Thought experiment-will this work?

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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.

Re: Thought experiment-will this work?
On Wed, 03 Jul 2013 07:51:10 -0700, wrote:

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Won't work. Which direction would it turn?


John Larkin                  Highland Technology Inc   jlarkin at highlandtechnology dot com    

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Re: Thought experiment-will this work?
On Wed, 03 Jul 2013 07:51:10 -0700, etpm wrote:

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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.

Tim Wescott
Control system and signal processing consulting
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Re: Thought experiment-will this work?
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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.

?? 100% natural

Re: Thought experiment-will this work?

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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.

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