digital ampmeter question

[dlenox]

I recently purchased a digital ampmeter that came with it's own shunt.  It was originally designed to be put into a 12v automotive system and I have it in a 48v wind turbine.

Are ampmeters calibrated for a specific voltage, or is it simply the voltage drop across the shunt that is used for reference?

Dan

[Flux]

The current measuring bit is not affected by voltage. If it was a nice simple analogue thing then as long as the meter fsd matched the shunt it could be used on any voltage system.

With digital life is not so easy. The actual circuitry needs power and that can't be derived from the tiny shunt drop and has to be supplied separately.

Most require the supply to be independent of the measuring circuit and that can be a real nuisance. A few are designed to accept power from the circuit being measured and that is possibly the case with your unit. It will be intended to draw its power from the 12v supply and it probably won't have much latitude in the supply requirements.

At the very least you will need to reduce the 48v to 12v and that is out of the range of the cheap voltage regulators ( 7812 etc).

I really can't see the virtue of digital current metering ( it does make sense with volts). Analogue meters are simple and the moving pointer means something, the ranting digital things just don't suit the type of current from wind power.

Flux

[dlenox]

Flux,

My ampmeter uses an external 9v battery for power.

I agree with you that with digital ampmeters it's constant reporting of the varying amps is a nuisance but price was right at the time, already have digital voltmeter on inverter.

As a side note - One of the reasons that I ask is that at higher winds (14+ mph) the amp output from the turbine and rpm's are low with the new stator that I installed (59 turns of #14 wire two in hand) per DanB's recommendations.

In 8mph winds I am seeing about 6 amps (~350 watts) but at 16mph I only see about 10amps (~580 watts).  We had a 44mph wind blow through last night and I measured 24amps (~1400 watts) so I believe that I need to add resistance into the line to bring up the higher wind output.

The turbine furls properly, starts just below 20mph and fully furled at 25mph.

Dan

[Flux]

If it has an external 9v battery for power then the system voltage doesn't matter at all.

From your description you have all the symptoms of running too far stalled. I agree about adding resistance. This is a good fault and it is far better to loose the heat in the line resistance than in the stator.At 48v you may want an ohm or more to get it to liven up.

Flux

[dlenox]

Flux,

Yes I agree, I have read all your previous posts about this and believe that I understood the issue.  I would have expected to see 24 amps at about 16mph winds.

With the 17' diameter blades I assume that the 1ohm resister has to be able to withstand the total output of the turbine, I saw DanB's comment that at one time he saw 6kw in very high winds so I assume that this resister must be rated for this amount of output.

Will adding this resistance change the speed of furling, or is that constant due to the amount of lift on the tail?

Dan

[Flux]

The resistor must be able to stand the power dissipated in it. If you have 24A flowing in it then the dissipation is 24^2 x 1 or about 600W.

If Dan has seen 6kW then that is about 120A Power into the resistor would be 120^2 or 14 kW.

I don't seriously think this is realistic and if it is then clearly 1 ohm is far too high. More realistically I would think 60A would be near the long term limit and that would need the resistor to handle 3600W if 1 ohm was needed.

Now you can see the problems of letting most of this current be dissipated in the stator. I don't know enough about your stator or line resistance to give you much idea how much you need to add. Just try some lengths of scrap cable until you find the best value rather than buy expensive resistors to test.

Adding resistance will have some effect on the furling but the exact effect is nearly impossible to predict. For modest values of added resistance you will extract more energy with faster blades and the thrust will increase so it ought to furl earlier but for the same wind speed you will get much more out so you may have to furl even earlier to keep within safe limits.

If you let the blades run away with too much resistance the thing may behave very differently, I would only add enough to get the power you want and furling should be ok.

Flux

[dlenox]

Flux,

My stator is 3-phase, each leg has four coils, each coil has 60' of #14 copper (2 in hand), I then run 80' down the tower with #6/3 SO cable, to 85' of #8 copper THHN before I get to the 180' runs of #2 copper into the house.  So each phase has .606 + .0632 + .1068 + .0563 = .8323 ohms total.

I know that the resistive load is normally put after the rectifier on the DC side, but if I replaced the 180' of the three #2 copper (wild AC) runs say with #4 aluminum I could introduce about .1 ohms in each of the lines.

Would taking this approach be acceptable? I've only seen references to adding resistance on the DC side.

Dan

[Flux]

Yes you can add the resistance in the ac cables, there is no real difference in the value whether you add it in 2 dc cables or 3 ac cables, the current in any 2 ac cables at any instant is effectively that in the dc line, that is how the rectifier works.

Unless you can use the heat it seems a far simpler solution to just add the loss in cables rather than obtain awkward value heavy duty resistors.

Just try adding some cable to find what you need then your wire tables will show the prospects of substituting the aluminium.

I am sure that for these higher voltage units people use heavier cable than necessary in the belief that it helps output and it doesn't always work. Just as using a lower than necessary cut in speed is very detrimental to output but they all do it.

Flux