two phase, 2/3 phase transformer

sgeiger · Mar 28, 2007

I bought a warehouse in Philly and it has 2 phase power. It has a 20 hp two phase motor for the elevator.
I want to connect my 3 phase lathe, mill, compressor.
My compressor is 10 hp and other tools don]t exceed 2 hp. I have a 15 KVA 2/3 phase transformer but I don't know how many hp this will handle. Does anyone know anything about two phase and 2/3 phase transformers?

JST · Mar 28, 2007

The "scott" connection will convert 2phase from and to 3phase.

It involves two transformers with specific tapping, and it may be illustrated in electricians info booklets. It isn't very common now, except where you are.

peterh5322 · Mar 28, 2007

The "Scott T" is usually used to supply an old two-phase customer from a new three-phase service, or a new three-phase customer from an old two-phase service.

Theoretically, it is possible to perform any combination of two- and three-phase phase and/or voltage conversions.

Lately, specialized "T transformers" have been designed which are primarily three-phase to three-phase, with the primaries always being high voltage, and ∆-connected, and the secondaries being Y, with a neutral, or the equivalent of ∆, without a neutral.

These T transformers are available in sizes up to 300 KVA.

I have previously shown how it is possible to convert single-phase to three-phase and two-phase in a single RPC.

Jraef · Mar 29, 2007

The transformer rating is irrespective of the 2ph/3ph issue. A general rule is to size a transformer kVA at 2 - 2.5 times the connected HP in order to allow for starting capacity, maybe lower if you use a soft starter, but never lower than 1.5 x HP. So if you apply that in reverse, your 15kVA transformer would be capable of powering a 7-1/2HP motor load, maybe 10HP total under the best of circumstances, but 12HP is a severe stretch. What it might do if you use it is to cause a severe voltage drop whenever you attempt to start the 10HP motor, and since the motor torque drops by the square of the voltage, you might end up with the 10HP motor not having enough torque to fully accelerate the load.

peterh5322 · Mar 29, 2007

"It isn't very common now, except where you are."

Niagara Falls (Ontario and New York)/Buffalo, Philadelphia/Camden, and Reading, PA.

Reading was a surprise to me, but a correspondent purchased a commercial plant there, which he later moved to another state, and for which he now required two-phase power.

Not only that, but there was no source of three-phase power.

This prompted me to design a single-phase to three-phase and two-phase RPC, and which requires no transformers other than an autotransformer to boost the X phase voltage from 208 to 240.

Here is that thread.

reggie_obe · Mar 29, 2007

Peter, hate to be a hijacker, but what autotransformer specs. would I be looking for for 208v to 240vac? I have a 20KW generator that is 3ph 208v, and I'd like to be able to run some 1ph 240v loads off of it.

peterh5322 · Mar 29, 2007

"I have a 20KW generator that is 3ph 208v, and I'd like to be able to run some 1ph 240v loads off of it."

Normally, one would tap any two phases, A and B, B and C, or C and A, and get 208 single-phase.

But, you want 240 single-phase. And, possibly 120/240 single-phase.

In order to maintain 120 to ground from either line, you would need two autotransformers, each one having an overall 120:138 transformation ratio.

Now, this ratio can be achieved with a 120:18 buck/boost transformer, operated in boost mode.

But, because 18 is not a common secondary, you may have to make do with a 16 or 24 secondary, in which case your 120/240 would be a little low or a little high.

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