DC motor/generator

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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



Re: DC motor/generator


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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
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Re: DC motor/generator



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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.

-

Re: DC motor/generator


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



Re: DC motor/generator


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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


Re: DC motor/generator



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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.

-

Re: DC motor/generator


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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: snipped-for-privacy@Hovnanian.com
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Re: DC motor/generator



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I have an orange that I want to use as a Ferrari, but have resigned myself
to an intermediate solution:  I bought an MX-5.  It gets me from point a to
point b with much less effort than the orange.

Cheers,
Alf



Re: DC motor/generator


finger to keyboard and composed:

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DSE have one for $4.

P8951, 920mV @ 31mA to 9.1V @ 220mA, 1000 - 10000 RPM:
http://www.dse.com.au/cgi-bin/dse.storefront/4285bd4b0c1f9aea2740c0a87f9c0752/Product/View/P8951


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

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