Homebrewed Electricity > Controls

Needing help with resistors

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makenzie71:
So keeping resistors straight is a constant struggle for me so I like to bounce my ideas off other people smarter than I am to confirm my thoughts.

The turbine I'm working with has a nominal output of around 4200w (350v@12a).

I have two ideas I'm entertaining:

First, I would like to try and build a staged brake...something that comes on at 425v to try and control the turbine, then at 450v it just shuts everything down.  Is this even worth considering?  My thought process here is that absorbing a little extra power might keep it in production if it gets hit suddenly, but the full brake would lock it down if things just can't be contained.  My thinking here is that stage 1 would be three 15ohm resistors off the three phases, and stage 2 would be three 5ohm resistors.  The ohms is where I'm confused here.  I know that fully braking the turbine at 425v would be 30ohms...ish...but amperage per leg on the three phases is about a third and I'm using three resistors, and then when the second stage closes there's going to be essentially two resistors in parallel per phase.  How is that math suppose to work?  I think 15 and 5 are correct, but I'd appreciate someone else's input on it.

Second, and this kind of is a continuation of the first, if I just do one stage at 450 to bring the turbine down then what should the appropriate resistors be?  I think it's 10ohms on each phase, but again I'm not 100% certain I'm doing that math correctly.

I appreciate the help with the math!

Adriaan Kragten:
The standard formulas for the dissipated power in a resitor for a DC current are given in chapter 2 of my public report KD 378. For the dump load of a battery charge controller, the resistor is often connected in series with a transistor. The calculation of the dissipated power in the resistor and the transistor is then more difficult. An example of the calculation of for such a situation is given in: "Manual of a 27.6 V, 200 W battery charge controller" which you can find at the bottom of the list with KD-reports at my website: www.kdwindturbines.nl.

If three different resistor R1, R2 and R3 are connected in series, the total resistance Rs is the sum of all resistors, so Rs = R1 + R2 + R3. If three different resistors R1, R2 and R3 are connected in parallel, the formula to calculate Rs is: 1/Rs = 1/R1 + 1/R2 + 1/R3.

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