It's a house on a Scottish estate.
Rotary converters and their close relatives can be quite interesting things. Available in a vast range of sizes, from tiddlers of a dozen watts that supplied HT in valve-based mobile radio comms units, up to the behemoths that powered the NYC subway until recently. They can convert AC-DC, DC-AC and DC-DC of a different voltage. Multiple voltages and numbers of phases are possible, but normal converters cannot convert AC from one frequency to another.
Conceptually, rotary converters can provide the same facilities for DC that a transformer can for AC, but they are more complex, maintenance intensive and have a practical upper limit for voltage. Thus, the supremacy of AC high voltage transmission was assured by the simple, reliable transformer, although there were a few DC schemes using local rotary stepdowns, IIRC in Oxford and Wolverhampton.
Rotary converters are sometimes confused with motor-generators. A M-G set is instead what its name implies, two separate machines, a motor (to suit the supply) driving a generator (to suit the local requirements). Rotaries are less versatile but smaller and more efficient, as they consist of only one electrical machine that need not even convert all the power once, let alone twice as in a motor generator.
AC-DC rotaries worked best at low frequencies and with larger numbers of phases (6-phase was common). Lots Road power station, the London Underground's own generation facility, used to make 33 1/3Hz rather than 50Hz, transmit this at HV and convert locally to DC with mercury arcs and rotaries. Lower frequencies are better still - NYC subway used 25Hz - but the transformers become unwieldy.
