Caution - Burned out CIM blows Victor
 

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.

Re: Caution - Burned out CIM blows Victor
 

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.

Re: Caution - Burned out CIM blows Victor
 

Where you practicing going over the bump a lot?

Eugene

Re: Caution - Burned out CIM blows Victor
 

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)

Re: Caution - Burned out CIM blows Victor
 

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


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Re: Caution - Burned out CIM blows Victor
 

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.

Re: Caution - Burned out CIM blows Victor
 

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


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Re: Caution - Burned out CIM blows Victor
 

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

Re: Caution - Burned out CIM blows Victor
 

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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Re: Caution - Burned out CIM blows Victor
 

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

Re: Caution - Burned out CIM blows Victor
 

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.

Re: Caution - Burned out CIM blows Victor
 

Interesting information. I wonder how the jags respond to this, given their internal fault protections. Really I just want to know so if something weird happens electrically we don't immediately blame the Jaguar.

Re: Caution - Burned out CIM blows Victor
 

There is really only one way to find out, and due to the RMA policy, it is free.


To measure CIM resistance, you really need a 4 wire set up and you need to check across the range of shaft positions. Hook up the 4 wire resistance meter and SLOWLY turn the shaft in small degree increments, pausing for a few seconds each time. The brushes can easily double / halve (or more) the resistance for a given position, and the back emf (generator voltage) of a moving CIM will overwhelm the ohmmeter.

Re: Caution - Burned out CIM blows Victor
 

Be careful with HiPot testing. The test can weaken or destroy the insulation. If you are going to do "periodic" testing, you'll want to use a lower voltage than the one-time test voltage used at the factory.


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Re: Caution - Burned out CIM blows Victor
 

Could you please elaborate on this statement? What is free, and why is it free "due to the RMA policy" ?

If you have access to some high-precision resistors, you can use a wheatstone bridge instead.

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