Originally Posted by
msbbarratt
. A lot of power generators actually work at 3000Hz, and the power station has to do a conversion to 50Hz
You may be thinking of a typical power generator rotating at 3000 revolutions per
minute, thereby generating 50Hz (cycles per
second) electricity. 3600rpm in the USA, parts of Japan, and so on for 60Hz power.
There's typically no frequency conversion in large-scale power generation. Instead, the spinning alternator shaft is brought to the correct frequency and phase for grid synchronisation, and maintained at that frequency and phase while supplying a load to the grid. Large electricity generating turbines and alternators produce electricity at a voltage of around 15kV, which is then stepped up by transformers to the grid voltage (275kV or 400kV in the UK).
If the load is too great, the shafts in all generators in the UK spin more slowly, causing a drop in frequency. Therefore total grid load can be signalled to the consumer by a small drop in frequency, and load shedding on the UK National Grid is performed partly by very large electricity consumers (such as heavy industry plants) having frequency-sensitive switchgear which disconnects their equipment from the grid as the frequency decreases, thereby decreasing grid load.
We're a bit off the topic of aircraft now
On-topic: I am somewhat surprised that the 787 would use the frequency-wild 235V AC to run things that are not in the engines or wings. The starter and wing anti ice makes some sense, but the cabin recirculation fans being run from 235V AC frequency-wild when there is either 110V AC 400Hz or the +/- 270V DC available seems a bit odd to me. It would be a very fine level of optimisation to find that extra wiring and complexity of the recirc fans being frequency-wild AC would be lighter and cheaper than providing additional stabilised 110V AC or +/- 270V DC to run those fans.