As a former employee and current owner of an industrial generator rental company and electrician with 15 years of generator troubleshooting experience, I’ve had many situations where the engine powering the genset is running great, but the generator isn’t producing any electricity. There are plenty of articles on the internet explaining how to flash the field in order to re-magnetize the generator, but this one will go a step further and also explain how artificially exciting a generator can aid in narrowing down the cause of the failure.
A WORD OF CAUTION…
Just because you don’t see any volts on your panel meter doesn’t mean that you won’t be electrocuted. Remember, the panel meter can be flawed, check it with a hand held multimeter. Secondly, flashing a field results in a build up of magnetism within the generator. Over time of inactivity, the magnetism may dissipate entirely and flashing the field “refills” the missing magnetism. It is, however, entirely possible that the reason for the generator losing output isn’t related to a lack of residual magnetism in the generator. If this is the case, then there will be a voltage present that is lower than the rated output voltage, but definitely enough to cause a painful electrocution.
CHECK THIS BEFORE YOU FLASH THE FIELD…
Use a multimeter to check if there is any voltage present at all. If there is, then it’s a good indication that the problem is not within the generator.
Shut the machine off and locate the voltage regulator. You’ll find 2 wires marked F+ and F-. Pay close attention to where each is and remove them. Put your multimeter on “Ohms” and measure the resistance across them and write down the result. Unfortunately each manufacturer has different values as normal, but 0 and infinite ohms are two bad news signs that you’ll require the services of a generator rewind shop. If you have ohms, leave the wires off and keep reading.
HOW TO FLASH THE FIELD…
Ideally you’ll have a variable DC supply available. If not, use a 9V battery, or even a car battery and carefully hook up the [+ to F+] and the [- to F-].
Start the engine, and keep it at low idle if possible. You should instantly notice a voltage is present. As you increase engine RPM, you’ll notice the voltage will increase. Do not go beyond rated voltage. Keep the machine running for 30 seconds and shut off the engine.
Re-connect the F+ and F- to their original location. Now start the engine and see if you have volts.
I STILL DON’T HAVE ANY VOLTS…
By flashing the field and bringing the machine up to proper voltage, you’ve effectively ruled out any problems within the generator’s main windings (the most expensive repair).
You should now focus on the AVR (automatic voltage regulator), a fuse in line with the AVR, or a wire or switch that supplies the AVR. Do “tug testing” of wires, visually inspect everything that you can. If nothing wrong is found, contact the manufacturer of the generator and find out what the ohms of the field should be and compare it to what you found earlier.
