Easy way to determine rpm and wattage prior to getting the genny airborne

[Mickel]

Im in the process of building my first genny and have read a number of articles of guys measuring there rpm before it goes up. The only measuring equipment I have is my digital voltmeter and ammeter and wondered if any body had any clever ways to measure the rpm.

[tecker]

I bought a strobe a year or so ago that I  can vary for 10 to 100 cycles . A little reflective tape and I can get pretty close . I have a tach to but with the unit in the air the strobe is usually the tool I choose .

[jmk]

 I wrote down a formula from a previous post. You can use a frequency setting on a multi meter to read the hertz. The formula they posted was cycles / seconds times 60 seconds / 1/2 the number of poles = rpm. 15 Hz times 60 / 6 = 150 rpm.  I'm not sure if it's correct but I want to try it on my machine when I get a meter with a frequency reading.

[finnsawyer]

You could use an ac electric motor of known RPM and gear it down to the RPM of interest (some people use a drill press).  You measure the voltage across one pair of leads of the star connection (open circuit voltage) at the speeds of interest.  You should also measure resistance between the lead pair.  The maximum power that the alternator could then deliver can be estimated by terminating each lead pair in that resistance and calculating the power into each load resistor and multiplying by three.  The problem is that you probably won't use the alternator in that way.  If you rectify the voltage and feed the current to a battery you have both the battery voltage and battery resistance being important as well as the voltage drop across the diodes.  Each lead pair will provide the battery current one third of the time.  While the lead pair current will vary somewhat from the DC average, you could assume the DC average to find the power out.  You have a circuit loop with a voltage source equal to the lead pair open circuit voltage in series with the lead pair resistance, two forward biased diodes, the battery resistance, and the battery voltage.  Note that the battery voltage will oppose the lead pair voltage as will the diode voltages.  From this you can calculate the current through the loop and the power delivered to the battery or the power lost in the alternator.    

[Mickel]

Thank god for you guys out there it makes my job a lot easier(and my learning)!! Thanks that should see me right for awhile.

[SparWeb]

Before getting a bicycle spedometer, I used my digital camera to film my old VAWTs spinning.  I counted the turns against the number of frames per second to get the RPM.  Doesn't work so well for things turning over 300 RPM.