Caution - Burned out CIM blows Victor

[sanddrag]

Had a bit of an interesting happening with some Victors the other night. We discovered the hard way (3x over) that a burned out CIM motors can easily blow the transistors on the Victor, before the breaker to the input will ever trip. I suspect that when the CIM motors run for too long, they overheat, causing the insulation on the windings to degrade, thereby causing an internal short. When we disassembled the motor, it was all black inside. So, be careful with how long you run your robot for at a time during practice, and check the temp of the motors often, or you may find yourself replacing a Victor.

[Kevin Sevcik]

Wait, let me be sure we're all clear on this. Your drive motors were CIMs on Victors with, I assume, 40 amp breakers? Did they start out as good CIMs, and end up as toasted CIMs from practice driving and THEN you blew up 3 Vics? Or do you think the CIM was toast before you started driving, and you finally pushed it over the edge?

Inquiring minds would like to know because even long term driving practice shouldn't be about to toast CIMs like that unless you're constantly blowing breakers. If it IS possible, then 20 or 30 amp breakers might be a good idea for practice driving.

[eugenebrooks]

Where you practicing going over the bump a lot?

Eugene

[Al Skierkiewicz]

Guys,
The fact that 40 amp breakers are in the circuit means nothing. These circuit breakers can withstand up to 600% overload for a few seconds and 200% for several seconds without trip. Even if they trip, they reset themselves within a second or two. The CIM motor is designed to be intermittent duty. Remember they are designed for trailer hitch lifts so at most they run a minute or two under load and then aren't used for days. They have no holes in the motor to let in cooling air or let out heat. The heat is transferred from the internal structures through the bearing end plates and through the magnet structure via convection and radiation. None of these are very efficient.
As such, when you run practice matches even with normal driving and loads, they eventually build up internal heat. We have toasted CIM motors in demos and long practice sessions. As Sanddrag has pointed out, they were run for quite a while. When the internal temperature runs hot on any motor, the varnish that covers the wire starts to melt. A few shorted windings, lower the power output or speed and lowers the series resistance. So this is a domino effect as the current rises which translates to more heat. At some point the Victor is trying to feed a very low resistance or a short. As great as they are, Victors are not miracle workers. Permanent failure is the result. (sometimes fire and smoke)

[Ether]

Originally Posted by Al Skierkiewicz

Guys,
The fact that 40 amp breakers are in the circuit means nothing. These circuit breakers can withstand up to 600% overload for a few seconds and 200% for several seconds without trip. Even if they trip, they reset themselves within a second or two. The CIM motor is designed to be intermittent duty. Remember they are designed for trailer hitch lifts so at most they run a minute or two under load and then aren't used for days. They have no holes in the motor to let in cooling air or let out heat. The heat is transferred from the internal structures through the bearing end plates and through the magnet structure via convection and radiation. None of these are very efficient.
As such, when you run practice matches even with normal driving and loads, they eventually build up internal heat. We have toasted CIM motors in demos and long practice sessions. As Sanddrag has pointed out, they were run for quite a while. When the internal temperature runs hot on any motor, the varnish that covers the wire starts to melt. A few shorted windings, lower the power output or speed and lowers the series resistance. So this is a domino effect as the current rises which translates to more heat. At some point the Victor is trying to feed a very low resistance or a short. As great as they are, Victors are not miracle workers. Permanent failure is the result. (sometimes fire and smoke)

Nice summary Al, thanks. Minor point: I think you meant "conduction" not "convection".


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[sanddrag]

Story goes like this. Once upon a time the CIMs were new (probably a couple years ago). They've been transfered between several robots over time. Put them in this year's practice robot. Drove it, smoked a Victor. Not knowing why, we replaced that Victor, smoked that one. Replaced the motor and the same position victor, smoked a different Victor. Examined motors, blackened inside. We hardly ever trip a breaker, and this time was no different.

[Ether]

Originally Posted by sanddrag

Story goes like this. Once upon a time the CIMs were new (probably a couple years ago). They've been transfered between several robots over time. Put them in this year's practice robot. Drove it, smoked a Victor. Not knowing why, we replaced that Victor, smoked that one. Replaced the motor and the same position victor, smoked a different Victor. Examined motors, blackened inside. We hardly ever trip a breaker, and this time was no different.

Just curious, what was the DC resistance reading on your ohmmeter when you measured the CIM leads before dis-assembling ?


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[sanddrag]

Originally Posted by Ether

Just curious, what was the DC resistance reading on your ohmmeter when you measured the CIM leads before dis-assembling ?

About 0.3 Ohms. Not sure what a new CIM reads. Anyone?

[Ether]

Originally Posted by sanddrag

About 0.3 Ohms. Not sure what a new CIM reads. Anyone?

Well, the rated stall current for the CIM is 133 amps at 12VDC according to the drawing. That would imply that the DC resistance is a bit less than 0.1 ohm.

It takes a bit of care to measure low resistances to make sure you have good probe contact.


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[sanddrag]

Originally Posted by Ether

It takes a bit of care to measure low resistances to make sure you have good probe contact.

True. I forgot to mention that I do not at all trust the meter I was using.

[ayeckley]

Just curious - does anybody believe that a periodic HiPot test should be run on the motors as a health monitoring measure? Seems like the easiest way to detect impending winding shorts. On the other hand, I can just imagine the shenanigans that could happen if proper safety measures aren't observed.

[Al Skierkiewicz]

Originally Posted by Ether

Nice summary Al, thanks. Minor point: I think you meant "conduction" not "convection".


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Correct, I was thinking of two or three things at once, just ending a curious day at Wisconsin regional.
I wanted to add that the breakers may have been tripping but without actually observing this once, it is hard to recognize especially on a loud robot. The breakers reset almost immediately and with a sustained short may actually buzz. Most everyone is so caught up in the other things going on that the buzz is the last thing that grabs their attention.
As to hi pot testing, I am not sure that would catch a failure until it becomes a disaster. A few shorted windings may not indicate any resistance drops to the motor frame. By the time the damage was done, the motor was already very suspect. As Ether has point out, a milliohmeter or bridge would be the better choice but with production variances, you might find significant differences from several good motors.
For everyone else, the CIM motor is not an easy motor to disassemble without damage. Once you pull it apart, you might damage it beyond hope. If you suspect a bad motor or one that is very hot in use, try removing one of the screws that hold the motor together. Smell the screw and the hole in the motor. The burning smell is distinctive and if things are bad the screw make actually show signs of contamination from the damaged windings.

[NyCityKId]

has anyone considered a growler test? its a piece of test equipment thats used to test for short (and im assuming damage) in motor windings. they uses it in the airline industry but im not sure if its main stream equipment

[Ether]

Originally Posted by NyCityKId

has anyone considered a growler test?

It is not permitted to disassemble the motor.


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