How many Amps should I be getting?

Hi all, I think I'm suffering from "I can't see the wood for the trees" here,
I've got a Permanent magnet DC servomotor that produces 18V at 200RPM, the motor's measured resistance is about 6 Ohms, so how many Amps should it be producing approximately?

Cheers for your help,

Hef :)

The motor rating plate quotes :-
Max RPM 1600.
V140.
Pulse? Amps 37

Obviously these figures are when used as a motor, not a generator as I'm trying to do.
Anyone any ideas please?

Hef :?

hey hef,

some nice volts per rev with that. high resistance though. that resistance could be from many turns of tiny wire in a small package? (not so good) or many turns of larger wire in a larger package..have you looked at the wire? just a 2 wire brushed motor? with maybe some other wires for the encoder..was there a brake on the thing?

folks may be hesitant to comment with that "pulsed" amp rating? think that may be calling out a drive spec for running the thing and would guess that a brief (fraction of a second pulse) will not say much about the continious ratings of the motor.

could easily be all wet here, but using the 6 ohms we might expect a short circuit current of 3 amps @ 200 rpm. (18/6=3) and about 660mA for charging current into a 12v battery ...18voc-14v needed to charge=4volts/6ohms=.66amps and .66x.66x6=2watts (I*I*R) "heat" and .66x14=9watts charging

at 300 rpm it might look like this.. 300rpm x .087volts per rev =26voc and 26-14=12/6=2amps charge into a 12v battery. 24watts of "heat" in coils and 28watts charging.

at 500 rpm? 500x.087=43voc 43-14=29/6=4.8amps charge..with 138 watts of heat in the machine and a charge of 67 watts.

500 rpm at 24 volt charging? 43voc -28v=15/6=2.5amps and I*I*R=37 watts of generator heat and 70 watts of charging.

i'm sure there is more to it than that but a close guess?

perhaps a "more experienced eye" might jump in to help us both out in our understanding. i'm not afraid of being wrong as long as i can learn from it!

cheers, dave

Thank you very much Dave.
When I tested it i got 3 Amps showing, but i wasn't too convinced that i was testing it correctly!
It's a 25 pound motor, so fairly big and the wires are fairly thick too. It DID have a tacho, but that's been removed now.
I've got it on Ebay now, due to a change of plan unfortunately, and I didn't want to mislead anyone with incorrect info.
Cheers for your help, Hef :)

Link to Ebay> http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... :MESELX:IT

cheers hef,

however this has been bugging me since wednesday, seems i've missed a step and using that measured ohms and shorted amps (no power) is not giving an accurate indication of predicted power under load.

we went back to some notes while working on a pedal power rig and found an old discussion with info we used to figure out the internal resistance while under a known load..these are the kind of "ohms" needed to make any kind of guess for rpm and output. and would guess that it would be much less than the measured "6 ohms"

here's that informative discussion on "qualifying a motor as a generator"
http://www.fieldlines.com/board/index.p ... 39040.html
(ben, sorry to post links to other forums..this one is on topic help?)

this is nice because we can plug and play with the forumla. but it's still easier to just run it into a battery and just see what it does?

cheers, dave <<=guilty of too wide a brushstroke

edit: wacky spelling

Thanks again Dave, It'll be interesting to test stuff with that formula and see what I get.
There's sooo much stuff on the net, it's just finding it!
All the best,
Hef. :D