battery & motor voltages ? - RC Groups
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Mar 09, 2002, 12:27 AM
13brv3's Avatar

battery & motor voltages ?

Greetings again,

Please pardon the newbie question, but I'm not sure I quite understand the relationship of battery and motor voltages. In looking at speed 400 type planes, I notice that it's common to use a 6V motor with a 9.6V (8 cell) battery. I also know there's a 7.2V version of the speed 400, and it doesn't run as fast on the 9.6V pack. From this, I have to conclude that the controller is not stepping the voltage down for the motor, and that we're just running the motor with more voltage than it's supposed to have.

Is this correct? Does this overvoltage use burn up the motors quicker? I would assume so, but I'm guessing it's worth the extra performance since the motors are so cheap. What's the practical limit? 9 cells, 10 cells ??? Should you carry a fire extinguisher for anything over 8 cells

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Mar 09, 2002, 09:17 AM
Registered User
Nah, you don't need the fire extinguisher. The motor'll just be a molten mass of metal, no open flame!

But your instincts are good. The "reason" for higher than rated voltage is performance; and the higher voltage shortens life. Nothing is free eh?

There are limits to what makes sense as far as voltage goes. But it's probably going to vary with other factors as well. Largest of those variables will be the amperage being pulled by the motor.

The larger the diameter and/or pitch of your prop, the more amps the motor will draw. A gearbox will allow larger dia/pitch for the same amps.

I'm pretty new to all this as well. Started reading about 6 months ago. I've been flying glow for 6 - 7 years. A lot easier to understand engines than it is these electric motor things.

But, what I learned first is that there's no "right" answer to any question about power combinations. But there seem to be ranges. Keep reading and you'll get the gist.
Last edited by DBCherry; Mar 09, 2002 at 09:19 AM.
Mar 09, 2002, 09:21 AM
Registered User
By the way. We do the same thing to our battery packs. Most are rated for a particular amperage draw and charge rate. We generally exceed both by a wide margin!

Again, shorter life for the sake of performance.
Mar 09, 2002, 02:59 PM
13brv3's Avatar
Thanks for the confirmation. I hadn't put 2 and 2 together about the load/current relationship, but it makes perfect sense. I've read in review where the current is given for static tests, and I guess that would be a worst case for any particular combination. I have to say I'm surprised at the current draw of some of these motors. I would have never guessed it was that high.

I guess I'll just carry on the tradition of abusing motors then, and perhaps join battery abusers anonymous as well

Mar 09, 2002, 05:25 PM
Registered User
I guess I'll just carry on the tradition of abusing motors then
That's the spirit!!

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