Anyone ever set RPM with stator voltage
 

First Post, 1250 owner

Anyone ever set RPM with stator voltage or frequency using a multimeter?

301AQS with 15 amp stator.
Manual says 28 volts at 3600 RPM.

Never tried, works in theory.

I only had time to check voltage on this cold afternoon. It does work. As long as my carb settings are correct. Mine was 14.65 volts at idle which is 1883 RPM.

It looks like I had a lot of views, but no responses. My question was either too technical or already tried.

This is linear interpolation for all who want to know how to figure this out.

http://en.wikipedia.org/wiki/Linear_interpolation

I will try to confirm this over the weekend.

I understand it but haven't done it...I use a timing light with a tach on it.

BTW your low idle of 1883 rpm is WAY too high. I usually set mine to 1150 or 1200 rpm. I think the manual says 1000 rpm, but I've had very few engines that would like to stay running that slowly.

This might be a dumb question, but where do you measure the voltage? Across the generator terminals?

You'd have to measure it across the two wires coming out of the stator. There is no 'generator' on the QL series.

It should probably be noted that this will only work on an engine with a stator-type charging system and not a starter/generator.

I checked my idle with mechanical tach today and read 1200 on the nose and 14.8 volts. According to my math it should be 9.333 volts. Therefore my theory is not proven. I will try later with a timing light tach and induction tach to see if the results are similar.

Where did you get the table from, or is it your own creation?

3600 RPM = 60 Hz? Is this one cycle per engine revolution? I thought there were multiple coils and multiple magnets?

Interesting theory though.

I created the table in excel to make calculations easier. The wiki page I attached shows the formula. It only works for linear values, which may be my issue.

3600 revs per minute would also equal 60 revs per second. A rev is a cylce and a cycle per second is also know as a hertz. Therfore, 60Hz.

Confused? I am.

So, you're taking the manual statement that 3600 RPM is 28V. And saying that 0 RPM must be 0 Volts to create two points on a line.

I've always said (jokingly) that "If a linear fit is desired, only take two data points." However, in real life, it simply doesn't work that way. In your case, there will be a threshold RPM where it will begin to start generating current. This will be AC voltage upstream of the rectifier, converted to DC by the rectifier, and regulated by the regulator at about 14 to 15 volts to charge the battery.

I still don't get the 60 Hz part. I'm an EE, so I know a little about frequencies and things like that. But any stator I've seen has multiple coils. Try looking here for more information: http://www.simpletractors.com/servic..._electrics.htm (I apologize to the mods if linking to that site is undesirable).

In that link, I see they say "28VAC or more". I wonder if the "or more" part is throwing you. Each one is bound to be a bit different. Also, this voltage will likely vary with load. If you just started the tractor, the load will be higher since it is trying to recharge the battery. Turn on the lights, or run something electric and it will probably change too.

I suppose you could use a tach and measure the voltage at different RPMs for your particular machine, and create a table. Wouldn't it be easier to just use the tach though?

Since I only have one tach, and it's permanently mounted on a different tractor, I'll probably use my Fluke 187 DMM and measure HZ on the coil wire that attaches to the points. I'll multiply that value by 2 and have RPMs. I haven't done this yet, so no guarantees, but I think it will work.

I hope some of this helps you out a bit. Good to see you're thinking "out of the box" though.