# 3-phase Inverter in single phase operation

I'm working on a standard (AFAIK) 3-phase, 6-pulse inverter. I'm testing it under single-phase conditions to see what the de-rating will be.

My understanding: With everything else staying the same, if I pull one of the AC phases, I should end up with effectively 1/3 the RMS current out because I now only have 1/3 the number of pulses. But the peak currents seen by the transistors should stay pretty much the same because that's controlled by the differential voltage across the choke and transistors... and the voltages haven't changed.

My question is: Can I rewire the 3-phase choke within my system so that the third winding (which normally would not be used for single-phase operation) is in series with one of the other windings? Like so:

simulate this circuit – Schematic created using CircuitLab

This would allow me to increase the inductance value without needing to have different parts for 1 vs 3 phase operation.
My concern is whether the physical asymmetry in the inductance values will cause a problem. On paper, there is only one current path so everything is in series. In reality, the factory AC grid is in the middle... so who knows what goes on out there.

• You've shown how you want to wire it but to answer your question and attempt to understand what the choke is you ought to post wiring of how it is currently. In fact a circuit that shows all components that the choke connects to including voltage levels and part numbers is probably appropriate. At the moment it's just an unknown choke. Feb 18, 2014 at 18:05
• @Andyaka The choke I'm using is a RLW-001105 Feb 18, 2014 at 19:17

It's absolutely common to derate three-phase inverters for single-phase source operation. The front end (diode bridge) is under more stress because all the current now only comes from 2/3 of the diodes. The DC bus capacitor also takes more of a beating due to higher ripple current since you're missing a phase that helps keep the bus charged. When I worked in this industry we usually derated the drive 33%, possibly more if it was for a constant torque application.

There are usually line reactors or a DC choke as well, as you've mentioned, but I don't think I've ever heard of rewiring or trying to use the now-unused third phase coil for anything. Even if you were to try to put it in series with one of the other phases you would introduce imbalances in how the inverter is taking power from the line.

I guess the bigger question is what you're trying to achieve by using this unused third coil? I don't think anything useful can come from it, and in fact if you're going to go to all the trouble to rewind it for marginal gain you might just be better off using a DC link choke instead of the line reactor.

The choke you have linked is a three phase choke but in fact it must be regarded as a transformer because all three coils share the same magnetic core. Now consider this: -

The short story is that the core has very little magnetizing flux to drive it into saturation - this has got nothing to do with load, it has everything to do with the nature of the balanced 3-phase voltages that feed it - it doesn't matter that the device is used like three individual chokes - it has special characteristics that means: -

1) Not using it as a three phase device might overload its mag core and cause major issues on your inverter and possibly a fire.

2) Wiring windings in series on the same phase is going to be very problematic as well - you've got to think of it as a transformer.

I wouldn't do it.