Determining the correct voltage and watts of a motor?

[bajadudes]

Hello all,

I am starting an electric go kart project with my 8 year old son and had a question about electric motors. I read the excellent sticky and must admit that a lot of the math was over my head.

I know just enough about everything to get me in serious trouble.

I found the below Electrocraft motor in a Navy salvage yard back in 1991. I used it in a silly art project and ran it at 12 volts with a 6 amp simple bridge rectifier that I built. It seemed to run well with a **** of a lot of torque. What I want to try and figure out is what is the correct voltage and watts it was designed for.

It has 2 brushes.

Connecting an ohm meter and slowly turning it to felt magnetic stops I get anywhere from about 7 to 35 ohms.

If I spin the shaft at high speed with a drill it generates 69 VDC.

I looked at the electrocraft website but this motor is not listed not a P/N that is similar.

How can I go about determining the correct voltage and amperage?

I have a 35 amp 24VDC motor speed controller from an old electric scooter that I would like to use.

Any advice or comments would be greatly appreciated.

 

[itsid]

Hi,
I think you did all the necessary measurements already

if it produces 70V it can be powered with up to 70V
that is a not uncommon voltage for more adult servos so that'd fit

The resistance though.. well it's a servo and as such it needs a way to determin the rotation of the shaft, that's either done by more wires (more than two) or with the internal resistance

Anyways, the LOWEST internal resistance is what needs to be measured since that defines the maximum Amperage the motor can draw.
So with a min of 7Ohms that'd be ~10Amps

it may still peak at a higher value, but from what you measured it's nominal value should be 10Amps & 70Volts

leaving you with 700W

'sid

 

[bajadudes]

Thank you very much for confirmation of what I was thinking.

So slightly less than 1 HP. That does not seem like a lot especially since the thought, weight and expense of 6 12VDC gel cell batteries at $80 each is intimidating.

I bought 2 34 amphr ones today and tied them in series and connected them direct. The speed with the gear ratio was not that high and it was nearly impossible to stop the tire with my bare hands. It is probably appropriate for an 8 year old just starting. I am considering going to 48 volts but then I would have to buy a new controller.

Thank you for your response. It was greatly appreciated. Any other advice regarding an operating voltage would also be appreciated.

Right now the motor has a 15 tooth sprocket which drives a chain driven transfer shaft. The motor side of the transfer shaft has a 13 tooth sprocket and the wheel side is an 11 tooth sprocket. The wheel side sprocket chain drives a 70 tooth sprocket on the wheel.

 

[itsid]

well assuming you get it up to 2500 rpm,
with 11" rear wheels that'd be rougly 15mph..

overall ratio of 5.52:1 (15:13->11:70) (0.867 * 6.364)

powered by any voltage above it's threshold it should draw roughly the same amperage
and thus it's power would be 360W @ 36V or 480 W @ 48Vetc.

all electrical power consumption.
I assume that at half it's intended voltage it won't quite reach a good efficiency I'm afraid.
so it may not be able to provide more than 250W of mechanical power @ 36V or say 300W @48V

And to be honest with those values, there are much lighter and efficient motors out there for just a few bucks

don't get me wrong.. I don't say it wont work;
in fact I'm pretty sure it'll work well;
but it's not the most powerfull motor out there in the first place, and using it at half it's rated voltage will not exactly help

I'd say the motor that came with the electric scooter that your controller originated from,
would be a wiser decision ...
35 A @24V are 840W, since that's a peak value and electric power.. it's fair to assume it came with a 500W mechanical power motor;
which is most likely smaller and lighter than the one you have up there

and for the price of ONE of your batteries you can buy such motor new (my1020 unite motor)
or you get two more batteries and a new controller and end up with LESS power for more money

just sayin'

'sid

 

[bajadudes]

Wow, That's a very interesting way to look at it. Thank you for the clarity.

The motor from the scooter looks so small but I guess looks can be deceiving. I saw no electrical rating on the scooter at all. When I get home tonight I am going to see if I can look up the model. Can't remember what it is right now.

I did not realize you sell motors. I would be happy to support an individual such as your self. Is there a link or website that shows this 1020 unite motor?

 

[itsid]

No, I do not sell motors..

I said unite my1020, because it's the most used e-scooter motor today..
it comes in various sizes (one would be 24V 500W.. and that's mechanical power )
and since it's definitely what you'd call "mass production" it's really cheap. compared to servos like the one you have

I don't think you should spend more than ~$80 US on that (incl shipping)
here's one on ebay for $55
http://www.ebay.com/itm/500W-24V-DC...581?pt=LH_DefaultDomain_0&hash=item233f31303d

as you can see it's fairly small (roughly 4 x 5 inches)

'sid