Discussion:
Electric motor specs question.
(too old to reply)
bfairey
2008-02-07 13:26:33 UTC
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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.
Double Ace
2008-02-07 16:03:23 UTC
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Post by bfairey
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.
Motor RPM = 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.
Doug McLaren
2008-02-07 17:51:41 UTC
Permalink
In article <4581833f-39c6-44f7-834b-***@k39g2000hsf.googlegroups.com>,
bfairey <***@rogers.com> wrote:

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

Yes, that is a no-load rating, and your math is correct.

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 --

http://www.hobbylinc.com/htm/ast/ast600.htm

It's a bit dry, and often too technical, and somewhat out of date --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 ...)
--
Doug McLaren, ***@frenzied.us
Dear Lord: Please make my words sweet and tender, for tomorrow I may
have to eat them.
d***@yahoo.com
2015-10-11 13:43:13 UTC
Permalink
Post by Doug McLaren
If you want to know all about electric motors, get this book --
http://www.hobbylinc.com/htm/ast/ast600.htm
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.
The Natural Philosopher
2008-02-07 20:44:21 UTC
Permalink
Post by bfairey
The KV rating says 3900rpm/V I assume this is the no load rating, I
multiply the battery voltage times 3900?
Yes, and then multiply by the efficiency to get prop RPM under load.
Boradlty speaking. Its not totally exact, but close.
Post by bfairey
The rating for the ESC is 20v w/o bec what does this mean?
Thanks
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..:-)
Post by bfairey
Brian Fairey
btw this has nothing to do with model airplanes this is a piece of
test equipment.
Tim Wescott
2008-02-10 20:53:39 UTC
Permalink
Post by bfairey
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.
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.
--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
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