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Thought experiment-will this work?

I think I pretty much understand how induction motors work. A typical single phase motor has a start winding that is approximately in quadrature electrically to the run winding. This provides the phase shift that gets the motor spinning. Energizing just the run winding with the motor just makes the armature rotate back and forth a tiny amount. In a three phase motor the windings are 120 degrees apart which is why they will start spinning without using a start winding. So what would happen if the shafts of three single phase motors were connected together, end to end, and then wired together as if they were one three phase motor, and then powered with three phase? would the assembly start rotating? I know that windings in induction motors make the poles in the rotor but I don't know if the location of the poles has any relation to the shorted conductors in the rotor. For my experiment to work would the rotor conductors of the each motor need to be offset from the others by 120 degrees? Or maybe just 1/3 of the angular distance between the conductors? Or would the assembly never be self starting no matter what? I have enough single pahse motors kicking around that I just might try this. Thanks for reading. Eric

Reply to
etpm
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Won't work. Which direction would it turn?

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com
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Reply to
John Larkin

It's more complicated than that.

In a perfect world (which you come close to with a 3-phase motor) induction motors work because the shorted turn on the rotor acts like a transformer. It responds to the rotating magnetic field in the stator by setting up its own rotating magnetic field. Because the rotor has resistance, its rotating magnetic field lags in time. Because the stator field is spinning, the rotor field lags in space, and a torque is set up, and the motor turns.

Looking at this another way, the rotor turns because the _same_ rotor is exposed to the magnetic field in _all_ the windings. In a single-phase motor with no start winding, the magnetic field just pulsates; if the rotor isn't turning then it can't keep turning.

(You can analyze single-phase motor behavior quite well by modeling the pulsating magnetic field as a pair of counter-rotating fields. An induction motor tends to have torque that increases as it speeds up, and decreases if it is forced opposite of the direction of travel of the stator field. So as soon as that single-phase motor starts turning it sees more torque in the direction its already going, and up it spins.)

In your three-motor example you'd have three motors with three uncoupled rotors. So you'd just have three unhappy motors vibrating away, but never spinning unless someone gave them a start.

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Tim Wescott 
Control system and signal processing consulting
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Reply to
Tim Wescott

it won't start, induction morots work by eddy currents in the core interacting with the stator's magnetic field

you need the angular difference between the eddy current and the magnetic field for the motor to generate torque.

once you get it started it'll run, same as a single motor would.

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Reply to
Jasen Betts

At first I was thinking that my motor assembly scheme wouldn't work. Because each motor would just vibrate back and forth. But then I got to thinking that since the voltage peaks in each leg of the three phase are offset from the others by 120 degrees motor one would start to turn in one direction, then before the voltage had a chance to reverse motor two would start to turn a little in one direction, then motor three and by then maybe there would be enough momentum to keep to motor assembly spinning in one direction. But the more knowledgeable agree with my first thought, that it wouldn't work. I have enough motors laying around that I'm tempted to try it even though I know it won't work, but it would take a lot of time and even if it did work I wouldn't end up with anything useful. Thanks, Eric

Reply to
etpm

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