# 3 phase rectifier

Discussion in 'Electronic Design' started by EnigmaPaul, Dec 12, 2008.

1. ### EnigmaPaulGuest

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!

2. ### MooseFETGuest

What sort of filter is after the rectifier?

Assuming a standard 3 phase bridge:

The bottom of the ripple is at 3/4ths the peak to peak minus the drop
in the diodes. If you need to maintain a voltage without filtering,
this is the point you need to worry about.

The peaks of the ripple are at 0.866 of the peak to peak minus the
drop of the diodes. If this is a capacitive filtered case, this is
the peak of the ripple on the output.

A good estimate for the capacitive filtered ripple amplitude is:

V = I / ( 360 * C)

3. ### EeyoreGuest

It won't be solid. It'll still have ripple, but 3 phase is much better in
that respect than single phase.

Graham

4. ### EnigmaPaulGuest

There is nothing designed yet. What we're trying to do is to
determine how much 3 phase AC input we'll need to be able to rectify
down to 350VDC. We can design the rectifier and filter to be whatever
we like and we're probably use whether the best state-of-the-art might
be. Assuming that, what is the relationship between the best DC out
versus AC p-p in?

5. ### Rich GriseGuest

1. Do your own math - that's what school is for.

2. Don't top-post

Good Luck!
Rich

6. ### EeyoreGuest

Rectify DOWN ?

To be honest it doesn't sound like you should be playing with ANY
electricity, never mind 3 phase !

Graham

7. ### EeyoreGuest

WHICH output ? Phase to phase or phase to neutral ?

Graham

8. ### JamieGuest

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

http://webpages.charter.net/jamie_5"

9. ### Rich GriseGuest

Hey, Guy! What's this guy paying you to do his homework for him? ;-p

Cheers!
Rich