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Old 03-26-2009, 03:15 PM   #16
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In both the brush motor and the brushless, the resisitivity of copper goes down as temperature goes down:


this reduces the armature resistance Ra. According to Ohm's Law, a greater motor current flows at lower resistance:

Ia = Vs/Ra

The heat dissipation as power in an armature/stator coil:

Pdiss = Ia*Ia*Ra

where power is the square of current so smaller resistor can cause greater effort to dissipate power.

The temperature build-up depends on a thermal resistance that you might find in an industrial motor specification, and the power dissipation level. The insulation on the coils melts at some temperature.

Exactly how these factors add up to a thermal failure in a given system is a matter of further analysis.

Magnets can fail too due to thermal and magnetic circuit limits but I'm not prepared to describe any physics at this point.

Peak mechanical motor power does go up when motor resistance goes down, so the physics predicts an advantage, but the heat dissipation is the key to sustained advantage in most racing machines.

Last edited by SystemTheory; 03-26-2009 at 03:19 PM. Reason: Comment of power advantage
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Old 03-26-2009, 03:48 PM   #17
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Originally Posted by Pablo Diablo View Post
Try a google search on the "Mpemba effect". You would be surprised that warm water can freeze faster than cool water! It's true. If you apply that theory in reverse to motor cooling, it could be that the cooler motor heats up faster?
It's common to hear people talk about motors overheating after cooling them first. I can't see how a little bit of miosture on the surface after cooling could be the cause.
Try this with ice cubes, it is true, the warm water evaporates leaving less water to freeze (smaller ice cubes) and frost in your freeezer. I guess it could work in reverse on your motor if the motor housing evaporated, not likely. Colder motors mean less resistance which means more current flow. More current flow may be what is destroying the motors but they will have more power as well. It may also change the tolerances in the motor causing a seizure but I would doubt that. I would say that any damage would be from thermal cycling and the expansion and contraction of the motor causing flexing in the circuits. Thus began the debate of turning your computer on and off or leaving it on all the time.
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Old 03-26-2009, 04:40 PM   #18
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sounds like a cool idea to try it out again. you could also cool some beers at the track aswell
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Old 03-26-2009, 05:27 PM   #19
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Chances are, if you read about the failures noted above, the insulation on the copper coils (a thin hard varnish) will melt and short the coils. This is from an engineering paper dealing with thermal failure of motors:

Overheating namely is the most common failure mechanism for an electric motor which is dimensioned too tightly. Especially in the case of modern electric motors with strong magnets and a compact design the motor has trouble leading away its heat. Overheating can lead to:
1)Failure of the winding-isolation, what results in a shortcircuit and possibly can lead to a burnout of the motor;
2)Failure of the bearings, what results in a jamming motor;
3)Degradation of the magnets (the magnets permanently loose force), so that the motor will never be able to deliver the peak-torque it is designed for.
That is why it is important to prevent overheating.
The thermal cycling is probably not a primary failure mode for motors, but it is significant for light bulbs and other electronics. The Mythbusters found that turning on and off light bulbs rapidly caused failure sooner than continuous use.

My father used to do service calls for frozen pipes as a plumber. He told me years ago the hot water pipes were often frozen before the cold water pipes, now I can tell him why -- the Mpemba effect! Thanks for the reference.
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Old 03-26-2009, 05:37 PM   #20
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Originally Posted by kschu View Post
i saw hara freeze his BL motor during TITC in thailand last month....
But how long was he freezing it for?
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Old 03-26-2009, 06:14 PM   #21
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Originally Posted by Skiddins View Post
But how long was he freezing it for?
i think around 5min before he start the race..
Give the best while u can !
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Old 03-26-2009, 09:57 PM   #22
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I used to use ice cubes on my 27t motors in TC last year before going to brushless. It really helped the motor when running at six or seven minute mains! I have not done this with a brushless motor, but I do not think it would hurt it at all unless you got moisture back in the hall sensors for the pickups. Most do not do it to run a taller gearig either, actually it will keep things cooler for the first minute or two while racing, I could not believe the difference in the motor temps after six minutes.
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Old 03-26-2009, 09:58 PM   #23
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if you are fast at building a motor, just chill the rotor in dry ice or liquid nitrogen. That should keep the moisture from building up in the can.

I remember seeing dry ice heat sinks for motors a long time ago (1990's). I was just a clip on heat sink and had some shallow fins. The trick part was that it has a 5/8" dia x 3/4" chamber for sticking in a dry ice slug. Inster a rubber plug and the ice stayed put. Maybe someone could start making a veloicty heat sink ring with a dry ice chamber.
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