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How hot is safe for brushless motor question

Old 03-07-2004, 10:26 AM
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Wayne Miller
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Default How hot is safe for brushless motor question

Hi,

I have a MPJET 25/35/20 brushless motor with the matching 4.1:1 gearbox. The documentation says it is rated at 3850/volt, max rpm 40,000, max effiency 82% Max effiency current 7-15 amps, short term current max is 25, number of cells 6-9. The documentation recommended propeller is 8X4 to 10X8, no prop manufacturer is mentioned.

If I run with a 10X6 to 10X8 the motor gets very hot on 2 or 3 LiPo's, I can just hold the motor, but does not smoke or smell.

I want to run the motor with either 2 or 3 1500 LiPo's instead of the NiCd's or NiMh's.

Can anyone tell me how hot is "too hot" for this motor?

Thanks for ytour help.

Best regards,

Wayne Miller
Old 03-07-2004, 09:24 PM
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Matt Kirsch
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Default RE: How hot is safe for brushless motor question

If you feel uncomfortable about how hot the motor is getting, it's probably too hot. Sounds like you might be working it a bit too hard, though. It's bordering on too hot to hold.

3S LiPoly is equivalent to 10 NiMH cells, so you'll probably want to stick with props toward the lower end of the range to keep the Amps down. Start conservatively, and gradually work your way up in prop size until the motor starts getting hot again, if you don't have any means to measure the current.
Old 03-07-2004, 09:50 PM
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Wayne Miller
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Default RE: How hot is safe for brushless motor question

Hi Matt,

I found some more specs on the motor at Hobby-Lobby. The example they show is:
MP Jet AC 25/35-20 = 154 watts with the following: 9.6 volts, 4.1:1 gearbox, prop 10 X 6, 16 amps giving 26 oz thrust.

Motocalc states 12.9 amps and 18.9 oz thrust for the same data.

I can check the amps and watts with the watt meter I built. I'll try different props and check the heat "by hand".

My thoughts on your reply is that the motor should get warm, but not hot.

Thanks for your help.

Best regards,

Wayne Miller
Old 03-08-2004, 11:41 AM
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Greg Covey
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Default RE: How hot is safe for brushless motor question

Wayne,

The definition of "hot" is subjective. Unless you are calibrated to what a brushless motor can really handle when hot to the touch, I would think that you are still ok. When I think a motor is hot, my motor designer friend, Tom Cimato says otherwise.

Remember to add an air cooling system to your plane to help keep it cooler in flight.

I'm using a 9x6 prop on my 25/35 with a 3-cell Kokam 1500 pack. If you decide to use a 10x6 prop, I would stick to the 2-cell pack.

Since brushless motors like voltage (over current), I feel that your better setup is what I'm using above.

Good luck!
Old 03-08-2004, 11:27 PM
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Wayne Miller
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Default RE: How hot is safe for brushless motor question

Hi Greg,

Thanks for your input, I tried different prop and motor configurations and I think your suggestion best fits my flying needs, it has the best current draw and flight time with out a lot of heat from the motor.

Thanks for your help,

Wayne Miller

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