On Wed, 4 Jan 2006 11:42:34 -0600, "Robert Swinney"
wrote:

Hey, Don, it sounds like you are beginning to go off half cocked, sort of
"Iggy style".


Look at
http://users.goldengate.net/~dforeman/delta_3D/

Colored lines are windings, white lines are connections.
'Splain to me how the windings of same colors are not in parallel...

Don "Half-cocked" Foreman

Don, I think you have a great insight here. The two motors are clearly
in parallel; swap a lead on either motor and the "bigger" one wins. I
still am not sure if a flywheel has any significant effect on which one
is bigger (though it seems like it would).

Steve

Don Young wrote:

Since the running idler and load motors are directly connected in parallel,
wouldn't plug reversing with identical motors and no mechanical load have an
equal chance of reversing either motor? When running free, it seems to me
that either motor could be considered to be the source or load for the third
phase leg. I tend to believe that the idler requires more mechanical inertia
than the load to maintain the best functioning.

If an induction motor does not "generate", is induced counter EMF imaginary
and the use of common induction motors as generators impossible? There are
many ways to understand and describe how things work and I like to think of
the RPC as simply a running induction motor with the magnetized rotor
inducing EMF not only into the line energized windings (counter EMF) but
also into the unenergized and phase displaced windings. Note that, when
disconnected and still turning, an induction motor still has voltage across
its windings and loading this voltage with "braking" resistors will
mechanically load the rotor. I do not claim that this is the only way to
describe it or that any description can change the operating principles
involved.

Don Young
"Christopher Tidy" wrote in message
...


Hi all,

I'm trying to figure out if there is any benefit in adding a flywheel to a
rotary phase convertor. I've heard varying opinions on the subject. Having
thought about it myself, I've reached the following conclusions:

(i) The sag in voltage on the third line is caused by the fact that it is
not connected directly to the supply. The flywheel doesn't change this.
Nor will it change the steady speed at which the rotor turns, so unless it
has some averaging effect on a cycle-by-cycle basis which I haven't
considered, it won't affect the quality of the three phase output when the
convertor is running in a steady state.

(ii) It might be an advantage when trying to plug reverse the load motor.
As far as I can see (on the most simplistic level), the motor with the
most kinetic energy will win.

I can't seem to find any used flywheels to fit my motor, but I can get a
brand new flywheel for £40. I'm not sure if it is worth it in order to
satisfy my scientific curiousity. If I get a different motor, I can get a
flywheel for next to nothing, but that will involve lots of effort,
bartering and deals in order to get a motor which isn't quite so cool.

Any opinions and arguments? Thoughts would be appreciated...

Best wishes,

Chris

On Tue, 3 Jan 2006 23:14:07 -0600, "Don Young"
wrote:

Since the running idler and load motors are directly connected in parallel,
wouldn't plug reversing with identical motors and no mechanical load have an
equal chance of reversing either motor? When running free, it seems to me
that either motor could be considered to be the source or load for the third
phase leg. I tend to believe that the idler requires more mechanical inertia
than the load to maintain the best functioning.

Interesting! The relative impedances are also important here. The
larger motor with lower impedance (and probably higher inertia) will
govern. Look at the terminal voltage where the two third legs are
connected. If the motors were perfectly matched, their effects would
cancel and this terminal voltage would be zero. If they are not
matched, the voltage (phase) of that terminal will be determined by
the motor with the lower impedance, and the phase of this voltage
determines (or indicates) the direction in which both motors turn.

If an induction motor does not "generate", is induced counter EMF imaginary
and the use of common induction motors as generators impossible? There are
many ways to understand and describe how things work and I like to think of
the RPC as simply a running induction motor with the magnetized rotor
inducing EMF not only into the line energized windings (counter EMF) but
also into the unenergized and phase displaced windings.

Right, up to here.

Note that, when
disconnected and still turning, an induction motor still has voltage across
its windings and loading this voltage with "braking" resistors will
mechanically load the rotor.

Only if the rotor has some significant permanent magnetism -- not
usually the case.

Don,

See my previous post, where I tried to show 2 induction motors operating
from single phase current in a RPC configuration cannot be in parallel.

Bob Swinney
"Don Foreman" wrote in message
...
On Tue, 3 Jan 2006 23:14:07 -0600, "Don Young"
wrote:

Since the running idler and load motors are directly connected in
parallel,
wouldn't plug reversing with identical motors and no mechanical load have
an
equal chance of reversing either motor? When running free, it seems to me
that either motor could be considered to be the source or load for the
third
phase leg. I tend to believe that the idler requires more mechanical
inertia
than the load to maintain the best functioning.

Interesting! The relative impedances are also important here. The
larger motor with lower impedance (and probably higher inertia) will
govern. Look at the terminal voltage where the two third legs are
connected. If the motors were perfectly matched, their effects would
cancel and this terminal voltage would be zero. If they are not
matched, the voltage (phase) of that terminal will be determined by
the motor with the lower impedance, and the phase of this voltage
determines (or indicates) the direction in which both motors turn.

If an induction motor does not "generate", is induced counter EMF
imaginary
and the use of common induction motors as generators impossible? There are
many ways to understand and describe how things work and I like to think
of
the RPC as simply a running induction motor with the magnetized rotor
inducing EMF not only into the line energized windings (counter EMF) but
also into the unenergized and phase displaced windings.

