There has been anecdotal evidence that if the IRU's are in alignment and AC power is tripped off, that the IRU's will not always complete alignment. This is why a lot of pilots will turn off their IRU's.
The nosewheel horn will sound for any IRU on Hot Battery only. The same horn is used for loss of equipment cooling in general (not necessarily because the IRUs have lost equipment cooling).
Re the generator. The engine spins, mechanically turning the accessory gearbox to which the generator is attached.
In the generator, there are 6 permanent magnets which induce an AC (alternating) current in an electrical circuit (permanent magnets are used so that no additional power from the airplane is required .. as long as the engine N2 rotor is spinning fast enough)
The electrical circuit goes to the respective Generator Control Unit (in the Equipment Centre). Here, the AC current is rectified to produce DC current. The Field Switches simply interrupt this process (like a light switch).
Normally, the DC is fed back to the generators, where the DC current is converted to aircraft AC power.
How such small magnets and tonnes of coils ultimately produce high voltage
and current is a complete mystery to me (I'll leave that to the scientists here)
