Author Topic: Dc motor output calculations  (Read 2700 times)

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MN Ben

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Dc motor output calculations
« on: December 02, 2012, 02:20:46 PM »
I found this motor that I am thinking about purchasing for $25. I think I did the math correct according to the FAQ sticky ( http://www.fieldlines.com/index.php/topic,143574.0.html ). The tag of the motor is shown in the pic below. It reads 4400 rpm , 100 v means my 12v cut in should be at 528 rpm, correct?

Not to see what my little PVC blades will make in regards to rpm's. if you know of a links let me know.

Just dabbling with little windmills.

taylorp035

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Re: Dc motor output calculations
« Reply #1 on: December 02, 2012, 02:41:42 PM »
That's 44 rpm/volt, which a bit on the high side.  528 rpm is really fast for a cut in speed for even a 4' diameter blade set.  Mine was ~35 rpm/volt and it took a good amount of wind to get it to reach cut in speed.  Something closer to 25 rpm/volt would be better, but obviously harder to find.

Bluevitz-RS

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Re: Dc motor output calculations
« Reply #2 on: December 02, 2012, 10:56:00 PM »
I've found the calculation also needs to have the efficiency added into the mix.  I got myself an AMETEK 40V 1050RPM motor and though that calculates out to 26.25rpm/V it actually needs about 45rpm/V to reach 13.8V. or about 600RPM instead of 360 due to the motor's inefficiency.

I had previously got myself a treadmill motor rated at 90V and 6500rpm.  It cogs really bad and so all I use it for is a generator on the back of my bike trainer.  I'd skip the treadmill motor if I were you.

Edit, so you basically take your RPM÷V but also ÷ it by the motor efficiency to get a more realistic value.
« Last Edit: December 02, 2012, 11:00:10 PM by Bluevitz-RS »

Flux

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Re: Dc motor output calculations
« Reply #3 on: December 03, 2012, 04:06:54 AM »
That is right, the quoted speed will be with 100V and producing full output. The no load speed that gives a better idea of the emf will be quite a bit above this.

What is already very fast from your calculation will probably turn out to be too fast for anything except very fast wooden blades and if it cogs or drags significantly it may not start in sensible winds with fast blades.

Flux

MN Ben

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Re: Dc motor output calculations
« Reply #4 on: December 03, 2012, 01:27:20 PM »
thanks. I'll save my $25.
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MN Ben

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Re: Dc motor output calculations
« Reply #5 on: December 03, 2012, 01:39:06 PM »
.....unless I set up some pullies to increase the motor speed. I'm not looking for much, just something to tinker with over the winter.
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Bluevitz-RS

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Re: Dc motor output calculations
« Reply #6 on: December 03, 2012, 01:50:17 PM »
I thought of that too but by the time you gear it up fast enough, the cogging gets so bad you need a massive machine to even start it.