testing AC Motors
Thanks, for the help
RE: Testing AC Motors
Also I've seen temperature related failures where there's a breakdown in the insulation when hot and then looks fine when cold.
If your motors are getting that hot, I'm assuming you're not using forced air cooling or a high duty-cycle[?]. If upgrading isn't an option, perhaps add air cooling to increase the longevity of the motors.
Additionally what quality bearings are you using? I hope they're high-quality, full seal bearing with a hi-temp non-migrating grease.
RE: Testing AC Motors
RE: Testing AC Motors
Zogzog, thanks for the tip I will look into the book and possibly getting a megger.
RE: Testing AC Motors
If the people problem persists use management techniques.
This would hold after the upgrades as well.
RE: Testing AC Motors
Note that these are all HOT. You can boil water on the lowest rated motor!
Just because a motor "feels" hot doesn't make it too hot. Human skin blisters around 65degrees C!