Re: 7.5 HP Lathe on a 10 HP idler?

Jim,

Thanks for lending some clarification to a confusing issue. Perhaps I am totally dense and
hopelessly lost in an overly-simplistic way of looking at RPCs. Please, if you will bear with me, I
would like further clarification on your response "back to basics" below:

{ To begin let's label the respective terminals of 2 motors as A B C . . and . . A B C. In an
idler / load set up, one side of a single-phase line is connected to A, A and the other side of the
single phase-line is connected to B, B; and terminals C,C are together. This, I believe, defines
basic connectivity of a "RPC". The term "plug reversing" is taken to mean reversing (turning over)
a pair of leads feeding the load motor.}

Jim wrote: "This is in all respects a true parallel connected system. Both
motors receive power from the two single phase input lines and
power is exchanged on the common phantom phase link line.
If either or both the motors is mechanically loaded, current
will flow in the phantom phase line, power flowing from the
lightly loaded motor to the more heavily loaded." Agreed. This would be the case whether we are
considering a practical RPC or a perfect pair of identical motors.

Jim wrote: "In this fairy tale case of plug reversing precisely identical
motors the two torques will oppose in their respective rotors
reducing the spin speed of both motors to below minimum start
speed stalling BOTH motors." This is where I am confused, Jim: Your statement seems to imply plug
reversing both ("precisely identical") motors.

The scenario, depicted in the opening, is with the motors connected A B C to respective A B C as
described in the opening paragraph above: The single-phase input, A A and B B is reversed. This
would reverse inuts to both motors and torques, thus direction of rotation, of both motors would
reverse. In essense, plug reversing of both idler and load.

Jim, my confusion arises in that AFAIK if we were to "plug reverse" only the load motor, the
connections would be single-phase line to A, B of the idler, A of idler to B of load, and B of idler
to A of load, With C terminals together. It seems to me this connectivity would reverse only the
load motor, not both motors.

Please explain if this is not the case.

Sincerely,

Bob Swinney








<pentagrid@xxxxxxxxx> wrote in message news:ftp215lqqm7ocd50r18971lk12m2i7e575@xxxxxxxxxx
On Sun, 17 May 2009 11:32:33 -0500, "Robert Swinney"
<judybob@xxxxxxxxx> wrote:

Don Sez:

"Yes, I do. Of course, that depends in part on the relative
horsepower ratings of the idler motor and the lathe's own motor. I have
read here postings by people who have experienced that with equal
horsepower on each. And *that* might be something which could be
improved somewhat by a flywheel on the idler motor -- but expect the
housing to jump around during the reversal. :-)

Tuning and balancing capacitors might change the threshold at
which this reversal happens in that they will be applying a bit of
rotational bias -- but with equal sized or smaller idlers, you still
will have the idler reverse at some set of parameters." " Indeed so."

That "set of parameters" would have to be a reversal of polarity on the idler's two input line
leads. I'm afraid you are caught up in the erroneous premise that an idler motor and load motor
constitute a generator and load. They do not. Irrespective of rotational mass and any series
capacitance present, current flow (and torque) in each motor depends on the polarity of applied
voltage. Swap the two input (line) leads in either idler motor or load motor and that motor will
change direction. The other motor will continue to spin in the same direction as before.

In a RPC, the idler motor and load motor are not in parallel with respect to current flow. This
because of the way current must flow in the "3rd" leg to return to either side of the single-phase
line. There is no "parallel" way to connect a single phase source to a 3-phase load. For that
reason, only the inut line leads of either motor can be considered for polarity swapping in order
to achieve plug reversal.

Bob Swinney


This thread is getting a bit tangled. Back to basics

Take an equal sized "idler" plus "motor" system. No starting or
balancing capacitors. Mechanical or pony motor start.


If the motors are identical and zero, or equally loaded, the
phantom phase voltages are identical. No current flows in this
line so there is no power exchange.

The ONLY differentiation in this system is the historical
fact that one of the motor was spun up to speed. Once that has
happened, the motors are identical twins.


If, in this perfect twin motor system, ANY pair of the
interconnecting wires is reversed the relative direction of
TORQUE is also reversed.

In a practical case various mismatch factors may be sufficient
to enable one motor to remain above minimum start speed and this
will eventually reverse the second motor. With roughly equal
sized motors this can be either motor.

Run capacitors don't make any difference because the phase
shift they provide is equally valid for both motors in both
normal and reverse connection.

A high inertia load (heavy chuck) favours reversing the idler
because equal reverse torque is more successful in reversing the
lower inertia idler. For the same reason a flywheel on the idler
favours the idler. However these are minor differences - safe
plug reversing needs a big idler.

Jim

.

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