Ship DC vs AC electrical systems?

[Rule of cool]

Something that comes up, in particularly with modernising British warships, is DC and AC electrical systems. IIUC British WW2 era and beyond ships were DC and during some major refits some, most or all DC systems were replaced with AC systems. The Brazilian Minas Gerais appears to have been wholly converted from DC to AC at great cost in order to be compatible with US systems, but HMS Eagle kept her DC electrical system but had an AC system upgraded to run certain things.

Given a warship is a self-contained electrical grid what are the advantages and disadvantages of AC vs DC and are certain items linked to one or another eg 1950s US radars are AC but1950s British and Dutch radars are DC?

 

[XRNWEO]

Something that comes up, in particularly with modernising British warships, is DC and AC electrical systems. IIUC British WW2 era and beyond ships were DC and during some major refits some, most or all DC systems were replaced with AC systems. The Brazilian Minas Gerais appears to have been wholly converted from DC to AC at great cost in order to be compatible with US systems, but HMS Eagle kept her DC electrical system but had an AC system upgraded to run certain things.

Given a warship is a self-contained electrical grid what are the advantages and disadvantages of AC vs DC and are certain items linked to one or another eg 1950s US radars are AC but1950s British and Dutch radars are DC?

RoC. In the post war era, the move to AC was driven by the expansion of electric equipment in ships, that AC motors and distribution equipment was (then) lighter than their DC equivalents, plus valve based electronics used AC, and in a DC ship, would then have required conversion machinery (DC to AC). This may no longer be true, given that rare earth motors have lightened DC motors, but these weren't available in the 1940's. This is the short version. There may well be journal or science articles in JNE or J-RNSS that would give a 1940's contemporary view.

 

[XRNWEO]

RoC. In the post war era, the move to AC was driven by the expansion of electric equipment in ships, that AC motors and distribution equipment was (then) lighter than their DC equivalents, plus valve based electronics used AC, and in a DC ship, would then have required conversion machinery (DC to AC). This may no longer be true, given that rare earth motors have lightened DC motors, but these weren't available in the 1940's. This is the short version. There may well be journal or science articles in JNE or J-RNSS that would give a 1940's contemporary view.

As a postscript, DC equipment required rather more skilled maintenance, particularly skimming the commutators, to retain performance. Regards - XRNWEO

 

[DWG]

Something that comes up, in particularly with modernising British warships, is DC and AC electrical systems. IIUC British WW2 era and beyond ships were DC and during some major refits some, most or all DC systems were replaced with AC systems.

The other thing to remember is that pre-WWII RN electricals turned out to be inadequate in both capacity and damage resistance. They needed significantly expanding, and they needed significantly increased redundancy, rather than relying on turbogenerators in the engine rooms. So if you need to rework everything anyway, then the DC to AC switch is less challenging than it might have been if you were switching for that reason alone.

 

[XRNWEO]

The other thing to remember is that pre-WWII RN electricals turned out to be inadequate in both capacity and damage resistance. They needed significantly expanding, and they needed significantly increased redundancy, rather than relying on turbogenerators in the engine rooms. So if you need to rework everything anyway, then the DC to AC switch is less challenging than it might have been if you were switching for that reason alone.

As another postscript, large DC ships (cruisers upwards) had ring main systems, able to isolate damage. All AC ships had ring mains, but AC allows much easier load equalisation and sharing, via synchronising generators, then transferring load between them.

 

[Rule of cool]

For large ships with long lives that start out as DC, like the British aircraft carriers that were exported after WW2, is simply adding more and more AC generation as newer systems keep getting added a reasonable course of action?

When did DC stop getting installed in new ships?

 

[XRNWEO]

The Tiger class cruisers we're completed as AC only ships. Type12, 41 and 61 frigates and County class DLG were AC. The Darings were a mixture, some DC, and the later ones AC. Carrier conversions like Eagle and Ark Royal were a composite, both AC and original DC

 

[Rule of cool]

So the change was quite early, pretty much immediately pist ww2. This is really only a issue for these ww2 carriers retained past the 50s.

 

[XRNWEO]

The last DC ship I'm aware of was Matapan, which ran as a sonar trials ship with both DC and AC generation and distribution systems.

It's worth adding that full electrical damage control facilities were built into all the post war AC ships, but was entirely ad-hoc in the pre-war and wartime DC ships.

 

[Rule of cool]

The Dutch Karel Doorman cost USD $6 million to convert from axial deck to a definitive angled deck in 1955-58, but I believe remained basically a DC ship. The Dutch converted the Brazilian Minas Gerais 1957-60 to a similar standard in terms of flight deck, although it used a US sourced catapult rather than the KDs British cat and cost USD $27m.

However, internal electricity was converted to AC through the installation of four turbo generators and one diesel generator.

Can the wholesale conversion of this aircraft carrier from DC to AC account for a good chunk of the USD $21m extra the MGs refit cost over the KD which remained a fundamentally DC ship?

 

[SeaslugMk2]

Another reason for the change to AC is the ability to provide any required voltage with transformers rather than motor/generator units.
A lesser factor is that alternators are more efficient than generators at low rpm.

SRJ

 

[tychosis]

Another reason for the change to AC is the ability to provide any required voltage with transformers rather than motor/generator units.

Yeah, and why you'll find oddities like 400Hz AC in maritime and aviation, because you can do this with smaller and lighter transformers.

(400Hz AC was also useful for accurate synchros and servos, largely obsolete now but still out there on some older units.)

 

[XRNWEO]

RoC. It would be difficult to compare costs. KD would have been a Dutch naval project, whilst MG was likely to be an export. As such, it could have racked up a lot of government on-costs, not applied to KD. Changing to AC would have required the ships side opened to fit TG's and the DG, but the rewiring labour costs are much more substantial, because all the generator to switchboard, and switchboard to load centre cables have to be changed, involving glands at each watertight bulkhead, and manual reeving of heavy cables, each (one per phase) likely to be 2" dia across the outer insulation. You'd also have to add the cost of switchboards themselves, however it's likely that work on the main machinery and combat systems would have been comparable in both cases. P.S. AC ships generally generate at 440v three phase, 60 Hz.

 

[XRNWEO]

You may find additional answers in PM Rippon's book 'The Evolution of Engineering in the RN, Vol 2'. I'm away from home, so I can't look up my copy to check.

 

[Rule of cool]

but the rewiring labour costs are much more substantial, because all the generator to switchboard, and switchboard to load centre cables have to be changed, involving glands at each watertight bulkhead, and manual reeving of heavy cables. You'd also have to add the cost of switchboards themselves,

This is what I suspected, I'm familiar enough with wiring in cars and houses to imagine this would be a massive but very fiddly and detailed job.

 

[tychosis]

the rewiring labour costs are much more substantial, because all the generator to switchboard, and switchboard to load centre cables have to be changed, involving glands at each watertight bulkhead, and manual reeving of heavy cables

Yeah, that sort of shipboard work is exceedingly painful.

I don't work with shipboard power generation but I work with sonar engineering and integration. We're always working to keep the fleet updated but are naturally constrained by the actual front-end sensors on older platforms (688i/Seawolf/Ohio/VABLK12) that are effectively obsolete and on life-support at this point.

Our new systems are cut in at a defined line of demarcation and everything "forward" of that is original just because it's cost-prohibitive to tear that stuff out. As you mention, it'd be a lot of work. (And as everyone knows, Murphy is gonna get you on those big jobs.)

 

[XRNWEO]

As mentioned above #14, see copies of Rippon, p280-282 attached. He covers RN electrical changes, post war. I posted from memory, but I think I covered all the main points. Regards - XRNWEO.