Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Induction motor generator capacitor

  1. Oct 22, 2014 #1
    Hope I can get some help here. I have a 1-1/2 hp. 115/230 volt single phase induction motor I want to use as a small generator. currently it's wired for 115 volt I need help with the math to
    calculate the proper capacitor/s to use. Running no load amps as a motor draws 18 amps.
    Name plate states:
    110/230 volt single phase
    UF: 500
    Rpm 1720
    1.1 kv.
    duty cycle 100%
    Amps 18/9
    4 pole
    I want to use this for 110 volt output. This motor is commercial grade. I understand slip so the rpm needs to run at around 1875. Please any help would be grateful.
  2. jcsd
  3. Oct 22, 2014 #2

    P = 1100 W
    V = 110 V
    f = 60 Hz
  4. Oct 22, 2014 #3
    Capacitors for what ? Starting or power-factor? In your data I believe the UF:500 means it needs a 500uF Cap for starting. Also if the Gen is dedicated to this motor - switch both to the HV ( 230V ) taps. It will start easier. NEMA motor name plates have a specific requirement - so a picture may actually tell a little more.
  5. Oct 22, 2014 #4
    As far as I understand, he wants to use the motor in an async generator regime. So this isn't a "start-up" cap but cap that provides necessary reactive power in a continious regime for wanted generator output. Value of the cap isn't critical (can be somewhat higher or smaller than given by formula), but there is quite a big diference regarding voltage output: for 230 V it is cca 55 μF and for 110 V it is 240 μF(!). Low voltage costs more money :D
  6. Oct 22, 2014 #5


    User Avatar
    Science Advisor

    I have never seen an easy way to run a single phase induction motor as an AC generator. There is no question that it can be done with a symmetrical three phase motor.

    With a 3ph motor the inductance of the phase windings can be measured by reading the inductive current flowing while the motor is stalled, rotor locked. Given the frequency required to be generated, a delta of capacitors can be connected to the motor that will be LC resonant with that motor's inductance. The motor must be spinning at a rate slightly faster than the self resonant frequency before it will self excite and begin to generate power at the resonant frequency. Overloading the motor/generator will destroy the self excitation and so self protect the generating motor.

    A single phase motor has two different phase windings in an “L” arrangement that can be arranged as an asymmetrical three phase topology, so I guess it should also be possible. Getting three different capacitor values that resonate at a similar frequency might pose quite a challenge.
  7. Oct 22, 2014 #6
    Ohh - re-read and I agree, Induction motor as generator is more trouble than it is worth... esp as a one off project
  8. Oct 22, 2014 #7


    User Avatar
    Science Advisor

    I would experiment by connecting two motor shafts, powering one and placing AC capacitors on the other until it self excited. Initially I would do it with 3ph motors. If you really must do this then be careful of the high voltages involved. Procede carefully as follows.
    Physically lock the rotor so it cannot move.
    Connect the motor to the AC supply. Note the supply frequency, 60 Hz ?
    Apply power for a very short period, read the motor voltage, Vm, and the motor current, Im.
    Compute the inductive reactance XL = Vm / Im.
    Resonance occurs when XL = – Xc. So you can avoid computation of the actual inductance.
    Knowing that Xc = –1 / ( 2 * Pi * Hz * C )
    Solve for the required C = 1 / ( 2 * Pi * Hz * XL )
  9. Oct 23, 2014 #8
    I agree this motor would be better if was a 3 phase. I just want this to generate auxiliary power for the LED lighting
    in my house. My house is totally LED lighting with a total wattage 90 watts. I have my main generator for my freezers
    and other "have to" utilities. Great input thanks to everytone.
  10. Oct 23, 2014 #9


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Why not acquire an old motor car alternator? They are more plentiful than induction motors and easy to replace. Moreover - they were designed to do the job you want.
  11. Oct 23, 2014 #10
    So you wanna power 90 watts load with 1 kW+ rated rotating electrical machine? Most likely the losses in generator will be as large as total wattage of your LED load. It can work, but you're not very economic :D
  12. Oct 23, 2014 #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    So i guess you have separate wiring for the LED lights ?
    Read the nameplate data of your LED lights carefully.
    What range of supply volts and frequency is acceptable to them? Modern switching supplies are surprisingly broad-minded about input power.
    Sophie's car alternator might well be an economical solution.
  13. Oct 24, 2014 #12
    Here in new jersey any alternator with a sufficient wattage for my needs are outrageous in price. Even used.
    I think I will do some looking for a 3 phase to experiment with. Seems from my reads that 3 phase is the chose of DIY'ers
    Thanks again for all the input.
  14. Oct 24, 2014 #13
    Looking for a 3 phase what?
  15. Oct 24, 2014 #14
    The LED lights are a different voltage than the "main generator" ? What do the LEDs need?
  16. Oct 24, 2014 #15
    Buck converter electronics you get with them?
  17. Oct 24, 2014 #16
    Yeas - some type of DC supply is what I was thinking - need the OP to update us....
  18. Oct 24, 2014 #17
    Really no need for that. The simplest driver circuit topology which rectify has been integrated in LED bulbs since early days of commercial LED technology (looks like this) . And today you'll find more advanced topologies integrated. Hopefully OP is not looking for 100W 3-phase induction motor to power his LEDs. He will have a hard time finding such.
  19. Oct 25, 2014 #18
    Believe it or not a buddy of mine who works for a water company told me last night that pumping stations change out motors
    as part of maintenance that some of the motors are good it;s just they are changed after so many hours. He said I could probably
    get a 10 hp 3 phase motor for free if I pick it up. As cost is a consideration. I'm not looking to spend more than it would cost just to buy a generator. I would look foolish. Getting the voltage and wave to be stable is something I can do especially with info. from guys with your knowledge. Don't know when I will update you guys depends when I get a motor. As I work a 3- 11pm shift hampers my motor shopping.
  20. Oct 25, 2014 #19
    Are you serious?
    In order to use a 10 hp 3 phase induction motor as a generator, you'll need a very large cap bank, or 2-3 Hp sync generator to supply it with reactive power. Not worth trying & money knowing your load is some 0.1 kW LED lightning
  21. Oct 26, 2014 #20
    I remember a project in the 70's where a guy used a simple single phase induction motor to kick power back into the grid. Back then, people put all manner of bad power factor on the grid, so I don't think a capacitor was even used.
    But, you did need the grid to sync into. Otherwise, the motor spun, but wasn't generating.

    A couple of interesting alternatives are the motors used in tread mills and old brush-less-DC motors from air conditioners.

    With the 13 SEER mandate, I'd be surprised if there are any air handlers sold without one. The electrical drives are more likely to pop than the motor, so if you have a buddy that's an AC repairman, these are great. Just pull off the control and you'll have a 3 phase motor. Get a couple of full-bridge rectifiers, and you can cobble a three phase rectifier cheaper than buying one.
    Careful though. These parts were intended to reach peak speeds at around 300 volts p-p (ouch!). Running the shaft slower gives less voltage and less power, so a 3/4 hp, 900 RPM type should be good for about 90 watts at about 48 volts and 90 RPM. The limiting factor is the wiring, which is only intended to take so much current.

    Note that you can also use three 220 V transformers arranged in either a delta or wye to take better advantage of it's capability. Even though it's frequency decreases with speed, so does it's voltage, so the transformers remain happy. Also, rectifying three phase gives a fairly smooth current to your system.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook