First, I think you mean 'DC' generator, assuming the motor is a PM type with commutator that runs on DC.
Roughly speaking, if the motor's no-load speed is, say, 10,000rpm at 8V input, then you need to turn it at 10,000 rpm to get 8V out as a generator.
You can rewind the motor, but it looks like you'll need 10 times as many turns. Maybe better to start with a motor that has a no-load speed somewhat lower than the speed you'll be turning it when it's connected to 8V. Paul Mathews
Assuming that you have a PM motor...your DC motor will generate a DC voltage when turned from an external force. The 0.8V AC that you're seeing is the ripple on the DC output from the motor. You won't get AC from a DC PM motor. In order to get AC from it, you'll have to rewind it so that you have the windings connected to a pair of slip rings, which will replace the commutator segments. Realistically, unless you want to do this for the learning experience, I suggest that you find a small AC generator that will give the output that you want. Cheaper and much less painful in the long run.
-- Dave M MasonDG44 at comcast dot net (Just subsitute the appropriate characters in the address) Never take a laxative and a sleeping pill at the same time!!
Hi all
I have a DC motor from a old RC car and I want to make it into a AC generator with an output of around 8V or greater. But when I run it from a drill the output that I get at the terminals is 0.8V. If I were to rewind the motor with a finer wire would that increase the output or just the current produced? Also is you have any other suggestions on how to get a greater voltage thats will be great.
Thanks in advance
It is unlikely your drill spins as fast as the motor did when running as a motor, so your output voltage will be lower. More turns of finer wire will raise the voltage (and lower the safe current).
You can't get AC from a DC motor because the motor has a commutator. The commutator is a series of segments that essentially keep the armature changing magnetic polarity (make it act as if it were driven with AC) with respect to the field. Reverse the process, and DC comes out.
Most small AC generators spin permanent magnet(s) in a stationary field (easier to make and no brushes or slip rings to worry about).
If you want to tinker and have a source of parts, stepper motors make ac when you spin them. They come in a wide range of voltages and currents. They usually have a lot of poles so the output frequency will be high.
Split phase capacitor motors (AC mains types) can be run as AC generators with outputs similar to the mains and relatively high currents.
Synchro Selsyns can be used to make AC. Armature and field have windings and armature has slip rings. They are a sort of rotating transformer. Feed DC to the armature/rotor (or field) and take AC from the Field (or armature) while spinning the shaft. Synchro's are dinosaurs that were used as computers and remote positioning applications in WW2.
You don't say why you want AC, but if it is a frequency sensitive application, frequency changes with speed and the number of poles, so things like transformers or AC motors may not be happy with higher or lower frequencies than they are designed for.
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Then, as been mentioned, a drill will go nowhere near that fast - I'd even be surprised if a drill ran at 800 RPM.
But if you want to futz around with mechanical stuff, you could make a pulley to run the with the drill, and a belt to go around the motor shaft. A 10:1 diameter ratio will give you a 10X speed ratio. It could be done in an afternoon with a wooden disk and fat rubber band.
Good Luck! Rich
Hi all
In regard to my original post. The motor that is being used is a PM with a speed rating of 8100rpm. This is just going to be used for the purpose of a learning experience and so current is not important also I had made a mistake in the stating AC (like you have picked up it was meant to be DC).
Thanks in advance
Your drill may be more like 650 rpm. Electric drills are geared down for torque at the expense of speed. When spun at the same speed the motor turns, as a motor, it outputs a similar voltage. That's what "back EMF" is about.
So, if current isn't important, winding with fine wire is the answer in raising voltage at lower spin speeds. Turns ratio should be reasonably close - double the turns and double the voltage for the same speed. Inductance and resistance increase and brush losses probably go up too.
As Rich points out, mechanically increasing the speed with a pulley will work. With an electric drill, chisel, and some wood, it is fairly easy to "turn" (make it on a homemade lathe) a pulley. Making a hub to fit the pulley to the motor - generator could be a little harder.
Rubber bands are fine for slow speeds or ramping up to speed. "O" rings have less stretch and will last longer.
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What's the rated no load RPM (running as a motor) and how fast were you spinning it with your drill? Was that 0.8V output AC, DC or an AC ripple superimposed on DC?
Its not going to be easy to convert a commutated (DC) motor into an AC generator. You are going to have to replace the commutator with slip rings or otherwise make some major changes to the motor.
Why not run it as a DC generator and build an inverter?
-- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ If the first attempt at making a drawing board had been a failure, what would they go back to?
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