# 3 Phase Full wave rectifier Design

Discussion in 'Homework Help' started by Xavier Pacheco Paulino, Feb 20, 2016.

1. ### Xavier Pacheco Paulino Thread Starter New Member

Oct 21, 2015
1
0
Hello,
I am designing a 3 phase full-wave AC-DC rectifier. The output DC voltage is Vdc= 350 V. The output may have load variations: 5 ≤ Idc ≤ 10 A but the voltage can't change. My question is: Can I fix the current at 10 A for design purposes?

Thank you.

2. ### #12 Expert

Nov 30, 2010
16,685
7,327
Can you assume that the required current is the current you design for?
I can't think of any other method.
What other amount of current would you use for the design calculations?

3. ### Ramussons Active Member

May 3, 2013
562
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design for a 20% overload i.e. 12 Amps

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
What does it mean when you say "the voltage can't change"?

Does the voltage have to be 350.000000000000 Vdc and if it is 350.000000000001 Vdc then the design is a failure?

You need allowed tolerances on your specs otherwise they are largely meaningless (i.e., there is no way to know if your design is satisfactory or not).

While you CAN pick 10 A for design purposes, you then need to ensure that the design still meets spec over the entire range. In general it is not sufficient to just check at the other end (5 A) since it is possible that the output voltage is right at 350 V at the ends but that it peaks or sags in between enough to be unacceptable.

It is also generally good practice to build in some headroom, so you might design your system to that it is within voltage spec from 4 A (20% below 5%) to 12 A (20% above 10 A), or some other headroom factor -- I like using 50% unless there's a good reason not to (and sometimes there is).

If you are super thorough (though it seldom is justified), you can work your design in terms of the spec'ed ranges mathematically. What I mean is that you find Vout as a function of Idc and then you have a spec

Vlow < Vout < Vhigh

and then you substitute into that the function for Vout in terms of Idc and then substitute in the range for Idc.

Generally an easier way is to invert the function and figure out Idc in terms of Vout and then substitute in Vlow and Vhigh to see what the operating currents are at which you fall out of voltage spec.

5. ### MrAl Distinguished Member

Jun 17, 2014
2,555
515
Hi,

My question is: is the 'voltage' that cant change the input voltage or the rectified (output) voltage?
I ask because the rectified voltage will always change with load for a number of reasons, and the ripple will increase with load also. So at 5 amps the output will be one voltage, and at 10 amps it will be somewhat lower, and the ripple will increase if the output is filtered. That's the basic nature of rectifier circuits with or without filter caps.
Also, i am not sure what kind of other components you might be using, which could change things too. For example, are you using filter caps or a series inductor for filtering, or just the rectifiers?
Also, are you using a transformer and can you design it yourself or are there any specs for the one you might have to use if you are using one?