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

 

[John Larkin]

On Wed, 03 Jul 2013 07:51:10 -0700, etpm@whidbey.com wrote:

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

Won't work. Which direction would it turn?


--

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

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

 

[Tim Wescott]

On Wed, 03 Jul 2013 07:51:10 -0700, etpm wrote:

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

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
www.wescottdesign.com

 

[Jasen Betts]

On 2013-07-03, etpm@whidbey.com <etpm@whidbey.com> wrote:

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?

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

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

 

On 4 Jul 2013 08:24:50 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2013-07-03, etpm@whidbey.com <etpm@whidbey.com> wrote:
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?

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.
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