24V brushed motor power ??

[bentech]

Hi, i already built myself a e-bike using a power steering motor and 15:1 reduction gearbox from a chevrolet cobalt.

Now i want to built one with 2 motor. I'd like to know how to mesure the maximum amp or watt
of thah motor on 24V ???

I want to get two 24v controller (one for each motor) and the spec of the controller is 24V 500watt and 28 amp max.

How can i mesure if the motor will pull too much and blow the controller ??

I alerady mesure the resistance of the motor that is 3 ohm.

 

[major]

So your motor is 3Ω. You have 24V. Ohm's Law is useful to find the maximum current.

But somehow I suspect 3Ω is not the correct motor resistance. Where did that come from? Also, if it is a decent controller, it will limit the current to its maximum (current limit).

 

[bentech]

I have rechecked with a fluke and its more like 1.1 ohm.
That make more sense.
So this motor is a 524w and can pull 22 amp right ??
Is 22 amps is the max amp it can pull (stall) ??


Here the controller i want to buy:

http://www.cafr.ebay.ca/itm/e-bike-Motor-Brush-Speed-Controller-for-Electric-Bike-Bicycle-Scooter-24V-500W-/171271528995?hash=item27e0942223&vxp=mtr

 

[major]

The Fluke is likely a very good meter but I wonder if you are getting an accurate resistance measurement. The brushes and commutator can give a false reading to the instrument. But if 1.1Ω was correct, then yes, at 24V applied to the motor, 22A would be stall current and the maximum it could draw. Maximum power output would be at half that current, so 11A, or 264W input. There would be 133W in resistive loss, so output power would be less than 131W, about 1/6th hp. Rated power is typically less than maximum power. So quite unfair to call that motor 524W as you said.

If the motor resistance is actually much smaller, then the controller current limit should keep the maximum current draw at the 28A as specified.

 

[bentech]

i have try my setup on a bike last year with only one motor on 24 volt.
I have put a amp clamp to see what the motor draw and without a controller (only a on / off switch)
when i start the bike, amp go up to 45. than drop rapidly to about 12-14.

How can amps raise to 45 if the stall current is suppose to be 22 ???

 

[major]

How can amps raise to 45 if the stall current is suppose to be 22 ???

I mentioned several times that your resistance measurements are erroneous.

 

[parabellum]

I suppose, resistance of your motor is low enough to push "28A+-1AShort Voltege" , now we multiply it by 24V and get 672W in to motor! Now 672W, less copper loses, less iron loses, less brushes to commutator electrical and mechanical resistance and whatever loses else (maybe centrifugal fan inside for cooling).

 

[bentech]

Ok so what about make the engine stall in a vise and check the amps with a amp clamp ??

i can't go wrong with this method to find the real specs of this motor ??

 

[major]

Ok so what about make the engine stall in a vise and check the amps with a amp clamp ??

i can't go wrong with this method to find the real specs of this motor ??

Well, you can damage the commutator, so be careful. Use a lower voltage and keep the duration at stall (locked rotor) short. Do it at for just a second or two; no longer than enough time to take the V and I readings. I'd use maybe 5 Volts and then 10 Volts running the motor for a minute or so in between tests to even out heat on the comm and letting it cool afterwards. Then take the average of those two tests (calculated equivalent resistance). That should give you a reasonable resistance value to then calculate the stall current at higher voltage.

 

[Hillhater]

Even when you have the accurate resistance and max current draw, you still wont know what the motor "power" actually is.
You can find what the max power TO the motor is, but you wont know the efficiency, losses, or power output....which could be much less than the power input.
Also you wont know what the useable safe input or output power is without some complex testing.
So unless you have a lot of time and motors to test, i suggest you accept Majors advice that the controller will most likely limit the current to 28 A max ,as specified.
Its more likely that the motor will overheat before the controller suffers.
You should plan on monitoring the motor temperature closely in normal use ,until you are comfortable with its stable temp levels.
Note, ..I have used those (or very similar) controllers, and can confirm that they do limit the current ( mine at 29A precisely) even when using voltages above the 24 v rating ( 36v and 42 v used)