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Old 06-01-2009, 01:39 PM   #1 (permalink)
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Question Electric motor questions

As Paul's Open ReVolt controller project nears completion, I am thinking it's time to get going on my EV journey. Several guys have mentioned doing an electric drive bicycle as a small project to get their feet wet. I'm thinking an electric go-kart would be fun. It would also get my 13 year old son into it, as well.
I've been around the forum for a while now, and I've read a lot of threads on EV conversions. But I still have a lot of questions. Mostly about power output of motors.
Is the wattage of a motor a good way of comparing them?
Is a 1200 watt motor going to give me twice as much speed/acceleration as a 600 watt motor?
How big of a motor would I need for a go-kart, anyway?
Anyone have some good links where this stuff is spelled out succinctly?

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Old 06-01-2009, 02:04 PM   #2 (permalink)
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Most electric motors have a horsepower rating, but wattage works fine too. You can convert it to horsepower (I like Online Conversion) However, its kind of a rule of thumb deal. Take your gas engine's horsepower and divide by 4 to get your electric motor horsepower. This is a rule of thumb because a 600W electric motor can put out just as much power as the 1200W motor. It just can't do it indefinitely, at least not without cooling (assuming brushes don't blow or something else). The 1200W motor will be able to sustain twice the power of a 600W motor without overheating or causing damage to the brushes or commutator.

I can tell you, with my ecorider mower, I'm using a 24 volt 1.6hp motor. This weekend I hooked it all up and drove it around and I was drawing 100 amps fully load @ 36 volts. This translates to 3600W, or 4.8 horsepower. Now, my motor is fairly large and didn't even really warm up for the short periods that I ran it around the yard. So, weight also has something to do with it. This is kind of why most guys measure their motors in diameter. Mine is about 7" in diameter. I'd also imagine a motor like mine being plenty for a go kart, especially if you have multiple gears. In third gear that thing got moving.

Of course, there is always a simple single geared setup like Ben's motorcycle too. That thing goes plenty fast too since its geared correctly. It still has plenty of torque too!

Have fun with the project! If you make it up here for an EV build day we might even be able to get you some parts. I know I have extra contactors.
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Old 06-01-2009, 02:47 PM   #3 (permalink)
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Thanks, Daox. That's very informative.
What's confusing is when there's no horsepower OR wattage rating. I'm looking at one that just says, "36 volts" on the data tag, and the seller says is 6.5" diameter and almost 11" long. I'm trying to get specs from the manufacturer, but it's just kinda frustrating when there's nothing familiar to reference to, like horsepower.
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Old 06-02-2009, 07:14 AM   #4 (permalink)
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It sounds like a DC Series motor. They're essentially mechanical-transformers i.e. they'll try to make as much power as your controller and batteries can throw at it. It may break or burn-up or something but it will try to convert all the electrical energy into mechanical energy.

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