You do not say what type/model generator you have, so I will just give you some basics.
There is nothing wrong with the magnets. Their power did not change at all, their pwer never changes, except to degrade slightly over time (hundreds of years.)
The generator rotor is hitting the stator. In other words, the alignment of the rotor inside the generator is off center, making the rotor bind on the inside of the stator, resulting in a lock-up.
With the generator attached to the motor, remove the generator end, and slowly try to turn the engine over by hand. REMOVE THE PLUG WIRE FROM THE PLUG FIRST, so that the engine cannot start. Watch inside the generator, and discover why the generator rotor is not aligned. Pehaps the bearings were installed incorrectly, or the housing is installed incorrectly. Most generator engines have a tapered power-takeoff shaft, where the gen. shaft connects to it. Is this Bolt tightened to specifications? Is there some debris floating around in the gen. (such as a screw, or a pebble, or a nut) that would impede its movement?
good luck
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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