electronic ballasts, THD, and non-linear loads

[GoldDigger]

I see where we differ. The current on one line-to-line phase (ordered phase wire pair) will be 60 degrees but a single phase wire will be contributing current for two of those 60 degree intervals.
Under your reasoning the diode for that line will conduct for 120 degrees, but the diode it is paired with will switch halfway through.
Note however that the condition for conduction is NOT that one line is positive with respect to another. It is that it is more positive than any other two wire pair.
You have to rise above the steady DC voltage before you get conduction.

I keep thinking of a capacitor input filter with or without an output choke, not a choke input or choke only filter.

Tapatalk!

 

[Besoeker]

I see where we differ. The current on one line-to-line phase (ordered phase wire pair) will be 60 degrees but a single phase wire will be contributing current for two of those 60 degree intervals.
Under your reasoning the diode for that line will conduct for 120 degrees, but the diode it is paired with will switch halfway through
Note however that the condition for conduction is NOT that one line is positive with respect to another. It is that it is more positive than any other two wire pair.
You have to rise above the steady DC voltage before you get conduction.

Tapatalk!

Believe me, ALL the diodes conduct for 120 deg!
Suppose we use the RYB names for the three phases and R+, Y+, and B+ for the diodes in the top half of the B6U and R-, Y-, and B- for those in the lower half.
Then R+ conducts through the load and back through Y- then B- each for 60 deg in sequence so that accounts for the 120 deg R+ pulse. But it doesn't mean that say, B- conducts for only 60 deg. After finishing with it's R+ mate, it then goes to Y+ for the next 60 deg.

And what you need for conduction is a couple of forward biased diodes. Vdc is a consequence of that.

 

[GoldDigger]

I agree that in your scheme each diode conducts for 120 degrees. Just saying that each PAIR conducts for only 60 degrees at a time. There are six possible pairs of diodes in the bridge

Tapatalk!

 

[Besoeker]

I agree that in your scheme each diode conducts for 120 degrees.

Much as I might like the accolades it isn't mine! The credit is due to Karol Pollak for the invention and Leo Graetz. B6U is just the three phase version.

We sometimes do a hexaphase arrangement.



This is actually a 24-pulse arrangement but each of the four circuits is a hexaphase.
Typically this is used for low voltage rectifiers. Each device (SCR in this case) conducts for 60 deg but it means just one in series with the current at any one time as opposed to two with the B6 arrangement.
It reduces the voltage drop and the losses in each device since it carries DC current for one sixth of the period instead of one third.

The 24-pulse gets the supply THD down to minimal levels.