Folks,

I have a motor control system which is working quite nicely and is fed by single phase mains. It drives three phase motors using a micro and an IGBT pack. The mains is rectified and smoothed to feed the IGBT module and off-line power supply.

We have been asked to create a version that runs from three phase mains as well as single phase. Having looked at the rectifier circuits page of the main site, it looks like it's pretty simple to have an extra pair of diodes in a bridge to give the option of:

1. Single phase rectify live and neutral using four diodes.
2. Three phase rectify using the three phases (no neutral) and use an extra pair of diodes.

If we were to do that, am I correct in assuming that the d.c. voltage from the three phase bridge would be greater than root two times 230? I assume that you would get voltages from the different phases adding together to give a d.c. voltage larger than ~325V you get from 230V mains?

If my assumption above is correct, this would mean the IGBT voltage and input voltage to the off-line controller would be too large. If this is the case, is there a way to have a product where you can have both three phase and single phase input that both rectify to the correct d.c voltage?

Many thanks for your help.

 

What discrepancy in the IGBT DC bus voltage can you tolerate?

Consider a 1-to-1 3-phase transformer in which the secondary can be optionally configured in either star or delta at the terminal block.

With a 240V line-to-neutral system you could have star primary with 240V on the winding. With the secondary also connected in star the voltage between any two winding terminals would be 240*√3=415V. Using this as the single phase source feed to a single phase diode bridge would give a nominal DC of 374V.

For the 3-phase case, connect the primary in star and the secondary in delta. With a 3-phase bridge this would yield a nominal DC of 324V.

Of course the former single phase case has a higher ripple than the latter 3-phase case.

 

The motors are rated at 230V, so I suspect that is the most important point really rather than the IGBT ratings.

I'm afraid I don't understand what you mean with primary and secondary, we aren't using big old step down transformers, it's an off-line PSU. We're rectifying single phase mains to feed the IGBT and the off-line power supply. Really, what I need - I think - is to be able to produce approximately the same d.c. voltage from both three phase and single phase supplies when rectified. I suspect that's not possible.

I'm probably asking hopelessly naive questions, so thanks for your patience.

 

No transformer ....OK. Sorry it didn't occur to me.

The only other simple option you really have then is to use something like phase control on the rectifier bridge to bring the 3-phase rectified DC down to a value commensurate with the single phase DC output case.

Problem with phase control is the harmonics you introduce and the attendant EMC issues.

You could implement a more sophisticated active control on the rectifier side or on the DC bus, but that will increase cost and complexity.

 

I think the only option if you want to keep with just switchmode operation is to use a dc/dc to get from rectified 3ph DC down to 300-340VDC and go in to your existing converter. You may have to consider a PFC stage to get your rectified 3ph DC - which would then make 3 stages of 'relatively simple/mature' switchmode conversion. No really simple workarounds if you want to keep using the same motor.

 

Look up three phase rectifier, you usually use six diodes in a bridge configuration.