On Thu, 6 May 2010 02:28:48 -0400, "Ed Huntress"
wrote:


"Don Foreman" wrote in message
.. .
On Wed, 5 May 2010 08:37:10 -0400, "Wild_Bill"
wrote:

This description you used Don, sounds like a PSC peramanent split
capacitor
motor.

Is that what you were referring to?

Apologies to Ed, I'd never seen the term "permanent split capacitor"
but apparently there is such a term. I looked it up. The capacitor
phase could either be a weak "helper" phase as Ed described, or it
could be an essentially equal phase contributing significant torque. I
would call the latter a capacitor-run motor. In such a motor, the
torques contributed by the two phases are in quadrature so torque
never goes thru zero which significantly reduces torque ripple and
vibration.

Thank you, Don. Now, since I'm not sitting here angry about your remarks,
g and since you forced me to go back to the books, there are a few other
things to clean up. Maybe you and Wild Bill already covered them. If so, I'm
sorry for repeating them. I tuned out of the conversation.

First, Winston was talking (he would admit it: he was babblefishing) about
variable-frequency motors. I was talking about all of the conditions under
which a motor may operate. You were talking about a motor with a fixed load
at a fixed rpm, and then you were talking about balanced two-phase motors
operating in time quadrature, which was not the subject that I thought we
were talking about.

Right. I mentioned it only to note that an electronic two-phase drive
(replacing the cap) could handle variable speed and varying loads
while the cap can be optimal for only one speed at rated load.

As you probably noticed when you looked them up, permanent split capacitor
(PSC) motors are split-phase motors with a small cap partially providing the
"synthesized" second phase. The small-diameter windings on the secondary
poles operate partially like a regular split-phase motor, except that
they're not switched out after starting, and they provide the phase shift
for starting. THOSE WINDINGS ARE NOT IN TIME QUADRATURE, except at ONE
specific load/rpm combination,

Correct to here, it would be good to stop here! G

and they're only in quadrature because of the
combination of resistance in the secondary windings and the capacitance in
the secondary circuit.

Resistance may enter in but isn't necessarily significant. You don't
want resistance to be significant because resistance --- dissipation
--- heat. But don't forget that transformation of electrical power
to mechanical power appears electrically as a resistance too, one that
does not produce heat in the motor because the power is delivered to
the load.



At any other operating condition, the voltage peaks
are NOT 90 degrees out of phase. The windings are in *space* quadrature, but
the reactance of the winding/cap combination can be "tuned" for only one
load/rpm combination.

The windings aren't necessarily in space quadrature, but you have the
right idea.

At least, unless someone has come up with something
new. I didn't look for anything new. I realize their are fancy new
electronic controls, but I didn't think we were talking about them.

Oh! I did. I thought that's how this thread started.



(There
are motors of this type that are not in space quadrature, either, but that's
another subject.)

Indeed!

They're very popular for fans, blowers, etc. When I last studied motors,
they were the dominant type for medium-sized industrial blowers. My Googling
suggests that they still are -- as I said before you jumped on me. g

I don't know what you were thinking of concerning
capacitor-start/capacitor-run motors, but the term always used to refer to a
motor with a big-mother special electrolytic cap for starting, which is
switched out at around 75% of running rpms by a centrifugal switch, and a
small running capacitor. After this motor is started, it operates about the
same as a PSC motor. From my Googling, it appears that this is still what it
means.

And neither of them are BALANCED TWO-PHASE MOTORS! Their windings are
unbalanced, in terms of winding resistance, and generally in the number of
windings on the secondary, plus their placement nearer the pole-ends. As
unbalanced two-phase motors, THEY NEVER, EVER PRODUCE EQUAL TORQUE ON BOTH
PHASES! They are in space quadrature. At one specific load/rpm, they can be
in time quadrature. BUT THE CURRENT IN THE SECONDARY WINDINGS IS NEVER, EVER
THE SAME AS IN THE PRIMARY WINDINGS!

Thus, the second phase NEVER, EVER produces the same torque as the primary
windings. That's a basic characteristic of an unbalanced two-phase motor,
which these are. To quote one of the scientific papers on this subject that
I just consulted, to make sure of what I remembered, the primary and
secondary windings of single-phase motors with permanently engaged
secondaries have an "asymmetry [that] causes an oscillating term in the
electromagnetic torque." It helps smooth out the overall torque of the
motor. But it is never as balanced as a true, balanced, two-phase motor.

All true for those that have start windings capable of delivering
brief periods of torque 4X to 10X rated run torque, which includes
most motors with start windings however the start windings are
disposed and excited.

There are also cap-run motors, which I suppose also fit the term
"permanent split capacitor" (try to order a split capacitor, permanent
or temporary) that do not use a separate switched start cap. These
really do work more like polyphase motors because they are polyphase
motors.

All of this contradicts several things you said in your somewhat snarky
first post, if I was reading it correctly. After I saw you question PSC, I
sort of tuned out and let it all go. And I'm in no mood to go back and read
the thread.

I've already apologized once. That's about infinitely many more times
than you ever have for snarkey and even abusive responses to some
others, so one is all you get here.

The term "permanent split capacitor" was not used in my EE courses
regarding electrial machinery but it apparently has become part of
the jargon and vernacular since then, however misleading it might be.

I don't know how you came up with two-phase motors and quadrature from all
of that, because the motors we were talking about -- capacitor-start, and
then PSC and cap-start/cap-run, are NOT balanced two-phase motors to begin
with.

That describes the motors you (not we) were talking about, I described
a cap-run two-phase motor. That's relevant because the thread was
about an electronic device that purports to be able to excite
single-phase motors at variable speeds. I mentioned cap-run
two-phase motors because they (sans capacitors) would indeed be
amenable to VF drive by such a device. Perhaps I should have made
that connection more clearly than I did.

Except for a few types, including the latter two types, they aren't
even two-phase at all, except when they're starting. Conventional
split-phase and cap-start motors switch out the secondary phase after
starting, and they run as straight single-phase motors.

They do indeed. As such, they're not of interest here.

Now, have I missed or misconstrued something you wrote? Because I admit that
I was only looking at your objections, and I was annoyed. If I missed what
you were talking about, I'll apologize in advance. But I've covered what *I*
was talking about.

Well, that's what counts! G