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Old Aug 13, 2012, 10:18 PM
Thermite + ice = Big boom.
boaterguy's Avatar
Canada, ON, Ottawa
Joined Apr 2009
2,211 Posts
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Generating current with a generator (brushed motor)

This doesn't really have an application yet, but I was mostly wondering.
How is current generated with a brushed motor? Not like how does a generator work (magnetic flux and blah blah blah) but more how much current will it generate. When running off of a power supply (lipo battery), the load (brushless motor) will draw current based on how it is loaded. If it is loaded to a higher current than the power supply can handle (ignoring motor and ESC limits right now) then the pack will heat up and drop in voltage significantly. So if I use a generator and spin it to a set RPM (more RPM = more voltage, correct?) and then load up my power motor to a set current, how will I know how much my generator can supply? What if I load up to higher than this?
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Old Aug 14, 2012, 04:20 AM
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Zen09's Avatar
Germany, HB, Bremen
Joined Jun 2010
919 Posts
OK, you have an idea how things work, but you should study basic electrical laws like Ohm's & Kirchhoff and some electromagnetism to understand what's going on there.

I'm not going to explain everything (I don't have a 100% understanding anyway), but:

Assuming the generator won't drop rpm under load (let's say it's turned by a powerful ic engine), the only difference between it and a battery will be the internal resistance.

Every generator (chemical, solar, electrostatic(capacitor) electromagnetic(spinning generator), etc.) consists of an ideal voltage supply (with unlimited supplied current potential) and a series internal resistance. The maximum current in the circuit (short circuit current) will be I = E/r.
The brushed generator will have a much higher internal resistance (the windings) than a lipo, so it's going to give you less current (and drop voltage faster under load).
Of course there's a current limit (determined by thermal transfer) that if you exceed, the generator will overheat and eventually burn.

If there's something else unclear please let us know.
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Old Aug 14, 2012, 08:28 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,159 Posts
Quote:
Originally Posted by boaterguy View Post
I use a generator and spin it to a set RPM (more RPM = more voltage, correct?) and then load up my power motor to a set current, how will I know how much my generator can supply? What if I load up to higher than this?
It should be able to supply close to its rated current when operating as a motor (provided that adequate cooling is supplied). However, the output voltage will drop substantially.
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Old Aug 15, 2012, 04:36 AM
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Joined Feb 2010
183 Posts
For motors and generators, there's usually this relationship:

voltage is proportional to rpm
current is proportional to torque

The operation characteristic of an electric motor is usually a constant current up to the maximum rpm, i.e. you can get up to the maximum torque from zero rpm up to the max specified rpm. If you go over the spec'ed rpm, torque drops (i.e. the motor can spin a bit faster than specified, but not with full torque).

Now for the other way round:

Spinning the motor at a constant rpm generates a constant voltage at the output. The required torque to keep it spinning then depends on the load. If it's a simple resistor, the flowing current (being proportional to the torque) follows Ohm's Law.
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Old Aug 15, 2012, 07:58 AM
Thermite + ice = Big boom.
boaterguy's Avatar
Canada, ON, Ottawa
Joined Apr 2009
2,211 Posts
My posts aren't going through on this thread anymore...
I'm starting to understand how and why it all works now, thanks guys.
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