## Motor Theory Head Check Needed

Electric Motors and Controllers

### Motor Theory Head Check Needed

Yo,
I tested a Chinese BLDC and am starting to "run" the numbers.

I measured:

Kv=27.97 rpm/V
Kt=.252 lb-ft/A
Rmotor=.043ohms
Noload: Io=1A, speed=1324rpm @ 48.6V (batt)

So, here's where I need my head checked - if tis motor is stalled then the current at 48V through the .043ohm motor will be 1116A. I base this on the equation V=E+IR (applied voltage=backemf+IR). Since at stall there is no backemf then V=IR.

I'm a bit stymied as the motor current at stall seems extremely high.

Also, maximun theoretical mechanical output is equal to V^2/(4R). In this case that equals 13395W. Again, this seems extremely high especially since the motor is rated for 500W at 48V.

Maybe I have this correct but need not be concerned since the current limit in my controller is what really governs the current and power of the motor.

Any thought are appreciated.

Cheers.

mclovin
100 W

Posts: 184
Joined: Sat Apr 04, 2009 1:19 pm
Location: Seattle

### Re: Motor Theory Head Check Needed

mclovin wrote:Yo,
Maybe I have this correct but need not be concerned since the current limit in my controller is what really governs the current and power of the motor.

Any thought are appreciated.

Cheers.

yup
Thanks Justin of http://www.ebikes.ca/
Also a thanks to Methy at http://www.methtek.com/
And Dave who has some good deals on STUF
RC lipo and most other types of Lithium batteries you MUST know your individual cell voltages while charging and discharging.
Batteries of all kinds need respect they can burn your house down, so don't sleep with them under your bed or any other were you cant afford smoke or fire!
[color=#FF0000][b][size=150]Never above 4.2v never below 2.7v EVER!!!

Arlo1
100 GW

Posts: 5227
Joined: Sun Apr 26, 2009 10:36 pm

### Re: Motor Theory Head Check Needed

Arlo1 wrote:
mclovin wrote:Yo,
Maybe I have this correct but need not be concerned since the current limit in my controller is what really governs the current and power of the motor.

Any thought are appreciated.

Cheers.

yup

and thank goodness it does too. I wouldn't even be able to ride my SuperV if the controller didn't tightly control the current.
John in CR
100 GW

Posts: 10389
Joined: Tue May 20, 2008 12:58 am