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How can I convert an AC motor to a generator? Answered

So for a school project, I am required to convert an AC motor to a generator. Actually, it is a table fan of diameter approximately 10-12 inches, and runs on AC power at 220V 50Hz.
I want to convert it into a windmill, but I'm stuck on the power generation part. So using materials easily available in a high school physics laboratory, how can I convert an AC motor table fan into a generator?
Also, what would the output of the generator made in volts and amperes?


Came across this while doing some research for induction motors as generators,

the basic idea is to match your rpm so that your value of slip goes from positive to negative. Say slip is 3%, then the rpm must spin faster then the synchronous speed so that slip become -3%


What you probably have here is a universal motor, so it has no permanent magnets only electromagnets. So what you really need to do is cut out the electromagnets wound into the case and replace them with permanent magnets.

That there is not an easy task, I have tried that before with a AC drill motor (also universal) and had little success (somewhat to do with my undersized magnets) and only produced 0.5V.

However If you separate the wiring from the case electromagnets and the rotor or commutator electromagnets you may be able to input power into the case electromagnets, spin the motor and produce additional energy out of the rotor electromagnets, in essence how a alternator works.

Sorry MAD Sci robot,  But a universal motor would be way too noisy
for a quiet whispering fan.

All table top fans are powered by an induction motor which is a perfect mate for the fan blade,
  • At low RPM where the  induction motor has lo torque the fan blade takes very little torque to start turning.
  • At running blowing speed the induction motor develops its maximum  torque just as the fan needs that torque.


The trouble with "fans" as generators is that you have to spin them slightly faster than they would have spun as motors, and they lack magnetic field to generate with, unless you add some external capacitors.

So 1.) Spin it very fast (you may need to use belts and pulleys) and 2.) add some capacitance between the two windings

Well I can deal with the problem of spinning fast, because I have a powerful 16,000 RPM blower (Black&Decker KTX5000) available at my disposal.
but what about the capacitance between the windings? I have never really worked with capacitors, so could you just break it up for me?

thanks for your quick reply man!

16K would be WAAY too fast.

Its really a bit "suck it and see" - there are plenty of videos under

"Ceiling fan alternator" which should help you.

The capacitor values depend greatly on the load and speeds.

Done right, this is a fascinating topic for an EE student.

this is for my EE experiment only!
thanks for the heads up about the tutorials, I'll try them out and i'll ask you again for help if i get stuck somewhere.

thanks again!