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Old 04-04-2013, 11:02 AM   #1 (permalink)
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Keeping a Motor Cool

I'm currently using a 7.5" diameter 9" long series wound motor for my conversion. I bought an Alltrax 72V 450A controller that I want to use but I'm worried about the motor overheating. I can program the amperage limit so that the motor will not overheat, but I would like to be able to fully utilize the controller. If necessary I will get a larger motor, possibly a large forklift motor, that can handle the amperage.

The current motor does not have any obvious cooling. I was thinking of taking the motor apart and drilling or cutting some holes for ventilation and adding a fan to the shaft to force some air through. What parts of a series wound motor do I have to worry most about overheating? It might be possible to water cool the stator using some flexible aluminum tubing, but I think the rotor would have to be fan cooled unless I could submerge it in a dielectric fluid. I don't think that's a good idea though because it could prevent the brushes from making an electrical connection with the commutator.

Please let me know if you have any ideas, suggestions, or experience to share.

Thank you,

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Old 04-05-2013, 12:00 AM   #2 (permalink)
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The brushes and commutator are what you want to keep cool, brushes will stay coolest if they have good contact with strong springs and are seated in, same with the commutator, seating brushes in is best done by running the motor without a load until they are wore in, this can take days, there are short cuts you can take by using pumice stone, but I've been told by people who design motor parts not to take short cuts.
Forcing air in to the motor is a good idea, make sure it's clean air so you aren't getting grit in there.

I ended up buying a thermocouple reader and some thermocouples then drilled a 1/16" hole in one of my brushes and JB weld'ed the thermocouple in there, that way I get a reading from the most sensitive part of the motor I can, 300F is the point that brushes and commutators start to become damaged.
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Old 04-05-2013, 03:48 AM   #3 (permalink)
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Thanks for the advice. I've heard that running a series wound motor without a load can cause damage, should I try to run it within a certain rpm range? Would you recommend placing some sort of filter over the inlets when I add fan cooling? Do I have to worry about water getting in to the motor? I plan to block off the engine bay as much as possible but some might still get in.
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Old 04-05-2013, 09:45 AM   #4 (permalink)
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If you keep the voltage down, running the motor unloaded is fine. Run it at 6 or 12V and you'll be fine. More voltage = more rpms, and they'll eventually fly apart if you get them spinning too fast.
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Old 04-05-2013, 10:44 AM   #5 (permalink)
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I run my motors at 12v or so to wear the brushes in, the motor for my car is an open face motor so it has to be on an axle to have the drive end support bearing so that does add some drag to it, for that motor I run it at 18v.
You of course don't want to over speed the motor (they will self destruct!) but I was told that nearing max speed will wear it in faster because you are looking for as many revolutions as possible, if you want to play it safe tho slowing the motor down will just cause it to take more time to wear in, you can tell when they are wore in by a nice even shine on the part of the brush that rides on the commutator, I get mine really close by using sand paper on the commutator with the grid facing out then sanding the carbon brush down till it fits snug, then let the motor wear it in the rest of the way.

I wouldn't worry as much about water as I would dirt and grit, my car has air intakes on the sides of the car, up away from the wheels, but if you are pulling air from the engine compartment, near the wheels then at the very least a chunk of stainless steel screen seams like a good idea, an over sized cone air filter like someone might put on their supped up pickup truck would get everything filtered out without costing a lot.

what brand of motor do you have? if it has holes for checking/replacing the brushes then that would be the ideal place to force air in, a number of companies make clamp on bands with a duct inlet for cooling motors.

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