8 replies to this topic

[SlowBeas]

What are some of the reasons that a motor would run extremely hot?

 

I have a JK Challenger motor, never been raced, that runs really hot. I've only put it on a power supply to break it in, but it runs really hot -- to the point that it was difficult to handle when I removed it from the power supply for break-in. FYI, I put it in water first for a minute or two to help seat the brushes, then oiled it and left it on the supply for about half an hour. When I returned to put it away, I thought it would burn the fingerprints off my fingertips.

 

Before removing from power, I observed that there was no arcing on the comm and brushes, so I think the brushes are properly seated. No buildup on the comm,either.

 

Any ideas for me to check would be most welcome. Maybe the springs are too light? Wrong brushes? I'm stumped.

[Dominator]

Bad brushes are a possiblity. If the comm has never been cut try that also. Most c-can arms come with a comm diameter of .202-.205. Cut the comm to .198 and should help performance. Check to make sure it is spinning the correct way also.

[SlowBeas]

Nice tips, Dom. Looking from the endbell end, it's running counter-clockwise -- like my other C and D cans. I had replaced the brushes with some Gold Dust that I had lying around. Will see what I can do to have the comm cut.

 

Thanks,

jb

[gotboostedvr6]

Comm slots could have gunk in them from the water breakin.

[Bill from NH]

Jim, is this one of JK's 'close-out' Mura Challenger 1's? Since these are unbalanced, you might want to send the arm out for reconditioning. This would include comm cutting & balancing. I don't know anything about the history of this particular batch of motors, but reconning the arm might help.

 

As an aside, I once hade a new Koford boxstock arm that drew more amps than volts on my power supply. That thing was a real heater too!

[zipper]

Timing has a very great influence on the current draw. Best to check it.

[Don Weaver]

A likely reason for the heat is a short between the windings, either between two of the poles or more likely turn-to-turn on one pole.  A good ohm meter will tell you.

 

Don Weaver

[SlowBeas]

Obviously, the problem could be much deeper than my personal knowledge. About the only things I can try myself would be changing springs and brushes, then cleaning with a good shot or two of Pure.

 

Don, I'll try to stick the motor in my racing box to take with me to Abbeville Saturday. Come join us, and I'll let you take a look.

 

Thanks for all the ideas.

jb

[Marty N]

Motors run hot for one of two reasons. 1.) Draws high current. 2.) Not converting the current it draws to torque.

The amp meter on the power supply can give a hint if combine with a little basic familiarity with the motor type and its state of tune. Fold in a measure of good old fashion common sense and a dash of caution.

Give it a good inspection even before you put it to power for the first time. Mark your brushes with a Sharpie on the near edge. Different color for each brush so that they will go back in the same holder, in the same orientation. Remove both brush and spring. As you unlatch that spring give it a careful look. Is it bowed over the end of the brush? Is the long leg anchor bent so far in that the spring must be contorted to mount it? Spring bind is a common cause of #1. With the springs loose spin the motor by hand. Is it free of drag? Tight bushings or ones misaligned are less common but happen as does not enough end play. Check, check and oil. Pop out the brushes and give the fit a feel. A hung brush is trouble. While out look to the commutator segment spaces with a loupe for bridging. Copper slivers, packed slots. Neither all that common unless broke in wet and not blown out. Is there insulation between the comm. And back of the bushing/bearing?

Give the business end of the brush a look. Is it “really” broken in and if so how well does the pattern look aligned? Misalignment of both brushes to the same side is a large increase in overlap that acts like a short…because it is.

Last check. Paperclip test on the magnets. It’s a feel thing. Compare to known motors of the same magnet type. A weak magnet is the leading cause of #2.

If it passes muster proceed to break in if needed or to the power supply if not.

On the bench each motor has a “base line” current it draws that is wind and state of tune based. Establishing a consistent point of reference helps nail that number down. Three volts is a common point of reference. I prefer two volts only as I work commonly with hotter winds and one thing for me to remember is a challenge. Working with a bag motor like this doesn’t leave you in the dark. As common as dirt and as finicky as most racers are, the question; “How much current does a bag Challenger draw at 3 volts?” alone should get you an “in the ball park” reference. If it’s a custom built motor from a reliable builder he will know exactly how much current that combination will draw and at what voltage.

Motor on the supply at your reference voltage you will be close or you won’t.

If you are close then increase the voltage slowly and observer the amp meter. If it rises at all it will be slight then level off or may even fall a hair. That is normal behavior.

If it spikes up and increases in step with the voltage increase you have an internal short somewhere. (Remember you already checked before power up for binding and obvious commutation issues.)

If it passes all test and still runs hot on the long run this isn’t your first rodeo and know what to do.