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
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