3 phase full wave bridge.

[elevmike]

Ok so I'm brain dead right now and cant find/remember the equation.

I need to calculate the output voltage of a 3 phase rectifier. I think it's 1.?? * the AC voltage...

Help...

 

[elevmike]

nevermind....1.732

 

[TConnolly]

A little trick to remember it by:

3 phase power
square root of 3 = 1.732.

 

[elevmike]

Thanks for replying Alaric; Been a long day....

 

[randylud]

Mike, you are not quite right if you are looking for the average DC output from a 3phase bridge rectifier. The calculation goes as follows:
Peak X 0.955 or RMS X 1.35
Given a 480VAC 3phase supply you would get:
(480 X 1.414) X .955 = 648.12VDC
480Vrms X 1.35 = 648VDC
The 1.73 relationship takes into account the phase shift between phase voltages and currents when you are calculating power and the like.

 

[TConnolly]

*wipes egg off face*

Randy's right. That what I get for firing off a knee jerk post without thinking.

 

[Rod]

Randy,

where did you get the .955 from?

Rod (too lazy to fire up Excel and play 'Poke-and-hope'

 

[SimonGoldsworthy]

Vdc=Vrms * (root 2) * 3/pi or Vpeak * 3/pi

So the 0.955 comes from 3/pi

 

[randylud]

Rod, the short answer to your question is from the textbook that I use for teaching industrial electronics. The long answer is probably a relationship based on the rectified sinewave and is more complex than is worth the effort to use. I do not know the derivation of the factors I quoted, just use them when necessary. It is the same way for the 1.73 factor, that is of course the square root of 3 but why it is it not as relevant as what to do with it and why the relationship is important. I use a very down to earth, practical approach in my courses.

p.s. Simon may be right in his statement but I can not confirm or refute it.

 

[elevmike]

Randy,

Thanks for saving me from what could have been an expensive blunder. We ordedrd a DC power supply, (460 3phase AC to 240 DC. 21 Amps), from a local tranformer company, and all they made was the tranformer...It's got 3 sets of secondary taps, so the rectifier needs to be connected to the correct taps. They usually provide the entire assembly w/enclosure, but dropped the ball on this one.

Thanks again.

 

[kamenges]

Mike-

Keep in mind that the 648 VDC value is a no-load value. As you start to load the DC side the voltage will start to drop. If you load it far enough you will sooner or later get the RMS voltage level of the AC supply out of the DC side.

Keith

 

[randylud]

Glad I could help Mike, and Keith is correct about the no-load value from the calculation. A good stiff set of filter caps and even a good voltage regulator on the output of the filter will give you a good, stable supply. One with out the filter/regulator will doop pretty severely as you load it. If the voltage value is not all that important, skip the regulator and just put some pretty good sized caps on there for a filter.

 

[elevmike]

I dont think regualation is an issue on this job. This is for one of our ol' Detroit Landmarks...about 80 years old. Im replacing a rotating generator used to power the ooollllldddd controller. The hoist motor is controlled by another MG set. This place is so busy that they cant be shut down for a full mod, so the obsolete equpment is starting to look like a Rube Goldburg contraption due to all the retro-fits.

Wish me luck.....

BTW, this particular unit is knowen as Christine. If I say Christine to anybody in the local trade, they know exactly what unit I'm talking about.

Off to the races.. Thanks again.

 

[randylud]

Good luck and tell Christine I said hello! LOL