Running 3 phase motors from single phase
 

In article , Ignoramus20106 says...

That made me think, I just used a way to run a 3 phase motor on single
phase power, by adding a capacitor. Can I make an electronic device
made of capacitors and perhaps a timeout or some other relay, that
would simply start 3 phase motors. That would obviate the need for the
idler.

Congratulations, you just invented single phase motors!

You need either a centrifugal switch, or a potential relay,
to implement the idea.

You cannot:

a) plug reverse motors like that, or
b) develop the entire nameplate horsepower

Jim


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In article , Ignoramus20106 says...

I could reverse them by changing to which leg the starting cap is
connected, though. Right?

Sure. But you cannot plug (instant) reverse it. If you just
quick swapped that over, it would keep right on running in the
same direction because your start switch or relay would never
drop out.

Jim


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please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

The so-called "static" phase converter, as sold by Phase-A-Matic and others
provides a method of starting 3-phase motors on single-phase current. A
3-phase motor, once started, will continue to run on single-phase current.
Essentially, it is running on 2/3 of its windings and accordingly it is said
to only produce around 2/3 of its name plate HP. As an aside, in the very
early days of motor manufacturing, single-phase motors were built as 3-phase
machines to take advantage of existing tooling. They still required some
sort of auxiliary means for starting. I would guess many of them used the
"spare" phase group as an auxiliary winding and fed it with a phase shifting
network in order to achieve start-up.

Spinning the motor shaft with a rope or pony motor arrangement (Rozen
system) is another way to get a 3-phase motor started before applying
single-phase current. Commercial static phase converters use some sort of
device, i.e., potential relay, timer, etc. to remove the starting capacitor
from the 3rd leg. Additionally, it is desirable for them to automatically
re-set themselves to be able to begin a new start cycle after a power
interruption.

Run capacitance added to the load motor (or idler motor and load
combination) provides a better path for the complex currents that flow in a
3-phase motor running on single-phase current. Run capacitors do not shift
phase; "phasing" is an inherent quality of the transformer action that
exists in a rotating mass of copper and iron. It is sometimes said, a RPC
is a rotating transformer. Run capacitance provides a lower impedance path
for the complex current, and in a manner of speaking, offsets the inductance
in those paths. This effect can be compared to the phenomenon of series
resonance.

A static phase converter (no run caps on the motor) is merely a starting
device. A RPC (idler motor) with correct amounts of run capacitance
simulates, no "emulates" true 3-phase power and allows full HP from the load
motor. This is true, theoretically, with only one specific amount of run
capacitance and idler motor load. Because of the broad nature of the series
resonant circuits formed by the run caps and motor(s) inductance, very good
performance is achievable over a fairly wide range of loads.

An idler motor alone, (no run caps) unless it is many times larger in HP
than its load motor, is only a little better than a simple static phase
converter. It does, however, provide a means to start the load motor and
thus tends to give the illusion of a proper RPC.

Bob Swinney




"Ignoramus20106" wrote in message
...
On 3 Aug 2005 11:54:42 -0700, jim rozen wrote:
In article , Ignoramus20106
says...

That made me think, I just used a way to run a 3 phase motor on single
phase power, by adding a capacitor. Can I make an electronic device
made of capacitors and perhaps a timeout or some other relay, that
would simply start 3 phase motors. That would obviate the need for the
idler.

Congratulations, you just invented single phase motors!

Aha, so I am about 100 years late...

You need either a centrifugal switch, or a potential relay,
to implement the idea.

You cannot:

a) plug reverse motors like that, or
b) develop the entire nameplate horsepower

I could reverse them by changing to which leg the starting cap is
connected, though. Right?

i

Ignoramus20106 wrote:
So, I made a self starting phase converter that starts an idler using
a start capacitor.

That made me think, I just used a way to run a 3 phase motor on single
phase power, by adding a capacitor. Can I make an electronic device
made of capacitors and perhaps a timeout or some other relay, that
would simply start 3 phase motors. That would obviate the need for the
idler.

My own thought here is to wire it like the self starting capacitor
works: a cap between leg 1 and 3.

After starting, the 3 phase motor can run on single phase.

The only disadvantage to this that I can think of, is possibly low
starting torque. Perhaps that can be rectified by wiring it using
some other method.

I am sure that people smarter than me have already thought about all
these things, but I am curious about this question.

i


I've got a couple of 3ph. pedestal grinders that I run with sort of low
starting capacitance (slow starting, the 12" takes about 3 seconds to
come up to speed) and no extra switches or anything, just leave the
capacitors in the circuit. They seem to no mind running that way, used
'em for a few years now.

John

Ignoramus20106 wrote:
So, I made a self starting phase converter that starts an idler using
a start capacitor.

That made me think, I just used a way to run a 3 phase motor on single
phase power, by adding a capacitor. Can I make an electronic device
made of capacitors and perhaps a timeout or some other relay, that
would simply start 3 phase motors. That would obviate the need for the
idler.

My own thought here is to wire it like the self starting capacitor
works: a cap between leg 1 and 3.

After starting, the 3 phase motor can run on single phase.

The only disadvantage to this that I can think of, is possibly low
starting torque. Perhaps that can be rectified by wiring it using
some other method.

I am sure that people smarter than me have already thought about all
these things, but I am curious about this question.

I converted a power hacksaw to single phase by this method five years
ago. I had separate start and run capacitors, a potential relay and a
contactor. It was also a dual wound motor (two completely separate
windings, a four pole and a six pole) so I had a rotary switch to change
between the two. It ended up being a fairly complicated project! The
worst part was that the motor had both sets of windings permanently
connected in the star configuration, so I had to open it up, cut the
star points and solder on new leads. The machine worked fine, except
that it needed a little more start capacitance than my book recommended,
probably because it has a lot of inertia. It uses a 1 3/4 hp motor,
which is unusually rated at 1 3/4 hp on both speeds. Unfortunately it
stopped working last winter, and I haven't yet had chance to figure out
what's wrong with it. I can post some pictures if you're interested.

Best wishes,

Chris