RPM from AC frequency.

[bigkahoonaa]

I'm trying to see if it's possible to measure RPM from AC frequency produce by a genny.  It's 5 phase with 10 coils and 12 magnets on either side.  Messy.  I wound like to change AC frequency to binary, something like 0 when < 5 V and 1 when > 5 V.  Has anyone tried this?

[Ungrounded Lightning Rod]

Yep.

Piece of cake.

[oztules]

Underground Lightning Rod,

Amongst your cake recipes, do you have a fix for this?

Using a multimeter,(set for hz) I measured the output into a resistive load no problems, but on hooking up a bridge rectifier and drawing load into batts, the freq readings went haywire. It appears that the non-linear load of the diodes sent spurious harmonics back up the line rendering this measuring system useless for any but linear loads.

Do you have a simple recipe for this sad development.

Hit the freq wall.

........oztules

[Gill]

Yes, familiar with the problem.

I have 3 meters each with different triggering levels.  The cheep meter works fine whereas the quality ones pick up the harmonics.  I found this simple method works fine.

Fit 2 x 100K resistors, one to each lead.  This attenuates your input, hopefully to a level where the harmonics no longer trigger a reading and you are left with the fundamental Fq.

[oztules]

Thanks Gill,

will try this tomorrow (suspect as the freq goes up and the power goes up, may require further attenuation....) but will give it a go.

........oztules

[Flux]

Everyone here seems to treat this as easy, I just wonder how many wrong answers are about.

I have found that almost invariably a filter of some sort is needed to make sense of almost all frequency measuring circuits. The 2917 tacho chip can be an absolute bitch to get to operate reliably, particularly when any form of switching from a mppt converter is present. Opto couplers seem to make life very much easier.

In many cases a simple filter is adequate but it is no use giving values as various meters have totally different input characteristics.

Flux

[watermanhfl]

See my diary post on my remote RPM meter.  Used Hughs plans for a simple circuit.

http://www.fieldlines.com/story/2007/2/25/171213/184

http://www.scoraigwind.com/circuits/index.htm

Anthony

[wind pirate]

If you use a microcontroller (PIC or Basic Stamp or??) you can count pulses with a few diodes and an opamp. See the information at http://thebackshed.com/Windmill/PicLog.asp

WP

[Ungrounded Lightning Rod]

Fit 2 x 100K resistors, one to each lead.

I'd use 1k resistors and put a capacitor across the input of the meter.

Size the capacitor so the time constant (2 pi R C where "R" is the sum of the resistors in ohms and "C" is in farads) is no more - but not a LOT less - than the period of the frequency at cutin, to be sure the harmonics are attenuated significantly but you still have a signal to measure.

See RC Time Constant on Wikipedia.

[BigBreaker]

Don't use the main stator coils to sense RPM.  Put a small neo on one of the rotors and use a coil to pick up the signal.  Use a small fermite core in the coil to saturate it at a reasonable voltage level at all meaningful speeds.  No harmonics and saturation will limit the voltage rise.

Alternatively use an optical pickup set up like an encoder.  You don't need the precise angular position of the rotor so it can be sloppy.

Using the power signal as an input to control logic is a bad, bad idea.  I'm assuming that you want RPM in digital for use in a load controller.  The harmonics that are in the line already are nothing compared to the feedback from using the power waveform directly in your controller.

The inductance of the stator coils combined with any load change is going to cause voltage spikes that will mess with your logic and potentially fry it.  If the controller to doing PWM dump control or charge management the load changes will feedback into the input and ring chaotically.

[kell]

One more idea.  You could use a variable reluctance sensor, like the crankshaft position sensor in a car.  You could score one for cheap at a junkyard.  A vrs senses the passing of some metal like a geartooth and puts out a pulse.

[bigkahoonaa]

I don't bake, I only fry electronics.

[bigkahoonaa]

Thank you all for your ideas.  I can see that there is a diversity of opinions re RPM from AC frequency. That's what I like about this site.  Many users have experimented.  Some have failed, and some can measure RPM from AC frequency.  The consensus seems to be that measuring RPM form AC frequency is possible, but spikes can happen when switching, and an AC line can have harmonics. Am I correct in saying that any circuit measuring RPM from AC frequency should account for possible line spikes and line harmonics?

Mau