# 6 phase vs 3 phase Inverters

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1. Oct 3, 2015

### tim9000

Simple question, What's the advantage of having more phases in an inverter? Like why have 6 or twelve phases? Is it something to do with harmonics? I know that the higher the switching frequency the further off in the freequency spectrum you can push the harmonics, but I can't see that being relevant here.
Cheers

2. Oct 4, 2015

3. Oct 4, 2015

### tim9000

thanks
Ok, so 'Higher phase order' does reduce harmonic generation, but that didn't say how.

4. Oct 4, 2015

### anorlunda

Any inverter approximates a sin wave. The more phases, the better the approximation.

5. Oct 4, 2015

### tim9000

Ah yeah, yeah so the inverter sine wave is like little discrete steps.

6. Oct 4, 2015

### sophiecentaur

I reckon this boils down to a specific application of basic sampling theory. The more samples per cycle (i.e higher the sampling rate) the higher the harmonic artefact frequencies and the lower the distortion in the waveform. In most cases, the distortion is referred to as Quantising Noise but, as the output frequency is locked to the sample frequency, it's simple harmonic distortion.

7. Oct 4, 2015

### Hesch

If the load of the inverter is a 6 phase motor or transformer, the inverter must have 6 phase output.

So maybe the question could be: Why do we have 6 phase motors/transformers ?

8. Oct 4, 2015

9. Oct 4, 2015

### tim9000

Well that's sort of what I thought, I mean I know that the MMF of each phase in a motor super impose to form one sine wave, so I can imagine 6 phases adding to form a cleaner MMF wace in a motor. But Yeah the switching frequency used in the inverter is what I thought would determine how close the inverter sin wave would be to an actual sin wave...not the number of phases?
the only real thing I remember about inverters is that hexagon where each point is represened by three binary digits to make the vector you want.

10. Oct 4, 2015

### anorlunda

Careful, it is easy to get confused. We could invert to 6 phase power. Motors designed for that are better than 3 phase.

6 phase transmission has been explored. It is superior to 3 phase. That is better if the whole grid was 6 phase. But if 6 is only a small part of the grid, every interface to the 3 phase grid would need an expensive 6 to 3 transformer. Therefore 6 phase never got used in practice to my knowledge. Below are experimental 6-phase towers. The one on the right is double circuit 6P, not 12P.

But what I believe we are discussing in this thread is 6 phase inverters to supply 3 phase power. If you design the switching logic properly to select from 6 sources instead of 3, you can do a better job of approximating the 3 sinusoids. Any number of inverter phases >=3 can be used to generate 3 phase power. I hope I explained that clearly. I wish I had a graphical depiction of the waveforms, but I couldn't find it.

11. Oct 4, 2015

### Hesch

Well, say you have a transformer with 6 phases, the output could be rectified more smoothly by means of a 12-diode bridge.

As I calculate it, the ripple ( peak-to-peak without LC-filter ) in theory will be:

1 phase: 100%
3 phase: 13.4%
6 phase: 3.4%

Last edited: Oct 4, 2015
12. Oct 5, 2015

### tim9000

out of curiosity could you show that rough calculation?

13. Oct 5, 2015

### tim9000

Out of interest, did you have any info on the physical construction of a 6 to 3 or 3 to 6 phase TX?

14. Oct 5, 2015

### Hesch

A 3-phase full wave rectifier will have 6 DC pulses ( green curve ) per 360 electrical degrees. So 1 pulse will have a width = 360°/6 = 60° = 2*30°.
The upper peak value of a pulse = Vpeak* sin(90°) = Vpeak*1.
The lower peak value of a pulse = Vpeak* sin(90°-30°) = Vpeak* sin(60°) = Vpeak*0.866.
Thus a peak-to-peak ripple = Vpeak*(1 - 0.866) = 13.4%.

A 6-phase full wave rectifier will have 12 DC pulses per 360 electrical degrees . . . . . . . . . . = 3.4%

Last edited: Oct 5, 2015
15. Oct 5, 2015

### anorlunda

If you just images.google.com "transformer core" together with words like three, six, single, polyphase. You'll see dozens of variations. Here is just one. You should do your own googling before posting questions here.

16. Oct 5, 2015

### tim9000

Aaah, cool.
Thanks

17. Oct 5, 2015

### tim9000

Thanks, I ddi a quick search but it was hard to get a schematic and picture together, that I liked.

18. Oct 5, 2015

Actually 3 to 6 Phase transformers are very common in large DC applications and large VFDs, and other power converters for exactly the reasons above. ( Look up 12 pulse rectifier) - typically these are going for 30 and 60 Degree phase shifts to manage harmonics due to the non-linear ( switching ) load. - Generally created by connecting various combinations of Wye and Delta windings. -- Pure 6 phase would just be a Y-Y and a Y-(inverted)Y - so Phase A of the 1st trans is 180 Deg off set from the Second Transformer - but this would not help the harmonics issues as much as the 30 and 60 degree phase shifts. For direct feed to a motor - there are technical befits - but the economics (cost to benefit) of these systems is almost never practical. (As like to say "Just because you CAN does not mean you SHOULD").
Above there are some comments abut the output of the inverter - really a different set of issues - there are number of ways to get a cleaner - or more pure sine ( Higher Switching Freq as mentioned above, but also Multilevel , and cascade topologies are used particularly in IGBT based systems where the losses associated with Switching, make the High Fsw impractical.

19. Oct 6, 2015

### tim9000

I enjoied reading your post, however you'll have to excuse me I'm a bit slow, and out of practice (a couple of years since I did a relevant subject at uni):
So what does the 180deg phase shift of Phase A accomplish?
All I pretty much remember from TXs is that a Delta Wye, traps the zero sequence current. I wouldn't mind a referesher on any other benefits of transformer combintations people cared to mention.

Sorry, could you elaborate on what those specifics are, please.

So using a transformer after an inverter, between it and the motor would or 'could' add a lot of life to the motor, yet it would cost a lot? Is that what you're getting at?

I may be out of line asking this, on this thread: I take it from you post that higher the frequency switching, the higher the losses (that might ring a bell but I can't remember why....if I had to guess I'd say somethign to do with high current from stored charge in on the junction during near instanious discharge on switching?)
But how does using 6 phases then converting to 3 phases, equal less losses than just 3 phase inverting at a higher switching rate? (as far as I can remember here all I've heard is that the AC ripple on the DC on rectification is smaller, not to say I didn't gloss over a realisation I should have made)
Did you have any links you'd recommend regarding "Multilevel or cascade topologies"?

So would I be right in inferring that a six phase to three phase transformer would do wonders in alleviating harmonics from a six phase supply? (from an isolation perspective)

Sorry to be a bother, I am interested in cleaner switching, I'm just pushed for time atm with a final year project..

20. Oct 6, 2015