Shunt resistor

[Boss]

I want to make a inline resistor to reduce the load our axial flux turbine sees to hopefully increase the RPMs and charge better.
I don't have most of the numbers I need to do the math, so this will be an exercise in guesstimation

I'll get the OtherPower book out and quote the basic numbers of the design: Star & number of windings, etcetera, ours is the standard ten foot turbine, with around 60 feet of SO 3 wire 8 AWG  cable" http://www.delcowireus.com/sow-cable-oil-and-water-resistance.php
I have this shunt which I still need to install as well as the 50 amp meter which measures the input to the inverter,

I'm hoping someone can tell me what the numbers mean.
 I read 30A which I assume is the overall current the shunt can carry, 75mV which I assume is what the meter needs for calibration, then there is 0.5 which I have no idea about
In understanding this shunt, I hope to make a couple resistors  in the 2 to 4 ohm range to test on the output of the alternator after the rectifiers and before the battery bank

Okay, my ohm meter says the shunt is .4 so that must be what the 0.5 number is, with a large tolerance, right?
I gotta run I will get those numbers for you in a while
 

[ghurd]

It all goes back to Ohm's Law.

The shunt is rated for 30A max.
At 30A, the voltage across it is 0.075V.

0.075V / 30A =0.0025 ohms

The 0.4 ohms the meter is reading is mostly the test leads.  It takes special meters to read below maybe 5 or 10 ohms with much accuracy.

Not sure about the 0.5.  Possibly the tolerance of the shunt?  0.5%?

[Rover]

Boss, One thing to keep in mind with shunts of this type, is you do not want to run them continously above 60-65% of rate capacity or they derate. The 0.5 is the accuracy in percent (O.5%)

If you expect your current passing through the shunt to exceed 20A on a continuous basis, you should probably get a larger shunt.

You say you have a 50A meter, you will also need to know the voltage drop it will measure full scale in order to give you accurate readings (otherwise you may have to do a little math)

example: If your 50A meter is rated for 50mV full scale, then at 25A through a 50A shunt rated at 50mv drop will appear correctly as 25A. (50A would appears as 50A).

however if your shunt is 75mv drop and is rated at 30 amp, at 25A it wll drop 62.5 mV. So if the meter is calibarated a  for 50mV drop = 50A it will max out,  displaying above 50A

even simpler: If yout shunt is 30A/75mV and your meter is 50A/75mV, the meter will display 50A at 30A actual

 

[Boss]

cool more food for thought
I wondered about the 30A shunt and the 50 amp meter, no I must be mistaken, it is a 30 amp meter

[Rover]

I should have added this in the examples
ok... even simpler, your meter would need to be calibrated for 125mV drop full scale (50A=125mV) to display 30A as 30A on the meter you have. (50/30 * 75)

Typically you won't find meters like this because typically no one wants to increase the voltage drop for a meter more than needed

Somoene check my math I'm a little foggy.. no dinner yet

[Rover]

cool more food for thought
I wondered about the 30A shunt and the 50 amp meter, no I must be mistaken, it is a 30 amp meter

If they were bought at the same time as a pair, more than likely they are in sync. However if it was a meter hagning around and a shunt hanging around... if the voltage drops don't match (even though both are rated/calibrated at 30A) , you readings will be off. If it is a analog meter, the sometimes have the voltage drop on the panel, if digital.. you'll needto look up the docs.

[ghurd]

Fairly simple to make a shunt to match the meter,
and make the meter match the desired max reading, even if the meter needs a new label on the front..
http://fieldlines.com/board/index.php/topic,133114.0.html

[Rover]

Boss, I agree with Ghurd, but you already had the shunt.

(hmm I also have a hard time believing you can get 0.5 % accuracy on a typical homemade shunt, but at 20 amps .. 1% - 2% probably not that big of a deal for informative purposes)

This may all be moot if they match

[ghurd]

Rover, I do agree 101%.
But the meter sounds analog, '60% rating' max (as you pointed out earlier), etc.
So 50% of 30A is 15A, at 25V average, means 5% error is only 19W.

10' is rated at 800W peak, and usually a LOT less, say 200W and 8A, so its typically only a couple/few 100ma difference on a 30A scale?
No Way I could read the difference!

edit-  Not too hard to change the total shunt value with some wire either.
Sure would be easier with a factory made shut to start with.

[BrianSmith]

Sometimes shunts are marked with their current rating and the voltage you see across it at the current rating.  If that was the case:

V = I*R ==> V / I = R  so  0.5V / 30A ==> 0.0167 ohms

Power burning in shunt at 30A :  I * I * R = 30 * 30 * 0.0167 = 15 watts

I don't see how the shunt could be a much bigger resistor than that and not get too hot to handle.

Try measuring ohms on your meter probe to probe and see what it says.   

[BrianSmith]

Please ignore above, I missed the 75mV text.. 

[Rover]

I hate it when I do that too , and I've done it too many times

[joestue]

Negative on the de-rating, its only going to dissipate 2.25 watts, three if you add in the contact resistance of the ring terminals.

FYI: cheap dmm's generally read mv accurately, with a one amp current source made from an lm317 and two d batteries (or something more stable) its easy to make accurate 4 wire resistance measurements to 1milliohm.

[BrianSmith]

That's a good idea for measuring low ohms.  Thanks.