Running 3 phase motors from single phase

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

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

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Thanks for a great article. It should really be published somewhere.
I have a rotary converter with 2 run caps between leg 1 and 3.
Would you say that some sort of a system that would, after startup, connect more run caps between legs 1-2 and 2-3, do me some good?
If I measure plain voltage with a multimeter, voltages between legs on my _unloaded_ RPC are quite similar.
i still thinking.

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Ignoramus20106 wrote:

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
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Sounds great and simple. Since now, I feel very positive about using 3 phase equipment in my shop. Just today I was finishing up electrical wiring of my RPC. It's basically done, but I need a longer ground wire (stupid mistake) and I need to put incoming wires into flexible conduit. Otherwise the RPC is done. Everything is grounded etc.
I tested it with my 3 phase forklift, it worked great...
i
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Ignoramus20106 wrote:

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
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