AC motor as generator

Discussion in 'The Projects Forum' started by Fragger, Aug 21, 2009.

  1. Fragger

    Thread Starter New Member

    Aug 21, 2009
    Well my first post!
    I have read you can use an electric motor as a generator, I find this fascinating and want to build my own generator. To this end I have purchased this motor for the princely sum of $1 :D
    It is a 240 volt AC, 15 watt motor.
    My idea is to hand crank it and have it charge a small battery which is inside a solar/ac power pack, which can be connected to small appliances to recharge them.
    The problem is I know very little about electric circuits so I need some help desiging the circuit. Can I connect the motor to the AC adapter and charge the battery pack?
    I would be very grateful for for any help/advice.
    Thanks in advance.
    Hopefully this project will help me learn the secrects of electrics!
  2. JDT

    Well-Known Member

    Feb 12, 2009
    Although it is feasible to use an AC induction motor as a generator your motor is probably not the best type to use.

    If you want to charge a low-voltage battery your best bet would be to find a good quality, brushed, permanent magnet DC motor. It needs to have an operating voltage a bit higher than the voltage you want to generate. To test if the motor is any good, try shorting its terminals and try turning it by hand. You should feel some resistance to turning compared to when the terminals are open. I have found that "Maxon" motors are very good. When turned fast - via a gearbox - it should generate some useful current.

    To charge a battery, you will need to put a diode in series with the motor so that the current cannot flow backwards and run the motor!

    The only way to make your motor function as a generator would be to take it to pieces, remove the "shading rings" (the copper loops), and replace the rotor with a powerful permanent magnet. With the poles at opposite sides. Turn the motor at 3000 rpm and it will generate AC at 50HZ.
  3. russ_hensel

    AAC Fanatic!

    Jan 11, 2009
    What sort of appliances? Keep in mind that human output peaks out at about 250 w.
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    A lot of AC motors are of 'squirrel cage' design.

    The rotor is an array of copper bars without any form of magnetic element, the magnetic field is _induced_ by transformer effect while they are powered from the AC supply (hence induction motor).

    To use a motor as a generator, it needs some form of magnetic poles, whether these are wound field coils (so electromagnets) or actual permanent magnets.

    Many smaller motors (like electric drills etc) use series field coils, so they need lots of current to energise them. Ideally you want one with a 'shunt' field, where the field coils are designed to run off a separate supply.

    The only type I can think of like that which you may find in scrap is a washing machine motor?
    As these are mains voltage, you should be able to get some useable power out at relatively low speeds.

    The simplest option is a car alternator? Designed to give a regulated 15V out over quite a range of speed, though you will have to gear the input up to a decent output current, I'd guess a couple of thousand revs at least.
  5. Fragger

    Thread Starter New Member

    Aug 21, 2009
    Thanks so much for the replies guys!
    The washing machine motor sounds good and I should be able to find one cheap.
    What I really really need though is to know what the circuit should look like. Cheers
  6. Wendy


    Mar 24, 2008
  7. Fragger

    Thread Starter New Member

    Aug 21, 2009
    OK So I know I need a motor, a diode and some wire. What else do I need and in what order should they be? Someone here ought to know, please help me I really what to make this happen.
  8. jj_alukkas

    Distinguished Member

    Jan 8, 2009
    Not any diode, one rated for your motor current... And if its an AC induction motor, you will need a Bridge rectifier instead of that diode to get DC. Also some gearing and a crank will be necessary.
  9. Fragger

    Thread Starter New Member

    Aug 21, 2009
    Could someone please answer my quetion please.
  10. Bernard


    Aug 7, 2008
    The only stand alone AC motor that I know of is a synchronous with magnetic armature, like a clock motor that will generate an AC current. This I have used a variable frequency generator to test seismograph amplifiers. If a washing machine motor is connected to the AC lines & rotated faster than 1800 rpm, it will generate AC, maybe make meter run backwards? I've wanted to try this but it's a messy set-up with gasoline engine, otherwise could use a bigger AC motor to turn a smaller one stepped up with belt & pulleys. Would need a seperate meter to see any results.
  11. jj_alukkas

    Distinguished Member

    Jan 8, 2009
    Now that is Reverse Engineering !!
  12. awright

    AAC Fanatic!

    Jul 5, 2006
    The reason nobody has posted a circuit to turn your shaded pole AC motor into a generator is because it can't be used as you had hoped.

    Using a common squirrel cage motor as a generator is feasible, but not in the free-standing mode you imagine. An induction generator (same thing as an induction motor) functions by being connected to the AC power line, just as it would be as a motor. When it operates as a motor, the rotor spins at some speed less than the speed at which the magnetic flux created by the line current rotates (synchronous speed). That is why common AC induction motors are rated at (say) 1750 or 3450 RPM rather than at the synchronous speed of 1800 or 3600 RPM (for a 4-pole or 2-pole motor connected to a 60 Hz line). The difference between the operating speed of the rotor and the synchronous speed is called, "slip." The slip is not a result of poor motor design - it is a vital factor in the functioning of the motor since an induction motor running at synchronous speed cannot generate torque to drive a load! (Remember that we are talking about a squirrel cage induction motor here, not a synchronous motor which operates on a different principle.)

    Now, if you spin the motor at higher than synchronous speed, the motor will force energy back into the AC line, rather than taking energy from the line, thus acting as an induction generator. You can see that this is a far cry from a free-standing generator to charge a battery. It is, however, highly suited to wind turbines, probably the largest area of application of induction generators these days.

    The advice from the other posters that you start with a dc motor is good, in my opinion. It is difficult to get much energy out of a motor used as a generator unless you have an arrangement to spin it at fairly high speed or you have a special device with many poles made for the purpose. People have been experimenting with such home-brew generators for wind turbines for years and I gather that some work quite well.

    If you want to use a surplus motor, I recommend a servo motor rated for low RPM at the rated voltage. A brush-type servo motor is typically quite high quality and operates very smoothly. You will, however, need some kind of speed increaser if you are going to turn the generator yourself. Good application for a stationary bike! Charge batteries to power your TV that you watch while you pedal.

    Have fun.

    Last edited: Aug 29, 2009