# Electric motor specs question.

• posted

The KV rating says 3900rpm/V I assume this is the no load rating, I multiply the battery voltage times 3900? The rating for the ESC is 20v w/o bec what does this mean? Thanks Brian Fairey btw this has nothing to do with model airplanes this is a piece of test equipment.

• posted

Motor RPM =3D KV X Voltage X Motor Efficacy Factor (usually about 80% -

90%)

Most ESC's require, when used with batery packs having voltages great than 14 volts or so, that the internal BEC be disabled and a battery or external BEC be used to power the receiver. The internal BEC is disabled by cutting the red wire or removing the red wire pin connector from plug that goes to the throttle connection on the receiver.

• posted

| The KV rating says 3900rpm/V I assume this is the no load rating, I | multiply the battery voltage times 3900?

By itself, the kV rating assumes that your motor is perfect -- no resistance, no drag, etc. Such a motor does not exist, of course, but your math is probably pretty close. If you include specifications like idle current (Io) and internal resistance you can pretty much model exactly what the real-world motor will do under certain conditions.

In any event, the kV rating is perhaps the most important one, as it tells how `fast' the motor wants to go. If the kV rating is too high, you can reduce it by using a gearbox -- a 2000 rpm/V motor with a 2:1 gearbox is essentially the same as a 1000 rpm/V motor without the gearbox, all else being equal.

If you want to know all about electric motors, get this book --

It's a bit dry, and often too technical, and somewhat out of date -- (but the physics involved has not changed, of course -- it just barely mentions brushless motors, for example) -- but it's a good read.

| The rating for the ESC is 20v w/o bec what does this mean?

It means if you don't use the BEC, the ESC can handle up to 20 volts. If you do use the BEC, the voltage it can handle will go down, depending on just how much current you draw through the BEC.

(The BEC is probably just a 5v linear regulator on the ESC, designed to provide 5v of power for your servos and receiver. I'll assume that you understand what this means and the implications of how inefficient a linear regulator is ...)

• posted

Yes, and then multiply by the efficiency to get prop RPM under load. Boradlty speaking. Its not totally exact, but close.

It will handle 20V - about 5s LIPO*. It has no built in regulator to provide 5v for servos and the receiver. Or it might mean that you can use 5S LIPO to drive the motor,but the onboard BEC won't be useable at that voltage.

• peak fully charged 5s pack is actually 21V, but it won't stay that way once the motor starts..:-)
• posted

If your test equipment has a high starting torque a sensorless brushless motor controller may not work -- they start things rolling 'open loop', and don't have to generate much torque against a slowly-moving propeller. Depending on how your controller implements this starting up mode a high starting torque could confound it.

But try it out -- it may be fine.

• posted

Doug, it appears that the book you mentioned is "Discontinued, no longer available for purchase". Can you supply an ISBN or author(s) or publisher so that I could look for it elsewhere? Thanks.

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