Right, up to here.

Note that, when
disconnected and still turning, an induction motor still has voltage
across
its windings and loading this voltage with "braking" resistors will
mechanically load the rotor.

Only if the rotor has some significant permanent magnetism -- not
usually the case.

On Wed, 4 Jan 2006 11:45:55 -0600, "Robert Swinney"
wrote:

Don,

See my previous post, where I tried to show 2 induction motors operating
from single phase current in a RPC configuration cannot be in parallel.

I saw it. I just don't agree with it. See recent post showing things
in 3D.

Transform to Y using the usual Y-delta transforms if you like. See
any textbook on the subject.

In the Y case they don't look in parallel if there is no neutral
connection. However, since a delta depiction clearly shows that they
*ARE* in parallel, they are in freakin' parallel, BOB! Must I glue
up some popsicle sticks for you?

Can you explain the discrepancy? :)

Hint: if there is no potential between unconnected points (the
neutrals in a Y configuration) then they are effectively connected.

Don "half-cocked" Foreman
half cocked my arse....grumble mutter ....chuckle

Well, Don - you've missed the point again! What part of "2 induction
motors operating from single phase current in a RPC configuration cannot be
in parallel" did you fail to understand. Your well intentioned, and
colorful, drawings were not of a RPC configuration. Draw out a RPC and I
think you may understand. Oh! be sure to include some capacitors. They (in
electronic terms) might be considered as steering capacitors, for it is
their job to force the convoluted currents to flow in such a way as to
*emulate* true 3-phase. Note, I said *emulate* because current flow in a
RPC is not the same as current flow in parallel connected 3-phase motors, no
matter which transform is used.

Bob Swinney
"Don Foreman" wrote in message
...
On Wed, 4 Jan 2006 11:45:55 -0600, "Robert Swinney"
wrote:

Don,

See my previous post, where I tried to show 2 induction motors operating
from single phase current in a RPC configuration cannot be in parallel.

I saw it. I just don't agree with it. See recent post showing things
in 3D.

Transform to Y using the usual Y-delta transforms if you like. See
any textbook on the subject.

In the Y case they don't look in parallel if there is no neutral
connection. However, since a delta depiction clearly shows that they
*ARE* in parallel, they are in freakin' parallel, BOB! Must I glue
up some popsicle sticks for you?

Can you explain the discrepancy? :)

Hint: if there is no potential between unconnected points (the
neutrals in a Y configuration) then they are effectively connected.

Don "half-cocked" Foreman
half cocked my arse....grumble mutter ....chuckle

On Wed, 04 Jan 2006 11:04:20 -0600, Don Foreman
wrote:

On Tue, 3 Jan 2006 23:14:07 -0600, "Don Young"
wrote:

Since the running idler and load motors are directly connected in parallel,
wouldn't plug reversing with identical motors and no mechanical load have an
equal chance of reversing either motor? When running free, it seems to me
that either motor could be considered to be the source or load for the third
phase leg. I tend to believe that the idler requires more mechanical inertia
than the load to maintain the best functioning.

Interesting! The relative impedances are also important here. The
larger motor with lower impedance (and probably higher inertia) will
govern. Look at the terminal voltage where the two third legs are
connected. If the motors were perfectly matched, their effects would
cancel and this terminal voltage would be zero. If they are not
matched, the voltage (phase) of that terminal will be determined by
the motor with the lower impedance, and the phase of this voltage
determines (or indicates) the direction in which both motors turn.

If an induction motor does not "generate", is induced counter EMF imaginary
and the use of common induction motors as generators impossible? There are
many ways to understand and describe how things work and I like to think of
the RPC as simply a running induction motor with the magnetized rotor
inducing EMF not only into the line energized windings (counter EMF) but
also into the unenergized and phase displaced windings.

Right, up to here.

Note that, when
disconnected and still turning, an induction motor still has voltage across
its windings and loading this voltage with "braking" resistors will
mechanically load the rotor.

Only if the rotor has some significant permanent magnetism -- not
usually the case.




A squirrel cage motor DOES generate a back EMF and will continue
to do so for several revolutions after all power has been removed
even if it has zero permanent magnetism.

In the case of a 2 pole motor, at the instant of disconection,
the induced circulating currents in the short circuited rotor
inductance provide a diametrical NS field which decays at at the
rotor L/R rate for several tenths of a second. The voltage
generated by this decaying field is is easily observed with an
oscilloscope - the initial voltage is close to full supply
voltage.

Jim

On Sun, 08 Jan 2006 00:23:18 +0000, wrote:



A squirrel cage motor DOES generate a back EMF and will continue
to do so for several revolutions after all power has been removed
even if it has zero permanent magnetism.

In the case of a 2 pole motor, at the instant of disconection,
the induced circulating currents in the short circuited rotor
inductance provide a diametrical NS field which decays at at the
rotor L/R rate for several tenths of a second. The voltage
generated by this decaying field is is easily observed with an
oscilloscope - the initial voltage is close to full supply
voltage.

Jim

I stand corrected. It makes sense, now that you point it out. L/R
ratio is pretty high because R is quite small.

Thanks, Jim!