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Get Powered Up! Certified Energy Manager Jeff Yago answers your alternative energy questions

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AC generator voltage problem

Tuesday, March 31st, 2009


I have a 7500 Watt AC Generator and have a output voltage problem. If I set speed to 60 cycles my AC voltage climbs to 165 volts. Can you direct me to points of this problem that may help me correct same?

The normal rated output should be 60 cycles and 120 volts AC


Bob Libbon


There are many possible reasons you are having this problem and each generator brand and model have different designs that could cause this. First, what type voltage meter are you using? If you are using a $20 Radio Shack model, then your meter is giving you a good indication that there is a problem. If you are using a high dollar RMS volt meter, then this may be telling you something else. The actual “peak” voltage of the utility grid and a 120 volt nominal generator is actually 169 volts. This is because the voltage is changing every 1/60 of a second from a high of +169 volts to -169 volts and passing through 0 volts twice, so 120 volts is the “average” of this sinewave curve.

As you start adding loads to a lower cost generator, the internal voltage regulator allows the voltage to drop as it tries to maintain current for the load. This means the peak to peak voltage of 169 volts will start to drop. Since all battery chargers use only the “peak” part of the sinewave, as soon as this peak voltage drops, most battery chargers will stop charging, which is why you should not use a low cost generator to charge a battery bank like you would find in a solar home.

Some generator designs are based on a rotating coil that is supplied a DC current, and by varying this DC current the generator can control the AC voltage output from the fixed coil. Sometimes the voltage regulator device used to vary this voltage includes a control that you can adjust.

Some higher cost generators include a special “inverter” electronic circuit connected to the generator output to stabilize this voltage and maintain the 169 volts peak to peak at a perfect 60 cycles per second. For example, most Honda generators add a “i” to the end of the model number if they include this option.

Odds are your generator has a circuit board that is used to maintain the voltage output and something has gone wrong with this control board. Even lower cost generators have some type of control board to regulate the voltage output even if they are not as accurate as the inverter models.

Sorry, but sounds like this is going to cost,

Jeff Yago



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