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Electric Generator driven by Electric Motor

  1. Jun 17, 2008 #1

    I'm just an enthusiast. I want to generate electricity by driving a/c generator with the help of a motor and that motor will be powered by an inverter. Usually we see generator driven by an engine commonly known as "Electric Generator". But in my case it will be the motor which will drive it. Here in my country there is a serious problem of power shortage. I already have inverter (2000 watts) in place but it cannot turn the refrigerators on. So, i came up with this idea (perhaps stupid) to drive a generator(1.8hp) with an electric motor(2hp) and inverter(2000 watts) will feed electricity to the motor. IS IT POSSIBLE?

    HELP ME :) :rolleyes:
    Last edited: Jun 17, 2008
  2. jcsd
  3. Jun 17, 2008 #2


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    If I understand what you are asking then you are going to have the same problem starting the 2hp electric motor as you do starting the refrigerator motor.

    The average refrigerator draws less than 2000w while operating but the startup surge can be larger than 2000w.

    You need a bigger inverter.

    Or you could possibly drive your AC generator with a DC motor directly from the batteries.
    This setup is a mechanical inverter rather than an electronic inverter.
    A small flywheel could help get by the startup surge with this setup.

    Edit: There will be some problems getting the RPM right on the DC motor to get the correct frequency AC.
    Last edited: Jun 17, 2008
  4. Jun 17, 2008 #3
    What if i use capacitors to help and share the startup load?
    My refrigerator is around 1000 watts!
  5. Jun 17, 2008 #4


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    I doubt that a 1000w refrigerator draws more than 1500w on startup.
    I would think that your existing 2000w inverter should work.

    Do you really have enough batteries to drive the inverter to full power?
    That is around 180 to 200 amps from 12v batteries depending on efficiencies.
    The battery internal resistance and wiring resistance can drop the input voltage below shutdown trigger if the battery pack isn't right.
  6. Jun 18, 2008 #5
    I have two batteries 185amp each.
  7. Jun 18, 2008 #6


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    If you already have electrical power available, why would you use it to power a generator to then generate electrical power? That seems pointless.

    It sounds like you just need a soft start for your equipment instead. However, the power required for the equipment won't change. Instead, the dynamic start-up requirement will essentially be "spread-out" over a longer time (i.e. the in-rush current is reduced). I presume your source is tripping off-line or otherwise being overloaded yes?

  8. Jun 18, 2008 #7


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    My guess is that is the amp hour rating of the batteries.

    The amp hour rating is not valid when the battery draw is high.
    Amp hour rating is usually defined with a 20 amp load, not a 100 amp load (200/2).
    So even if you get the refrigerator to start it might work less than an hour before the batteries are dead.

    If your batteries are new the input voltage is probably dropping to about 10v when you try to start the refrigerator.
    If they are old batteries the input voltage will drop even more.
    This input voltage drop is enough to trigger the shutdown circuit on your inverter.

    You need more batteries and you need to have battery connections that can work with the 200 amps required by your inverter.
    I'm thinking that you need a minimum of six batteries to do this.
    10 batteries would be better.
    Wiring should be #4 or #6 depending on where and how long the wire is, it may need to be thicker.
    Last edited: Jun 18, 2008
  9. Jan 24, 2010 #8
    You may try a 1/3 hp motor with a smaller diameter pulley on the generator to assist in initial startup. 2000 watts is plenty for a refrigerator, but with a full charge, your batteries may last 30 minutes without recharging. The idea behind the 1/3 hp motor is you can find one that runs at 4.5 amps, so your draw is roughly at the most 550 watts, and 1/3 hp is enough to yank your arm off, once you get it going. The initial surge to start the motor connected to the generator is where your going to need a lot of juice, and this may require at high as 4000 watts. You can power the motor on and off, to slowly get it spinning, and have all other loads off until you get it started. Kind of funky, but it works.

    There are a few other factors to consider, like emf (electromotive force), and whether the motor generator combo is going to draw more power due to drag, etc.

    You may also need more than two batteries. 6 or 7 will give you more to work with. Once your batteries are fully charged, the fridge will run off the 2000 watt inverter with no problem, but for how long depends on your battery size and charge state. 185 amp hour batteries are an excellent start.

    Let us know if you make any progress.
  10. Jan 26, 2010 #9
    Thanks I'll try!!!
  11. Apr 26, 2011 #10
    Ba84 your problem is simple to recognize but may be a litter more difficult to diagnose. Well first you need to know that any air-conditioning, or refrigeration compressor draws a 5 to 6 times is nominal current on the star up for a compressor of 115V and a nominal current of 8.6 amps the energy for starting the compressor would be between 5000w and 6000w that is a lot more than 2000w so my best recommendation for you is to try with a starting kit or big capacitors just for the start and once the operational speed is attained the current draw will decrease to the normal range that is just a few seconds or less than a second but that instantaneous big current is what counts for the starting of the current. I know this because i am a a/c and refrigeration technician with some experience. By the way do not use that arrangement of electrical circuits, motor and generator because you will end with less power because they are not 100% efficient and for example a generator is usually in the 50 to 60% range of efficiency. Well i hope this help you.
  12. Apr 26, 2011 #11


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    no its not.... you cant produce more than 2000W from a motor and generator driven by 2000W because of all the losses in the system, you may end up only generating 1000 - 1500W Therefore you will be getting out much less power than you are putting into it...
    its the laws of physics!! :)

    if you batterys and 2000W inverter cant start up your fridge then the ~1000W from the generator surely isnt going to do it

  13. Apr 29, 2011 #12
    please design power & control circuit three motor working in sequentioal directonline and reversible
  14. Nov 22, 2011 #13
    i would like to build this as my final year project in a few years from now, i really need ideas...i plan to make the system efficient by letting or creating a pathway in which the inverters or batteries in the operation charge them selves which they operate just like that found in modern car batteries
  15. Nov 22, 2011 #14

    jim hardy

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    indeed starting current on a common ac motor is several times running current

    it is conceivable that a substantial flywheel would let you briefly overload a generator and achieve start
    but inverters are notoriously weak-kneed in the face of overload.

    since you are still in the study phase ---- try this
    energy in a flywheel = 1/2 I*w^2
    where I = moment of inertia, w = radians/sec
    figure 1/2 sec to start the fridge, size flywheel for 10% loss of speed or whatever the inverter will allow

    old jiim
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