# Split-phase induction motor as generator

Discussion in 'Homework Help' started by dlpearce74, Sep 15, 2013.

1. ### dlpearce74 Thread Starter New Member

Jun 5, 2007
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I know that the title of my post is possible, and I can do the calculation required for the capacitor that has to supply the reactive power to the run coil, to get the motor into power generation mode.

However, it struck me that it may be overkill on a small split phase induction motor. Would it be possible to supply rotor current via a DC bias of the start winding? I was thinking about this in relation to DC braking on induction motors.

Just curious if anyone has tried this or not. I have a few split phase motors, may have to drag them out and take a shot at it, but if someone has already tried it, I am curious about your findings.

2. ### MaxHeadRoom Expert

Jul 18, 2013
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How can you obtain induced rotor current with DC? This is the reason it is called an 'Induction' Motor.
DC braking is rectification of the BEMF which is another animal entirely.
3ph generation is done routinely with 3 phase motor off of 1 phase 240.
What generated supply are you hoping to achieve with a split phase motor, that requires 1 ph anyway to run?
Max.

3. ### dlpearce74 Thread Starter New Member

Jun 5, 2007
7
0
Thinking of building a very small generator, maybe with a weed eater engine at some point. Split phase induction motors are cheap and are in the mechanical power range of very small engines.

I don't think your explanation on "induction" is complete. When a induction motor uses DC injection braking, this induces a very large current in the rotor circuit (the squirrel cage). This field works against the dc-sourced field being injected into the run coils. The major problem with DC injection braking is that there is large heat build up in the rotor due to the high currents flowing in the rotor circuit. It's usually frowned upon unless the machine inertia is relatively small, and the braking duty cycle is relatively short.

So I am wondering, if I inject DC into the start winding at some current, with a external mechanical power source spinning the rotor, will the current in the rotor generate a smooth sine wave, or will it be intermodulated with the rotor's DC field crossings? I don't have modeling software to see this myself.

4. ### strantor AAC Fanatic!

Oct 3, 2010
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How do you know that the title is possible? Have you seen it done before? If so, where? I would like to see that, and if you could dredge it up, might be a good resource.

In order to regenerate with a 3ph induction motor, you have to feed power into it at twice the desired output frequency. It seems intuitive that you might be able to do something similar with a single phase induction motor, but I've never heard of it.

Oct 3, 2010
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6. ### dlpearce74 Thread Starter New Member

Jun 5, 2007
7
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Several folks on youtube have done this with a single phase motor and capacitor. While I don't have an example to give you measurements from, it will also work with a cframe induction motor with shaded pole, but you will have to cut the shaded pole off the motor.

I think MaxHeadRoom missed the fact that the rotor will be rotating. A rotating coil through a magnetic field that is not changing (permanent magnets or electromagnetic with DC power), develops a current. Hence while if you apply DC to a induction motor and rotate it's shaft, you will induce current into the rotor circuit (the squirrel cage).

For a split phase motor, you would just leave the start winding unconnected (or you could draw some power there, too, but it will be very limited), which is why I am wondering if I could apply DC to start winding, and forgo the reactive capacitor element in the circuit.

Here is a very good link on dc injection braking, and it's not Wikipedia, whose explanation appears that it may be wrong. The only time the static torque comes into effect, is when the rotor stops, and the dc field is still applied.

http://www.tmeic.com/Repository/Brochures/Drive_Time_-_Injection_Braking_1142532196.htm

7. ### dlpearce74 Thread Starter New Member

Jun 5, 2007
7
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Also, you don't need double frequency to get induction motor power generation. The best place you can start reading about this is application notes, etc., on induction motor control. You only need to apply slip to the frequency of the applied current to the motor. Ie., for a 60Hz motor that runs 1800RPM unloaded, you can apply a less than 60Hz frequency, and the motor will start returning power to your drive system. You can even return power to the grid if you slip the rotation speed over the synchronous speed of the motor while it is connected to the mains.

8. ### dlpearce74 Thread Starter New Member

Jun 5, 2007
7
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"Ie., for a 60Hz motor that runs 1800RPM unloaded, you can apply a less than 60Hz frequency, and the motor will start returning power to your drive system."

Somebody may get me here for not being pedantic, so this will only work if you are putting in mechanical energy to keep the motor spinning at it's synchronous frequency.

9. ### strantor AAC Fanatic!

Oct 3, 2010
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I'm on my phone so my research capacity is limited. But I think I got it wrong with what I said about the 3 phase; I think it's not the frequency you have to play with, it's the rotational speed. You reminded if it when you mentioned slip. When you motor the induction motor above synchronous speed, the slip goes negative and thus generates. typically the motor spun at twice synchronous speed to get as much power out of it as would be required to run it. Sorry for the inaccuracy.

I get what you're saying about spinning a conductor through a magnetic field, but how are you going to extract that power out of the rotor coil? Through the main winding? If that works, I don't understand how.

10. ### dlpearce74 Thread Starter New Member

Jun 5, 2007
7
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Yeah, I don't understand if this will work or not myself. The START winding is wound as a second phase, but I don't know at what angle to the RUN winding, and I don't know if the current pulses in the rotor circuit will generate the magnetic field at the phase angle required to induce current into the RUN winding.

I am unable to try the experiment currently, maybe in a couple of weeks, and I will post back here results as well.

I may try to build a shaded pole motor-based generator as well (driven by a sewing motor though, not a engine for now). If I do, I will post video/results.

11. ### strantor AAC Fanatic!

Oct 3, 2010
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I look forward to hearing the results of your experiment. It's an interesting concept.

12. ### MaxHeadRoom Expert

Jul 18, 2013
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Your O.P. just mentioned DC braking, not DC injection braking, which are two different methods.

I have built 3ph RPC's using the 240v 1ph supply, the 3rd phase is generated using the 60hz from the 1ph pair, the 3 phases have 120° separation from a virtual neutral point at 60Hz, same rpm, depending on pole count.
The motor is not separately driven.
Max.

Last edited: Sep 15, 2013
13. ### strantor AAC Fanatic!

Oct 3, 2010
4,302
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I corrected the reference to doubling frequency, not sure if you saw that. I'm not sure a RPC does the same thing as a 3phase induction generator, and I'm not sure why it was brought up. It doesn't generate any power that I'm aware of, and that's what we are talking about. What I've read about induction generators refers to spinning them above synchronous speed.

14. ### MaxHeadRoom Expert

Jul 18, 2013
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The RPC uses a standard 3 ph motor the 1ph pair generates the 3rd phase from the induced current in the rotor bars.
Capacitors are needed to 'Tune' for correct voltage balance.
See building information in the FRW- files.
http://www.mwdropbox.com/Dropbox/_1998_retired_files/
The motor can be started in a few ways, from automatic timer, manual P.B. or small 'Pony' motor.
With the exception of the last method, the motor is started in a similar fashion to a 1ph split phase motor.
Max.

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