........................as any mismatch in frequency will cause the alternator to act as a synchronous motor, and suddenly leap to the correct phase alignment as it absorbs a large inrush current from the grid, which may damage the rotor and other equipment.
A fact well known by anyone who has experience in the Boeing 707 and similar aircraft of that vintage. As each generator (alternator) is brought on line after engine start, the F/E turns a knob on his panel to carefully match the frequency of the unconnected generator with those already on line. When matched to nearly the exact same frequency, the the F/E then connects the generator with the others. Failure to properly match the frequency (by adjusting the constant speed drive) before bringing the new generator on line could lead to shearing the generator shaft.