Measuring rpm

[dgd]

In designing my own clipper for a 2kw turbine I would like to get rpm data directly from the turbine..
My initial thought is  to use a slotted opto switch statically mounted and a rotating blade, or maybe two or more pairs of these.
The immediate problem I have is powering the electronics at the turbine. Was considering a small li-ion battery powering the opto slot system, counter circuit and a short range 2.4 ghz data connection
Keeping the battery charged becomes an issue, small solar panel with simple controller.
My calculations using a 4ah 12v li-ion battery would keep the electronics alive for 24 to 36 days
So recharging becomes essential.
Has this already been done? Any pointers or suggestions would be welcomed
Dgd

[Frank S]

Heres a thought that sounds crazy even to me but how about mounting a bicycle dynamo to the rim of the rotor let that be the power course to keep the battery charged I'll bet that Flux would know of a way to make a module to charge the li-ion the whole thing could be mounted in the nacelle

[Flux]

There has been a fair bit of discussion in the last few days about measuring turbine rpm, it may be worth reading it to see the other options.

Personally I see no reason to have devices on the turbine when rpm is available from the terminal volts to your rectifier ( to an inverter I guess in your case).

Your opto chopper will work fine and some have done it. As rpm is not needed when the turbine is stationary I would have thought that a simple power supply using the turbine output would be better than a battery. If you want to use the battery then again a simple battery charger from the turbine output would be easy to do. I am sure the solar panel idea would also work if you want the added complication of mounting it .

You give no idea of the turbine or its output, but if starting from scratch with a home built machine you could add a small coil of a few turns to the stator for a power supply or for  battery charging.  This is based on the idea that Frank S suggested. The cycle dynamo would certainly work but I could never justify the complication of driving it as neither a commercial turbine of a home built axial would have a convenient drive point.

Flux

[dgd]

Thanks for reply and sorry I didn't see the other rpm message thread, I should have posted there 
The reason I ask is...
I picked up a turbine body at  a farm equipment auction, no blades hub, tail  or controller. The only marking is Y2000 and Xiaxian. I guess a Chinese 2kw unit. Cleaned it up, fitted hub with  3.2 metre blades, made tail unit, mounted it on 6 metre pole, wired to 3 phase rectifier and measured dc voltage out in range 42 to 72 volts. So it seems to be a 48v turbine.
Connected it to a Classic 200 and took a guess at a power curve using Classics existing power curves.
It all worked and I saw 700 watts in a 28km wind.
Now I would like to build a more accurate power curve hence the idea of getting some data on rpm so I could construct a more accurate output watts at various rpm.
The nacelle can be accessed and there seems room to get some rpm detection electronics in there with a small lightweight li-ion battery. I have a flexible 8 watt 12 v solar panel which would wrap nicely over nacelle top hence the solar panel idea. Although placing a small dc motor inside belt driven from main shaft would be nice but would need some fitting..
I can get the slotted opto inside too.

I was also thinking of the clipper I am building to work with this turbine and an rpm data into the clipper logic would help with getting dump resistor values accurate and the voltage width for the classics PWM aux output.
Dgd

[Flux]

The slotted opto and radio link will work fine. I don't know enough about lithium ion batteries and their charging requirements to comment on how much technologh you need to use the solar panel to charge it.  Something simpler and more robust such as NiCd would lend itself to a very simple current limit charger and choosing the right size panel may be all that is required.

Give it a try, should work fine. You may be a bit short on volts for the classic but it may programme well enough.

Flux

[Jason Wilkinson]

  "Personally I see no reason to have devices on the turbine when rpm is available from the terminal volts to your rectifier"
  Hi Flux  could you go into a bit more of this , meaning how can one do such   

    Jason

[Flux]

If you look back a couple of posts to something by midwoud1 measuring propeller rpm there was a fisr bit of discussion and one of the proposals was to measure the frequency of the incoming ac supply.

There is a nice convenient chip available under various numbers but it is generally known as a 2917 tacho chip. It converts frequency to a dc voltage which you can measure on a dc voltmeter and scale it to rpm.

I am fairly sure midwoud1 gives the circuit as designed by Hugh Piggott for a controller he did some years ago. This is a nice convenient circuit to use and it just uses a little isolating transformer to separate the ac line connections from the rest of the circuitry. You can use the 2917 without the isolating transformer but it is far easier to use it and it simplifies  things a great deal and quite often it is not necessary to filter the input and it provides the ideal input for the chip as the input needs to cross zero.If you try without the transformer you have to couple things via a capacitor and things are noisy electrically with the rectifier chopping the waveform.

You can use a multimeter if it has a frequency range but again you will need to filter the voltage to get consistent results as those multimeters will measure high frequencies as well as the low frequency you want to look at.

Flux