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Determining current draw of a motor
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Determining current draw of a motor

by mwallace on Fri Sep 24, 2010 4:58 pm

I recently acquired a number of (10-15 year-)old motorized wheelchairs. The chairs themselves are in pretty rough shape, but the motors and controllers seem to be okay.

How do I go about testing the current draw (max, etc) of these things? I want to build new motor controllers for them, but I haven't a clue what load I should be targeting.

I've contacted the manufacturer and the response was essentially "uhhh... we discontinued those years ago. We don't have any specs we can give out." ):

FWIW, these are dual-motor chairs, designed to carry up to about 300 pounds at 6ish MPH.
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Re: Determining current draw of a motor

by adafruit_support_bill on Fri Sep 24, 2010 5:13 pm

Is there any kind of a rating for the motors? From HP and volts you could back-calculate. Or you could just measure it.

Do you have meter capable of measuring amps? You would want to measure them under load. You would probably want to start at a fairly high range (20A or so) and work your way down.

Another approach is to look at similar products and see what they use. That will probably get you into the ballpark.

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Re: Determining current draw of a motor

by mwallace on Fri Sep 24, 2010 5:43 pm

Sadly, no. I have the part number, manufacturer name, etc, but those have led me nowhere. The chair is 24V - 2x 12V deep cycle batteries, but that's as much as I know so far. I'm disassembling one of them now to see if I can find any more information.
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Re: Determining current draw of a motor

by zener on Fri Sep 24, 2010 5:46 pm

Get an ammeter that can handle high current. There are cheap ones for cars that aren't super accurate but would do that job.

Something like this:

http://cgi.ebay.com/Veethree-50-Amp-Amm ... 3cb0b41b0c

or this:

http://cgi.ebay.com/CAT-Caterpillar-Bat ... 23099c49eb

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Re: Determining current draw of a motor

by uoip on Fri Sep 24, 2010 6:15 pm

How precise do you need to be, and how much do you want to spend?

A hall effect current clamp that can handle both DC and AC is ideal for this kind of thing. Search for PDI CA-60 or Tecmel CA-60 (same device in slightly different branding). These will handle up to 60 amps, and output a millivolt signal that your multimeter can read. Not nearly as cheap as the meters that Zener pointed to. But it's the ideal professional solution.

If you just neat a cheap rough ballpark, buy a fuse holder and a handful of automotive fuses at an auto parts store. By process of elimination, figure out the smallest fuse that won't blow, and the largest fuse that will blow. That'll get you into the ballpark of the current draw, but it won't be very precise, for various reasons.

Or make your own current measuring device. A current meter is just a shunt resistor with a voltmeter in parallel. You can buy a precision professional current shunt, or you can just use a short length of fairly narrow wire. If you roll your own, your challenge will be to calibrate it by passing a known current through it and measuring the voltage. Or alternatively, just precisely measuring its resistance. Of course, with high currents, be sure you're not putting so much power into the current shunt to get things hot and start a fire -- if in any doubt whatsoever, do this away from flammable substances and on a fire-safe surface like concrete.

And there's nothing wrong with Zener's suggestions, either.

Note that any motor's current draw will vary wildly with the motor load and speed. It will be dramatically higher when the motor is stalled or starting under heavy load than when the motor is free running at high speed.
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Re: Determining current draw of a motor

by mwallace on Fri Sep 24, 2010 6:21 pm

Zener: Thanks! Those are a great idea!

uiop: I don't need precision - I just need a ballpark. ie: Does it draw a max of 10ish/50ish/300ish amps? Thanks for the sugs. I'm headed to an auto parts store in a few mins to pick up some other bits and pieces, and I'll see what useful bits and bobs they have on hand when I get there.
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