Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric

Eric R Snow wrote:

Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric

Basically you have L1, L2, L3 each 120 degrees apart. If you feed motor 1 L1-L2
(which would be 208 volts) and motor 2 L2-L3 and motor 3 L1-L3 then if they run
on 208 volts they should all run OK but with no start windings they wouldn't
start, as there is no rotating magnetic field when connected to a single phase.

My intuition is they'd just sit there and hum, but if you didn't disconnect the
start windings they would all work OK.

The load would be balanced as long as each motor was loaded the same. Only way
to do that would be to have 3 208-230V motors each with shafts at both ends, and
use shaft couplers e.g. Lovejoy ..

GWE

Eric R Snow wrote:
Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric

No.

An induction motor works because there's a rotating magnetic field that
"drags" the rotor around with it. A 3-phase motor starts automatically
because the magnetic field rotation is unambiguous. A single-phase
motor keeps spinning without the start winding because a single
alternating magnetic field appears to be a pair of fields rotating in
opposite directions, and the rotor generates more torque to the one it's
rotating with than the one it's rotating against. A single-phase motor
needs a start winding (would 90 degrees out mechanically and with some
phase delay electrically) to give an unambiguous rotation to the
magnetic field at the start.

If you put three broken single-phase motors in line and excite them with
three single-phase lines that happen to be 120 degrees out of phase with
each other they'll just act like three broken motors.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Eric R Snow wrote:

Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric

I'm going to go against the grain here and say
that they might run.

You'd want them all to have similar or preferably
identical pole structures and you'd have to
"time" them by making sure that all of their rotor-
to-field relationships were the same after they
were coupled together (*not* 120 degrees out of
phase).

I think it would be an interesting experiment.
Unfortunately, I don't have 3 identical single
phase motors to try it.

Jim Stewart wrote:

Eric R Snow wrote:

Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric


I'm going to go against the grain here and say
that they might run.

You'd want them all to have similar or preferably
identical pole structures and you'd have to
"time" them by making sure that all of their rotor-
to-field relationships were the same after they
were coupled together (*not* 120 degrees out of
phase).

No, it won't work. The reason a 3-phase motor can start without a
start winding is because all three phases are applied to ONE ROTOR.

"Timing" the motors won't work. Induction motors don't have
any pre-defined poles on them until the rotor is excited magnetically
by the stator.

Jon

Jon Elson wrote:


Jim Stewart wrote:

Eric R Snow wrote:

Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric



I'm going to go against the grain here and say
that they might run.

You'd want them all to have similar or preferably
identical pole structures and you'd have to
"time" them by making sure that all of their rotor-
to-field relationships were the same after they
were coupled together (*not* 120 degrees out of
phase).


No, it won't work. The reason a 3-phase motor can start without a
start winding is because all three phases are applied to ONE ROTOR.

"Timing" the motors won't work. Induction motors don't have
any pre-defined poles on them until the rotor is excited magnetically
by the stator.

I'm still not convinced.

You may be right but I'm still not convinced.

Eric R Snow wrote:
Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric

I puzzled over this one for a while. Think about it this way. The
purpose of coupling the motors with shafts is so that torque can be
transmitted from one motor to another (you can say it is to "hold" the
120 degree spacing, but really that's the same thing). If torque can be
transmitted between the motors when they are starting, it means that
each motor is able to develop torque, which we know they can't do. But
if you could "morph" the three single phase stators into one, so that
their magnetic fields interact, I believe it would work.

Chris

Jim Stewart wrote:
Jon Elson wrote:



Jim Stewart wrote:

Eric R Snow wrote:

Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric




I'm going to go against the grain here and say
that they might run.

You'd want them all to have similar or preferably
identical pole structures and you'd have to
"time" them by making sure that all of their rotor-
to-field relationships were the same after they
were coupled together (*not* 120 degrees out of
phase).



No, it won't work. The reason a 3-phase motor can start without a
start winding is because all three phases are applied to ONE ROTOR.

"Timing" the motors won't work. Induction motors don't have
any pre-defined poles on them until the rotor is excited magnetically
by the stator.


I'm still not convinced.

You may be right but I'm still not convinced.



So, prove it to yourself. (If these motors don't have a centrifugal
start switch and start windings, then they would start by shaded pole
design, which could be confusing.) But, 3 single-phase motors tied
together on a shaft are not one three-phase motor.

Jon

On Thu, 01 Dec 2005 08:37:32 -0800, Eric R Snow
wrote:

Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric

It should work. The shafts would have to be rigidly joined
together so that there was no relative movement and the stators
lined up 120 electrical degrees apart (i.e 60 mechanical degrees
if it's a four pole motor).

A single phase single winding motor will not self start
because it is equally likely to rotate in either direction.
Without a starting winding to provide directional assymetry it
will just alternate about a stationary position.

With three coupled rotors with sequential three phase
oriented stators, the sequential torques will all add in the
phase rotation direction and provide the necessary starting
torque.

Once up to speed each motor will behave as a normal
single phase motor with the usual 100/120 hz vibration
transmitted to the stator. However if the the stators are rigidly
coupled the three sets of vibrations will cancel each other and
the complete assembly will have the same uniformity of torque and
freedom from vibration as normal three phase motor.

Jim

On Sun, 04 Dec 2005 00:26:06 +0000, wrote:

On Thu, 01 Dec 2005 08:37:32 -0800, Eric R Snow
wrote:

Just wondering. I'm not gonna try it out. No sense in it. But here it
is: Three 240 volt single phase motors are wired together in delta,
the shafts are connected together in line, and the starting windings
are disconnected. If connected to a VFD, or any three phase source for
that matter, will they spin up? Will they maybe need to be rotated at
assembly 120 degrees to each other? I think it will work. Just idle
curiosity. We all know how dangerous that is.
Thanks,
Eric

It should work. The shafts would have to be rigidly joined
together so that there was no relative movement and the stators
lined up 120 electrical degrees apart (i.e 60 mechanical degrees
if it's a four pole motor).

A single phase single winding motor will not self start
because it is equally likely to rotate in either direction.
Without a starting winding to provide directional assymetry it
will just alternate about a stationary position.

With three coupled rotors with sequential three phase
oriented stators, the sequential torques will all add in the
phase rotation direction and provide the necessary starting
torque.

Once up to speed each motor will behave as a normal
single phase motor with the usual 100/120 hz vibration
transmitted to the stator. However if the the stators are rigidly
coupled the three sets of vibrations will cancel each other and
the complete assembly will have the same uniformity of torque and
freedom from vibration as normal three phase motor.

Jim


That's what I think too Jim. I guess I'm gonna have to see if I can
find three 220 volt motors ant try it out.
ERS

Thought experiment:

Think of a normal 3 phase motor, with 3 sets of windings. Now imagine
that each set is only 1/3 the length of the rotor and separate axially
from the others (a set on each end and 1 in the middle). But still
having the same angular position. Very inefficient, but nothing's
changed magnetically, right? Further imagine that the rotor is divided
axially in three sections and each section is magnetically isolated from
the others. Each set of windings is still producing the same time
varying magnetic field, with the same phase relationship to the others.
Each field is still having the same effect on its section of the rotor
as it did on the rotor in the integrated motor. Overall the rotor is
still experiencing the same magnetic force.

This is equivalent to Eric's 3 separate motors with connected shafts,
isn't it?

Bob