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ghurd
Junior Member
USA
314 Posts |
Posted - 12/04/2008 : 19:24:29
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Sure is. But I'm not that smart. 
Don't push a lot past 18A, and it should be OK. HaHa.
They often won't go to 18A (or name plate rating) anyway. That's more like my luck. Some will go past double the name plate amps.
Some crazy things start happening at high RPMs.
The coil's inductive reactance can come into play. The brush losses change (I think). Eddy currents in the core. Stuff like that. Coil resistance (DC) is still important.
More important is the duty cycle, at the rated power. Heat builds up (the example needs a fan((the flywheel))). Windmills make power when it's windy, blowing away heat, meaning the amps can be pushed a little more than expected.
Enough of that.
130V/18A= 7.2 ohms. That's the best guess at the resistance. (Ohm's Law)
325RPM is 13V. 650RPM must be about 26V?
26V - 13V(bat) = 13V "extra".
13V "extra" voltage / 7.2 ohms = 1.8A. (Kirchoff's Law)
One step more, 975RPM... , ...= 3.6A.
Insult to injury? Treadmill motors are rated very strange. They may have a couple voltages, amps, HPs, and RPMs on the tag.
IE: Motor in the link.
130V and 2.5HP? 1HP=746W. 2.5x746= 1865W. 1865W / 130V= 14.3A.
130V and 18A. 130x18= 2340W. 2340W/746= 3.1 HP.
2.5HP and 18A. (2.5x746)/18= 103.6V
So what IS correct? Is it 14.3A or 18A? Is it 2.5HP or 3.1HP? 130V or 103V?
I don't know.
The inductance comes into play. A LOT.
Higher RPM = more reactive component. I have a feeling the higher the name plate RPM, the more it comes into play, but I can't prove it.
It's not very practicle to calculate inductance unless you already have the motor. If you already have the motor, connect it to a drill press (I can't recall what that's called in English-English)and battery and measure the amps at different speeds. Quicker, easier, and more accurate.
Most of that math doesn't hold up in real life, with a windmill, in my experience.
It sometimes seems we are using these motors at a very low RPM and voltage, compared to what they were designed for, and that helps a lot.
Fungus might be able to explain it better. My typing finger is tired.
G-
(PS- G., shoot me an email, G-)
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