HOW TO DETERMINE FREQUENCY OF AN AC GENERATOR (alternator)

 
The AC mains frequency in the US is 60 Hertz (cycles/second). AC mains frequency elsewhere may be 50 Hertz. The following calibration is intended for 60 Hertz devices.
 
If your generator engine runs too fast, the AC output frequency will be too high. Clocks and AC motors will run faster than designed; the converse is true if the engine runs too slow. Transformers and electric motors operating out of designed frequency may be not as efficient and may run hotter than normal. The ‘clock’ speed of digital devices may vary accordingly. For maximum efficiency, your generator should output AC mains frequency at 60 Hertz if it is used to power devices manufactured for use in the US.
 
There are several methods to determine if your generator is operating at the correct frequency. Some sophisticated DVMs such as the advanced Fluke meters will read frequency directly. Another option might be use of an oscilloscope.
 
A low tech option is as follows:
 
Connect an AC mains powered digital clock to the generator. Start the generator and record the time displayed by the clock when the minute changes. Using a independent time source such as a mechanical Rolex (true time), let the generator run for one hour. You may wish to calibrate the mechanical standard to WWV or WWVH. Record the time displayed by the digital clock after one hour of operation based upon the mechanical ‘standard’. Apply the following formula:
 
(Displayed digital elapsed time / true mechanical clock elapsed time) = ratio error caused by improper generator speed.
 
For example, if the digital elapsed time reads 1 h 5 m when the mechanical standard elapsed time reads 1 h 0 minutes, the difference is +5 minutes. This means the generator motor is running slightly too fast resulting in a frequency higher than 60 hertz.
 
For the example given we have:
 
65 minutes elapsed time displayed by digital clock / 60 minutes true time displayed by the mechanical clock = 1.08333
 
Generator motor speed/1.08333 = correct slower engine speed required to produce true 60 cycles/second (Hertz).
 
Use a tachometer to determine existing engine speed and then divide this speed by 1.08333 to obtain the correct slower engine speed that will produce a 60 Hertz output frequency. Depending upon the generator engine, its speed may be adjustable either mechanically or digitally (if computer controlled). Make the adjustment to slow the engine to the computed desired speed and repeat the experiment until the digital clock elapsed time and mechanical clock elapsed time are in agreement. Or alternatively, one could forgo the tachometer and incrementally adjust (see example following) the engine speed until the digital clock elapsed time is the same as the mechanical clock elapsed time.
 
If for instance the digital clock reads 56 minutes when the true time is 60 minutes then we have (56 minutes digital elapsed time/ 60 minutes true elapsed time) = 0.933333
 
Generator motor speed / 0.93333 = correct faster engine speed required to produce 60 cycles/second (hertz). Again, repeat the experiment until both elapsed time standards agree.
 
My various generators were factory set all within 5% of 60 Hz. The above simple procedure can be used to check your generator frequency.
 
Best wishes,
Panhandle Rancher

 

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