Tip & How-To about Electrical Supplies

Generator Not Producing Power - Field Winding

There are 4 requirements that have to be met before a generator is able to produce power. Due to number of questions that have been asked, a Coleman PowerMate 1500-1850 with a Briggs and Stratton Engine will be used here.

1) The Engine has to be turning at correct speed
2) Field Winding (Rotor) on the alternator must be energized
3) AC Winding (Stator) must create a voltage in presence of magnetic field
4) Output passes through Safety Devices before it reaches an outlet.

This Tip will cover the Field Winding.

The alternator of the generator is similar to the alternator in your vehicle. A current flows through the field winding, and creates a magnetic field. This magnetic field is rotated (by the engine), and the magnetic lines of force cut through the many coils of wire that are located in the stator.

First things first though. Lets see if the field winding (aka rotor) is any good. You will first have to disassemble the generator / alternator to get to the brushes. The brushes are how the current flows from the regulator, and into the rotor while it is spinning. Look closely, and you will see 2 rings on the rotor. Each ring is the end of the coil of wire that makes up the rotor. Using a multimeter, check for resistance between the rings, but make sure that you don't scratch or gouge them. You should have some kind of continuity here, compare your readings against what is published in the service manual. Also check each ring to the core or rod of the rotor as well. There should NOT be any continuity at all. If there is, this indicates a grounded rotor, and will have to be rewound (starter / alternator shop) or replaced.

Look at the brushes and brush holder assembly. Take a measurement of the length of each brush, and compare it to the minimum length in the service manual. If the brush is too short, or shows signs of overheating, shock, or otherwise, replace the brushes. If the brush holder is also darkened or burnt, it will also need to be replaces as well.

From the brush holder, follow the wires to the voltage regulator. In most low end generators, this is merely a capacitor that samples output voltage, and feeds it back into the rotor. There should also be a diode here as well. Again, using your multimeter, check the diode to see if it is open or shorted. If your meter has a diode check function, use it. Otherwise, check the diode using the resistance scale. You should have a resistance in direction, and infinite (open) in the other. If you have resistance in both directions, or infinite in both directions, then the diode is bad and will need to be replaced. Some meters will have a capacitor check, but the capacitor in the generator will likely be too large for this to work. Look for signs that the capacitor may be bad. Bulging, leaking, damaged terminals all indicate replacement is needed.

On generators that have an actual voltage regulator, you will need to consult the service manual for steps to check the regulator. Usually, regulators are not easily tested, and are replaced when other potential problems have been ruled out.

If a generator has sat for a long time, it may have lost its residual magnetism. When the engine is not turning, there remains a small magnetic field due to the properties of the iron / steel rotor core. It is possible that this field has dissipated over time. You can temporarily reestablish this field through a process called "Flashing." Basically, it involves connecting a lantern-type of battery between one of the brushes, and the core of the rotor. Consult your owners manual for the exact and recommended procedure.

If the field winding is testing good, its time to move onto the next tip:
Stator Winding.

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