480 RPC from single phase by connecting to the winding centre taps

Could a 480V RPC be built from a dual voltage motor by connecting 240 single phase to the winding centre taps? For example, on the wye connection, connect 240 at 6/9 and 4/7 and then take 480 off from 1,2 and 3?

it can be done in theory but in practice it would only be useful to transform 3 phase into 3 phase of double voltage, and your useable power might be 10% of the motor's nominal rating.

The reason why is because running a 3 phase motor on 2 of its 6 windings consumes the entire thermal budget of those 2 windings. leaving no overhead for the load to draw anything.

If you have a synchronous motor it can be done because a synchronous motor draws unity power factor and doesn't need amps flowing in the stator to magnetize the air gap.. those amps are flowing in the rotor.


also the coupling between the two sets of windings in an induction motor is rather crappy, but it depends on the motor. so your voltage regulation is going to be very poor.

Rotary transformers have been made, but I don't see many people using them.

Bill

johansen said:

it can be done in theory but in practice it would only be useful to transform 3 phase into 3 phase of double voltage, and your useable power might be 10% of the motor's nominal rating.

The reason why is because running a 3 phase motor on 2 of its 6 windings consumes the entire thermal budget of those 2 windings. leaving no overhead for the load to draw anything.

If you have a synchronous motor it can be done because a synchronous motor draws unity power factor and doesn't need amps flowing in the stator to magnetize the air gap.. those amps are flowing in the rotor.


also the coupling between the two sets of windings in an induction motor is rather crappy, but it depends on the motor. so your voltage regulation is going to be very poor.

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That seems pessimistic to me. I would expect the windings within the stator to function as an autotransformer regardless of the magic that happens in the rotor.

I would expect that the motor should be sized such that the single phase current draw is equal to the motor FLA. You may even get more than that, as the current flowing into and through the first half of the winding oppose, but it would be pushing the limits.

A nuisance of this configuration would be that you are half way between high leg delta and corner grounded delta, and you will have well over 400VAC from one phase to ground (I'm not doing the actual math), and only 120VAC on another leg and 360VAC on the remaining leg.




Another option with the motor mentioned is a running it in the wye configuration. Use a single phase boost transformer to turn your 240 into 277, then feed that into one winding. I would personally try this approach, as I would expect better efficiency and capacity. Without an isolation transformer your grounding is weird, but if you use a commonly available 240 to 277 single phase isolation transformer you'll end up with clean center grounded wye.

Perhaps even smarter would be to use the motor in a wye configuration and then feed it into a 400 to 480 transformer which is also commonly available, but at some point you just use a regular RPC and the right size transformer, though 240 to 480 transformers get bought up much quicker than other types.

johansen said:

a synchronous motor draws unity power factor

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This is sometimes true, but not universally true, or automatic. You can achieve unity power factor with synchronous motor by appropriate adjustment of the rotor excitation.

CarlBoyd

I've done it. It does work, but is very weak.

That makes sense, considering that the one winding has to supply the energy to the rotor to maintain a magnetic field for the rest of the turn.

You can use only half the ampere turns that you would have available if using the same motor as a same-voltage RPC. You MAY be able to go a little higher, perhaps 20%, because the other windings are contributing the heat from only one phase worth of current.

You may need to go up from 1.5-2 x HP to 4-5 x HP to get reasonable output. (That's a W.A.G. so don't take it as a recommendation.)

I wouldn't want to try it, you would need so many amps input to spin it up to speed plus the amp draw of the high side. basically think of the circuit in parallel, double the windings, double the amps, half the resistance. then trying to pull 480V out you would just end up with a dead short.

plus most RPC components are only rated for 300V for the contactors, capacitors and wiring. fire waiting to happen.