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Old 01-24-2008, 04:36 PM   #1 (permalink)
Nomadic Chicken
 
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quick FYI for me on testing electric DC motors?

I'm using Craig's List to find a motor.
" wanted - used motor. atleast 7.5 inch diamater, DC, ..." etc...

i've already gotten a few replies, people don't seem to know how much the motors could be worth so i may be able to get away with under $100 for a good motor.

Now the hard question -

what size battery should i take to test a random motor of atleast 7 inch diameter to be sure it's good and appears to do what i need for EV?

or is there some way i can test one with a meter?

if i blow $100 on a BAD motor then the wife test will flunk me for weeks!

the add that i'm using:
Craig's List ad



Last edited by WaxyChicken; 01-24-2008 at 05:05 PM..
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Old 01-24-2008, 07:00 PM   #2 (permalink)
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Hey, I recognize that motor image in your ad!

You can test a big DC motor just fine with a fully charged 12v battery. Jump it off your car battery, if you like.

The motor in the ForkenSwift, unloaded, will turn at no more than about 1200 RPM on 12v. Too much voltage and you risk getting into a runaway situation (DC motors will do that under no load) and could wreck it.

Here are some things you should look for (you may have to remove the brush service covers to see this stuff):
  • ideally, you want an 8 brush motor (4 brush pairs)
  • the brush leads (thick copper strand braided/twisted wire, usually) should be coppery, not dark red/purple or brown/black (indicating overloading/stalling)
  • an internal shaft driven fan on one end is useful, but not strictly necessary (you can add external cooling)
  • the bearings should spin freely & quietly
  • the commutator surface (which the brushes contact) should be smooth, not scored. It should be a shiny, coppery or deep brown in colour (not blackened)
I'm probably forgetting something. But those are 5 or 6 tips I wish we'd had when we were out motor shopping. All we brought was a tape measure ... and a lucky rabbit's foot.
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Old 01-24-2008, 07:02 PM   #3 (permalink)
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Oh - and an external shaft (smooth, keyed or splined) will be much easier to deal with than an internal splined shaft (which is what we ended up with).
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Old 01-24-2008, 07:02 PM   #4 (permalink)
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Thanks!
and thanks for not asking me to withdraw the reference pic.

I'll probably print out your post before i go buy one.
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Old 01-24-2008, 07:12 PM   #5 (permalink)
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Also, stress that you're looking for a BRUSHED DC motor. I suspect you'll get people who don't know any better inviting you to come over and look at AC stuff.
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Old 01-24-2008, 08:40 PM   #6 (permalink)
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Heh, i just hope i can recognize one. I know jack about motors. The only ones i ever took apart were the ones that came in toys when i was a kid.
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Old 01-25-2008, 02:04 PM   #7 (permalink)
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BTW, you said a 12v battery. Car battery will work, yes? i think the one in my car has 750 cranking amps. i can jumper it to the battery for a test. (don't wanna fry it on amps.- i know even less about electricity.)
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Old 01-25-2008, 02:15 PM   #8 (permalink)
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Winter beater Metro - '00 Chevrolet Metro
90 day: 61.98 mpg (US)

Fancy Metro - '14 Mitsubishi Mirage top spec
90 day: 58.72 mpg (US)

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In short (no pun intended), you'll be fine with your car battery:

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Ecodriving test: Manual vs. automatic transmission MPG showdown



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Old 01-25-2008, 06:27 PM   #9 (permalink)
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Thanks Metro (dan?).
I've had replies for everything from AirCond motors to washing machine motors to Golf Kart motors. Hopefully in all the spam I'll find what I'm looking for. No one has said Forklift yet, but not everyone knows what the motor they have came out of so I'm trying to stay optimistic. If there's anything else you can think of that i should know then I'll be happy to hear it. I start shopping tomorrow. (too bad a golf cart motor won't be strong enough for a 4 seater car)

I'll determine which donor car to target once I find out what motor I can get cheap.
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Old 01-25-2008, 08:17 PM   #10 (permalink)
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Quote:
Originally Posted by WaxyChicken View Post
BTW, you said a 12v battery. Car battery will work, yes? i think the one in my car has 750 cranking amps. i can jumper it to the battery for a test. (don't wanna fry it on amps.- i know even less about electricity.)
When matching supplies to components consuming electricity you need to be concerned with a couple things that cause problems. For motors specifically you want the voltage UNDER LOAD of the supply, the battery here to be equal to or lesser than the rating. You can overdrive motors but not forever. It's very safe to do it repeatedly if its short, and you won't harm anything. Anyways, when you supply any voltage to a motor, the motor will "demand" amps based on the load, if the battery cannot supply it, it will just supply as much as it can and the motor will make do with what it gets. Likewise, if you can supply 4x the amps of what the motor is rated for, that's perfectly fine, for the most part. You have a controller that will help you there in the actual car anyways.

With other electronics, you always need to match voltage under load to the rated voltage. For circuitry it is generally important that you have a source that can supply the rated amps or MORE. Just because a power supply can carry 5 A doesn't mean it will send 5 A - it will only send what the circuit desires.

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