freq meter

[mgtd]

I think a recent post slightly suggested this, but will one of the inexpensive DVMs with freq. range adequately measure the turbine wild AC for measuring RPM?   If so, is it a direct reading from one phase of the input to RPM. or would 3 coils in the phase necessitate dividing by 3?  Any model number/source references for such a meter if it will work?  

[dnix71]

I wouldn't trust a cheap dvm for measuring a home brew mill.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=41727

is $50 from HF. It uses a piece of reflective tape to bounce the light.

[zeusmorg]

 Cheapest way I've found for measuring rpm's....and yes I got one and it does work.

http://www.fieldlines.com/story/2008/10/18/134440/23

[mgtd]

I already have a Snap On (Blue point) digital tach, as per your suggestion, but after climbing my 103 foot tower, I can't seem to hold it steady enough to read the reflective tape on blade # 2's tip.

It would seem like a pulse counter and a divide by 3 network on the wild AC should do it

Back to my question, can the RPM be read from the wild AC prior to rectification, with a DVM that has frequency capabilities?

[Flux]

Yes it is possible to measure speed from the raw ac but it is not as easy as many assume.

I have not found a commercial dvm that you can directly connect on the frequency range and get a reliable answer.

Many of them will do it with some careful filtering of the input but that doesn't always work out easily. If they use analogue f/v conversion then filtering is usually ok. If they are true digital in the frequency measuring department you have a major project cleaning up the waveform.

I have an ancient transistor frequency meter designed round the very first junction transistors and that will work on the raw waveform and is foolproof.

I have used the 2917 tacho chip for all my other applications and that works perfectly well with a fairly simple filter if it is run from its own isolated supply. If you try to use the mill battery as the supply then the input filtering is challenging and an opto isolator gives the easiest and most reliable results.

Best of luck and double check the answer when you get it, it can be done, I would use the 2917 chip rather than a multimeter but others may not agree.

Flux

[Opera House]

These cheap meters should work if you filter the signal going to them.  Even expensive frequency counters give strange readings i there is any ringing from an inductive source.  Consider using a number of back to back diodes to form a fixed voltage and a resistor and capacitor to filter higher harmonics.

[Norm]

Hey! I told you it would!  LOL

[watermanhfl]

Hi,

I use an easy to build RPM meter using tacho chip off Hugh's site.  http://www.scoraigwind.com/circuits/index.htm

See my Diary.

Works great and allows me to see RPM and approx. Watts from same meter.  I use a wall mounted transformer for power to eliminate Flux's concern.

Enjoy

[mgtd]

In another recent post of mine, suggestions were made that filtering the raw AC would be necessary to obtain readings using a DVM with freq. capabilities.  I have on order an Elenco model LCM 1950 with autoranging frequency to 4 Mhz.  Resolution is supposed to 1 hz.

Can Flux, or others, provide a simple diagram of the filters proposed with component designations.  I'm not an electronic whiz and could use a little more direction.

I hope to get some meaningful output readings from my 10' Otherpower, at known windspeed (Lacross anneometer below turbine)and RPM of turbine (DVM w Freq capability).

If I'm sucessfull, I post and share.  Mill is almost ready to go back in the air after stator rebuild and other mods due to burnout a few weeks ago.