Overload? You understand the purpose of the exciter, right? If not, here is the simple explaination:
Electricity is produced by moving a magnetic field through a coil. In most generators, it is a series of coils, this matters not here. The issue is the magnetic field.
Permanent magnets are not generally used because of voltage regulation issues, electromagnets mean the output voltage can be precisely controlled. In most automotive alternators, the electromagnet's current is delivered through slip rings and brushes. Because these wear, most military generators are brushless. This is where the exciter comes into play.
The exciter is a second, smaller generator in the whole assembly, instead of the moving electromagnet, it has a moving coil while the electromagnet is stationary. This means the moving coil is now producing alternating current which is then rectified into DC by the diodes.
Just like the diodes in an automotive alternator, heat and current can cause diodes to fail. Running the generator at less than rated RPM can do it, current must increase to keep output voltage constant. Lower RPM then causes less cooling air. Bam, diodes are fried.
