Lucien's most challenging picture puzzle yet... what does this box do?

[Lucien Nunes]

You've seen some curious stuff in my vintage picture puzzles before but what can you make of this black box? I want to know not only what's inside but also its main electrical specifications; there are two in particular for which I need the actual figures.

There will be a few picture clues to help you along...

 

[Lucien Nunes]

Inside the box:

Data plate:

 

[Lucien Nunes]

Commutator at the DC end:

Sliprings at the AC end:

 

[Lucien Nunes]

Cooker from the same kitchen. This has been carefully maintained over the years, and was still in use, mainly for its huge ovens, until last year. It even came with a stock of spare elements (there are 20 in each oven) and switches, and we fully expect to cook in it again.

 

[marconi]

Are you restoring the equipment for a few more decades of sterling service on its original site?

 

[Lucien Nunes]

After three quarters of a century of use, they decided during some remodelling that the time had come to part with the old cooker, and kindly donated it and various equipment to the collection. Although in superb condition and probably fit for another 75 years use, even the best cookers of that era did not have thermostatic control which we have come to expect today. You had to keep an eye on the thermometer built into the oven door, or the contents of your pans, and manually switch the element power between low, medium and high to maintain your desired temperature. They had supplemented it with a modern cooker but those ovens were still ideal for the largest possible turkey at Christmas.

Three generations of their family have used it, plus the family before them who had it installed. He was a submarine captain, who would have been used to electrical solutions and methods when others still favoured solid fuel and gas. The original 60Hz fridge chiller unit had already been removed, but they are keeping and restoring the cabinet. When I saw the box on the wall above it, I said 'Oh look there's the old rotary for the fridge', realising afterwards just what an obscure thing it has become in the 21st century.

The house is still served by hydro-generation, but it was updated to a grid-tied AC plant a few years ago, giving the benefit of uninterrupted supply and utilisation of the resources. The owners calculated that the water resource and plumbing would support 70kW of generation, but they did a cost-benefit analysis of paying the DNO for their grid tie capacity to be increased and found it to be uneconomic because of the distances involved. FWIW the HV is single-phase with a 240/480 split-phase LV service.

 

[telectrix]

read this thread all the way through as it's all beyond any knowledge of mine. i undersatand now the what and why, but my eyeballs are aching. got to go for a lie down now.

 

[DPG]

I think last mention I saw of rotary converters was for lead-acid battery charging.

 

[Midwest]

Where & what is this property? Just curious.

 

[DPG]

Not Cragside is it?

 

[Lucien Nunes]

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.