# 3 phase - bridge rectifier input/output power

#### Skeebopstop

Joined Jan 9, 2009
358
Hi all,

generally when considering a 3 phase circuit it is consistent of sinusoidal currents and voltages under a balanced load. However once one introduces a 3 phase bridge rectifier this seems to break down and root(3)*Vline*Iline ceases to hold as the 'input power' into the BUS, in particular since one phase is 'not' conducting at any given point.

I have run some simulations and see the relationship, however fail to create an easy input to output power relationship.

For example, lets say I require 3kW of output power on a DC Bus being driven from a 3 phase rectifier. How can I easily determine what rating the rectifier must have? For now lets assume the DC Bus has no ripple, so that will automatically factor in a safety margin.

In reality I suppose I could just always simulate it and calculate the average currents in the rectifier diodes, but it would be nice to have something to reference in conversation.

Cheers!

#### t_n_k

Joined Mar 6, 2009
5,455
Hello Skeebopstop,

I guess my first consideration would be the DC load characteristic. Is it constant current fed for instance? Is it a constant output voltage case? As you have indicated - what is the application? You may then be able to build up a few device rating scenarios based on those considerations and the end user's requirements.

Have you considered looking at the power semiconductor manufacturer's websites - Semikron or Fastron say? - if they are still in production. I recollect that at least one of those two used to have quite detailed user information on their range of devices and typical applications.

I'd be interested in following up on this.

#### mik3

Joined Feb 4, 2008
4,843
P=V*I

If you solve for I:

I=P/V

I=current in Amps
V=voltage in Volts
P=power in Watts

Use the maximum input power to the inverter and the DC voltage out of the rectifier to find the current rating of the diodes.

#### Skeebopstop

Joined Jan 9, 2009
358
P=V*I

If you solve for I:

I=P/V

I=current in Amps
V=voltage in Volts
P=power in Watts

Use the maximum input power to the inverter and the DC voltage out of the rectifier to find the current rating of the diodes.
Well the tricky part is, that the DC Bus voltage will have the ripple of the rectifier. So the output power of 3kW will cause a variety of different currents.

I guess ideally, just find the low point of the ripple, assume that the voltage and therefore the maximum current can be determined.

The final trick to this question is, generally the DC Bus will be composite of some big ole caps, and thus the rectifier is actually just pulsing in currents ever few milliseconds. That being said, I suppose the 'peak' currents of the diodes is the most important.

These peak currents would have to be calculated from the equivalent ESR of the caps and the maximum voltage delta based off DC Bus ripple and line voltages.