EnigmaPaul said:

Hi,

If I need a solid 350VDC output from a 3-phase generator, can someone

point me to the equations to use to figure out how much peak to peak

voltage I must maintain on the output of the generator to achieve the

DC voltage out using a 3-phase rectifier?

Thanks!

This depends on the source at hand.

If you have a floating Delta supply, then you are stuck with

the phase to phase common practice which in general is a basic

DC peaks = RMS * 1.414

minus all the other little crap i'll explain later.

If you decide to use a grounded (Y)/Star type source, then you could

use a simple 3 Diode full wave with respect to ground/common.

Since 3 phase xformers are designed with a 120 degree offset

in mind, each leg is actually generating more than what you

measure from leg to leg. This is needed to make up for the degree

offset so that you can achieve required voltage leg to leg how ever,

in systems where the (Y)or 1 leg of a delta is ground and thus using

a common as the low side. You need to up the actual when converting

to single phase/DC.

in which case.

Vrms = Vrms * (1/0.8666);

for example.

in systems that employ 480V 3 phase to common point.

Vrms = (480 /2)*1.153 = (480 * 1.153) /2 = 276.72

We'll just round that off to 277.

systems are measured in RMS (mostly), you then calculate your Peak.

Pk = 277*1.414 = 391.68

diodes have a loss of ~ .7 per unit.

so, if you use a full bridge in a floating 3 phase, you'll

have ~ 1.5 loss which gives you a total of ~390.0

after that is all said and done, you have some ripple

from the caps..

I guess if one was to rely on the standard system

for better ripple control, one can use that extra voltage

to be dropped via a resistor on the reservoir capacitor.

So in the end, it all works out

P.S.

The dropping R must be calculated via the expected load and

the ripple error. The R will drive the reservoir cap. Also

keep in mind that crappy caps with high ESR can give you some

bad ripple.

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