15MSH 2 stroke alternator coil output?

[Hocus-Pocus]

Does anyone know what the voltage output of the alternator coil is over the complete RPM range of the 15 HP 2 stroke?

According to my shop manual, with the regulator disconnected, and the meter set for DC volts, it should produce 12 V @1500 RPM. I would have expected it to produce an AC voltage so the meter should be set on AC volts. It doesn't say what the voltage should be at any speed above that. As the engine speed rises, I would expect the coil voltage to rise.

Also, the manual states that that with the meter set for DC volts, the output from the rectifier, or rectifier/regulator should be 13 volts at 1500 RPM. If the coil only produces 12 volts at 1500 RPM it is beyond me where the extra volt would come from.

[99yam40]

Must be an after market manual you have and not the factory service manual from Yamaha.As the Yamaha manual will give voltages at cranking speed and other RPM also

It should also tell you to use a peak reading meter made for measuring these motors systems. Or use a DVA adapter with your regular meter set on DC

[Hocus-Pocus]

Looks to me like it is an official manual.

On the back cover it says "Yamaha Motor Corporation USA", "printed in USA". On the front cover it says "Yamaha", "Service Manual", "LIT-18616-02-59" and "63V-28197-1F-11".

[boats.net]

The 12V at 1500 rpm's is peak reading voltage, you will need a peak reading voltmeter or a DVA adapter to get correct reading.

[Hocus-Pocus]

OK, now I'm really confused.

There is no mention of a DVA adapter in the manual.

If the AC voltage produced by the coil at 1500 RPM is supposed to be 12 volts peak to peak, and RMS voltage is peak voltage x .707, that should result in an RMS voltage of about 8.5 volts. How is the rectifier supposed to get 13 volts from this?

Assuming that there is only one magnet on the flywheel, I should be able to read an AC voltage at 25 hz at 1500 RPM, and an AC voltage at about 92 hz at 5500 RPM. If I use an AC RMS meter I can just do the math to find peak voltage but I have no information of what voltages I should be looking for at 5500 or any RPM above 1500.

[99yam40]

If you will not take our advice then good luck.
You need the special peak reading meter made for taking these reading to compare to the specs. you cannot do as you think you can

http://issuu.com/cdielectronics/docs...owFlipBtn=true

[Hocus-Pocus]

I apologize if I've offended anyone by offering my opinion on how I might make meaningful observations.

Let's get back to my point, which is that my manual doesn't give any numbers that I should look for, peak or otherwise, beyond 1500 RPM.

Then there is the side issue of, if the numbers I am looking for are peak, how will these peak numbers result in an RMS rectified voltage less than peak?

I have a Fluke 87 VOM that will measure true RMS or peak. I also have a scope that will measure peak. It's not a matter of being able to measure a value, it's a matter of what value am I looking for at RPMs greater than 1500 and is it peak or RMS?

The thought just occurred to me that perhaps I didn't make myself clear in the original post. What I am trying to measure here is the alternator AC voltage which would be connected to a rectifier, or rectifier/regulator. Strictly for auxiliary electrics, nothing to do with ignition.

Does that make any difference?

[99yam40]

Your 87 will not read properly just like mine did not. Use the proper test equipment set to the proper setting as manual says.
All the coils under the flywheel are read with the same equipment, That is why I posted that link and it talks about the DVA

Maybe you need to find a different manual or the motor you have being manual start does not have a charging system like the electric start models and that is why it does not give all the specs

[rodbolt17]

true RMS is not peak.
its the root mean square of the A/C sine wave.
almost a working DC voltage.
most digital meters are simply not quick enough to measure peak voltage nor are they designed to.
RMS and working DC are roughly 70% of peak voltage.
any voltage spec in the manual is minimum voltage, maximum is normally not set.
its a rather simple setup thougfh and only makes about 6 amps output at 5000 RPM.

is your lighting coil a two wire or 3 wire coil?
inother words how many green wires go to the rectifier?

[Hocus-Pocus]

Rodbolt,
Thanks for he reply. We agree on RMS and peak.

For instance, I plugged my 87 into an AC convenience outlet. Using the Min/Max function it gave me a maximum value of 120.8 VAC, a minimum value of 118.4 VAC and an average value of 119.3 VAC. Using the Peak Min/Max function it gave me a peak max value of +165.2 VAC and a peak min value of -164.8 VAC. The numbers make sense and the meter has a range of up to 400 hz.

My manual tells me that the alternator is rated at 80 watts, so I figured that at full RPM , if the rectifier/regulator limits voltage to about 13 or so, it should produce a maximum of about 6 amps, which agrees with what you've stated.

The manual also mentions a rectifier and a rectifier/regulator. I'm not clear on what the differences are or which one is used in what application.

My engine is manual start. There are only two green wires coming from the coil which are not connected to anything. This tells me that it is a simple single phase coil. The object here is to have a battery charging circuit so I can power my sounder and running lights. I can buy an OEM or after market rectifier, or rectifier/regulator, if I knew which one I needed, but by the time the smoke clears it will cost me about CDN$100.

As you mentioned, this is a pretty simple circuit. Lots of older motorcycles, snowmobiles and the like used something like this with just a Zener diode to keep the voltage at a reasonable level and excess current was just dumped to ground. I'm looking into building something myself but I need to know what the maximum open circuit voltage is that I'm likely to see to select the right components. I was hoping that this information would be available in a shop manual, but you indicate that it is not so that answers my question.

Whenever we get a break in the weather I'll check the open circuit voltage myself and see what I've got. I probably won't be able to get the engine up to 5000 RPM in neutral, but if I take readings from idle to as high as I can go, the voltage rise should be linear and I can just interpolate for my expected maximum.

All of this won't be cost effective if I count my time of course, but it's more for the challenge and learning experience that I do things like this.

[Hocus-Pocus]

Rodbolt,
Today turned out to be a pretty nice day so I dropped the little boat in the water and got some open circuit voltage readings. I think you might find them interesting, I certainly did.

It was difficult to hold engine speed absolutely steady with no load, so the readings are all approximate, but fairly close non the less. They do show a definite trend.

With my true RMS Fluke 87 set on AC voltage at ~1500 RPM it indicated 12.1 VAC at 51 HZ. With it set on Peak Min/Max it showed a maximum value of +32.32 volts and a minimum value of -31.92 volts. I would have expected something around the 18 volt range, so I'm not sure what's up with that.

At ~2000 RPM I showed ~15.9 VAC RMS
At ~2500 RPM I showed ~18.2 VAC RMS
At ~3000 RPM I showed ~23.5 VAC RMS
At ~3500 RPM I showed ~26 VAC RMS
At ~4000 RPM I showed ~28 VAC RMS
At ~5000 RPM I showed ~37 VAC RMS

All of this tells me that the flywheel has two magnets (4 poles) and the voltage output is more or less linear in direct proportion to engine speed