How to measure the electrical current in testing a wind turbine performance?

[faqmar]

So I am doing research on blade design for small wind turbine and now I am testing my blade design with a 1000W/48V permanent magnet generator. The main objective of the testing would be: How much of the power could be generated by my blade design? and my problem is: How do I generate the electrical current? because then the current will be needed to measure the power (P=IxV)

My testing site is in South-Eastern Asia with a quite low wind speed (7-8 m/s, tops) and I have made several attemps for that:
1. I rented 4 12V batteries connected in serial circuits. The problem is that the voltage produced by the generator, Vgen, has never been greater than the Vbatt, hence no current is generated. The record shows the Vgen are around 35V tops. The owner of the batteries advised me not to discharge the batteries below 10V each, or 40V for the whole because it may generate heat on the generator. I've read the IEC 61400-12 Power Performance standard but I couldn't find any information in this matter.

2. I use 1000W/48V generator so I assume the max current will be around 20A. So I made a resistive cicuit with 20 48V light bulbs+switch conncected in paralel circuits. I managed to get some electrical current with this method. But then my concerns are:
(i) The cut in speed of the wind turbine raises
(ii) if I turn on too many switch the turbine will slows down and stop, thus I could not estimate the max current achievable by the generator
(iii) if I turn on and off the switch then I dont have a steady parameter for the load as I am not only testing one type of blade design this become a problem

Any advice will be greatly appreciated. And this is my first post so I am sorry if there are any mistake in the way I am posting this.

[dnix71]

You shouldn't discharge the batteries below 12.4v each unless they are made for deep discharge. I am assuming you rented flooded lead acid batteries. Some sealed batteries made for UPS's can tolerate discharge to 10v a few times and still recover. Those shouldn't be used for a windmill without some kind of over-voltage protection so you don't cause them to gas and pop the vents.

You aren't going to make much power if your windspeeds are low. Power rises with the cube of the wind speed.

One way to make some power in low winds is to take a mill made for higher voltage and connect it to an mppt. I have an AirX that was wound for 48v. It doesn't put out much power but I still get some because the battery is only 12v.

[faqmar]

thank you for your reply, dnix71
I might need a little further explanation on the "mill" in "..take a mill made for higher voltage and connect it to an mppt. I have an AirX that was wound for 48v..", what is it?

One of my testing objective is also making a power curve for the turbine with my blade design. I am always wondering how people made the power curve. Because in power curve usually shows us the power generated from 0 m/s until the rated and cut-out speed. How to measure the power when the wind speed is still low / below the rated speed.

[tanner0441]

Hi

You say you see 35V from the turbine. I would be tempted to connect the batteries for 24V and see if the turbine produces current at that voltage or if the turbine isn't getting enough power from your low wind speeds to overcome the load.

You don't mention the size of your blades, or the volts per RPM from the turbine, so you know speed you have to obtain to reach cuttin.

Brian.

[dnix71]

You say you are using a 48v generator but only see 35v max. That sounds like the generator is too big for the windmill.
Start over. The swept area determines the max power available, keeping in mind the Betz limit. Doesn't matter what the voltage is, except that higher voltages tend to be more efficient to handle. The Betz limit is 59.3% of the theoretical power available, 20% is pretty good home made.

You need to know up front how many watts you want to make. My AirX [1 meter diameter blades] never made more than 200 watts and that was in a gale force wind during a thunderstorm. In 5m/s wind 10 watts would have been normal. There isn't much point in playing around with small blade design unless you want a lawn ornament, especially in slack winds.

How big are your blades? Do you have clear air? Objective measurement is either trial and error on a real mill, or you need someone with a wind tunnel that is willing to help you.

[faqmar]

That's a good idea that I might try.
my blade lenght is 1,8 m and from my observation it is about 1 volts per 10 RPM.

The main objective is for experiment only as this is my final project for my degree. I have done wind tunnel testing for the smaller but same blade design (30cm). The testing site is pretty much has clear air because it is sited on a beach. I was aiming to reach 1000W but then it's quite impossible in this wind condition

thank you for sharing your experience, I'm a total newbie about this

[electronbaby]

Hello faqmar, if you are designing around 48v dc battery charging, and your blade length is 1.8m (diameter 3.6m) and you are carving wooden blades similar to Hugh Piggott design or Otherpower design, you will then want to cut in around roughly 125RPM. This means that at 125RPM you need to have at least your battery voltage after the rectifier. This means, for example if you are generating 50vdc, your turbine rotor will need to be spinning 125RPM, so that would be .4vdc/RPM. You claim that you are seeing .1vdc/RPM, so it appears that your alternator is wound with too few turns (more along the lines of a 12v turbine).

If you are in a pinch and NEED to get your testing done, I would re-wire the batteries in parallel, at which case you could possibly properly load the rotor at realistic RPM. Not sure what ampacity of wire you used in your stator, so beware of getting it too hot, as it will need to pass 80A to get the proper 1000w loading you require.

Hope this makes sense.

RoyR