Testing AC Motors 1

[Pivotman]

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.

 

[itsmoked]

Do you ever use a temp gun to see what kind of temperatures these motors are reaching? They should last many years. If they aren't then something should be changed. Either their temps should be monitored and their load/feed rates reduced or their current measured and again their load/feed rates reduced.

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

 

[Pivotman]

Quit offen that is the case. The motor checks out and they are fine once they cool down. We have checked the temp and what I have seen is about 110 degrees F. I have not checked them lately. But I will mention to them to check them when they see this problem.

Thanks

 

[itsmoked]

Use a clamp-on to measure the current as this will tell you if you are over-feeding.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

You don't have enough information posted to make a really good evaluation, i.e. 3 phase or single phase, how they are powered and controlled, gearing / belting etc., but here is my attemp with what you told us:

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
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[waross]

Buy a megger, not a hi-pot.
Build a test bench to energize the motors in the shop.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

There are two common types of test to check 3 phase motor windings.

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.

 

[Pivotman]

Sorry I should have given you more info, the motors are 3 phase and some are fix speed and some run on a vfd depending on the machine.

Thanks,

Jason

 

[sreid]

Perhaps the motors have internal thermal cutout switches. It might take a motor an hour to cool down enough for the temp switches to close again.

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.

 

[rovineye]

What happens when it stalls? Overloads/VFD trip? If so, it could be something binding when hot, like a bearing somewhere in the driven load. Check with a clamp-on as Keith recommended.
Why did your maintenance folks say the motor was smoked?

 

[fangas]

Jason,

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

 

[LionelHutz]

The others have somewhat jumped around this so I will flat out say it. You need to investigate if the motors are overheating and why. Then, if the motors are running OK, you need to investigate why maintenance believes they are bad and are constantly wasting time changing them. But, if the motors are really being overloaded and overheating, you need to figure out why and solve that problem. Then, possibly worry about buying test equipment.

 

[Pivotman]

We have a pretty good understanding on the cause of the motors malfunction. The drill heads over heat by being pushed to hard. The motor runs the spindle however the feed is controlled by air and a hydrocheck. If the operator runs to heavy of a feed the motors stall. But not consistantly and not always the same station. Some times the motor is just over heated and some times more damage has been done. What I am looking for is a good testing method so we can tell which case it is. The solution is we need bigger drill heads and with bigger motors. But our management has put off upgrading till we build new machines in afew years and the existing heads will not take anything over the 3/4hp that is currently running them.

Thanks, for the help

 

[Tech805]

I see you're checking case to phase for a possible short, but do you also check phase to phase resistance? The phases should be within ~10% of each other.
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.

 

[Zogzog]

Get a megger, go to AVO website, download (Free) "A stitch in time". Great book on megger testing. You will want to do a IR and PI test, this book will explain and give test voltages and expected results. Temperature correction (as always) is vital to interpreting the results.

 

[Pivotman]

Tech805, We have checked phase to phase and they seem to be within the 10% window. As for the motors TEFC rated for 100% duty. I am not sure what type bearings they are. The motors come direct from Suigino the mfg. of the drill head. I think some are Bosch.

Zogzog, thanks for the tip I will look into the book and possibly getting a megger.

 

[wmurphy]

Verify the motors are actually being overloaded and if the the machine operator are overloading them, tell the operators to ease up. Have a motor temp monitor and current monitor where the operator can see it, so they can tell how hard they are driving their machines.

If the people problem persists use management techniques.

This would hold after the upgrades as well.

 

[DickDV]

Just so you have the numbers correct, an insulation class B motor is rated up to 40degrees C ambient plus 85degrees C rise, a class F motor is 40 plus 110degrees, and a class H is 40 plus 130degrees C.

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!