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Vehicle: Car: Engine: Electric
Three phase electrical system   (+1, -1)  [vote for, against]
Because three phases are better than none.

Current vehicle electrical systems almost entirely use a 12 V nominal DC power distribution. An alternator generates three phase AC, which is then rectified to DC, which is then used to power accessories like brushed DC windshield wiper motors, fuel pumps, washer fluid pumps, window motors...wait, why are all these motors using DC? As demonstrated by none other than Nikola Tesla, AC motors are far more efficient and reliable.

Simply tap the alternator before rectification, and power all these motors with three phase AC. Do away with sparking commutators and rectifier voltage drops. Switching these loads is easier than DC as well. Mechanical switches can take advantage of the periodic zero volt crossing to quench arcs, and cheap TRIACs can be used for solid state switching.

Lights (as long as manufacturers keep using incandescents) can operate equally well on AC or DC, and the three phases can be balanced between bulbs. Computers and other electronics still require DC, but AC input allows efficient transformers to lower the voltage before rectification. Without all the inductive feedback from motor brushes, this low voltage rail will have less noise to filter out.

Unfortunately, one of the biggest electrical hogs, the starter motor, still requires DC. At least until someone develops an AC battery...
-- Aq_Bi, Jun 17 2014

Because most of the motors and such either need to be able to function without the engine running, or are used so infrequently that any net efficiency gain would be negligible. Designing a system that could switch them between AC and DC would probably be overall less efficient, and certainly less reliable. Also, hybrids and electric cars would be totally unable to use AC powered anything, since they operate powered entirely by batteries.

So it basically boils down to increased cost, added complexity (and possibly weight), decreased reliability, and virtually no benefit.
-- ytk, Jun 17 2014

Cost; most existing systems use the vehicle chassis as the return path. With 3 phase, that's a minimum 200% increase in cabling.

You can't just connect all the phases at a lamp and put the other terminal to earth; it has to be star or delta, or you need rectifiers ...
-- 8th of 7, Jun 17 2014

3-phase systems are more efficient mostly when the topic is "induction motors". Such motors "induce" the armature to rotate, and 3-phase induction motors are certainly a lot more efficient than single-phase induction motors. When DC motors aren't using permanent magnets, they send current to the armature to create magnetic fields that interact with the magnetic fields of the stator. This is directly a more-powerful interaction than the induction of AC motors, and therefore is also somewhat more energy-efficient than even 3-phase types.

On another topic altogether, it IS possible for a kind of "AC battery" to exist. An energy-storage flywheel, with an AC motor/generator can do that....
-- Vernon, Jun 17 2014

// wiper motors, fuel pumps, washer fluid pumps, window motors ..... power all these motors with three phase AC // Very funny: all these motors running synced with the combustion engine. Because you know three phase motors runs at proportional speed of the line frequency, don't you ?
-- piluso, Jun 17 2014

Ha! there's good money in this for Schneider and the like to start mass producing miniature VVVF drives for all of these little 3-phase motors everywhere.
-- Custardguts, Jun 17 2014

Maybe a 3 phase lamp could be manufactured, with 3 filaments inside it...
-- Ling, Jun 18 2014

Indeed it could, but 3 LEDs would be rather more efficient.

Star, or delta ? If star, neutral return ?
-- 8th of 7, Jun 18 2014

British Leyland had a three-phase electrical system. Phase One was everything worked, even the cassette player; this lasted about 4-8 weeks. Phase Two was all the essential things still worked. Phase Three was only some of the essential things worked, but by then the body had rusted anyway.
-- MaxwellBuchanan, Jun 20 2014

// cassette player //

Sp. "eight-track cartridge"

// all the essential things still worked. //

Unfortunately, in that era CERN had not yet developed sub-attosecond counters to the point where they were field-deployable, so definitive data could not be collected.

// Phase Three was only some of the essential things worked, but by then the body had rusted anyway. //

... and then it was rolled off the production line for shipment to the distributor, who would store it outside in the wind and the rain for months before shipping it to the main dealer, who would sell it to the final "customer" (Sp. "victim")

The Austin Princess ... the road vehicle that made the Ford Edsel look like a sound investment. And as for the Allegro ... well, let's not go there. It didn't. Go anywhere. Ever. (at least, under its own power).

Did you know that if you translate "Triumph Acclaim" into German, it comes out as "Sieg Heil" ... ?

mutter, mutter ... 6R4 ... mutter ... Group B ... mutter ... WHAT WHERE THEY THINKING ... mutter, grumble ... Quattro ... Lancia Delta ... Group B ... <nostalgic sigh>
-- 8th of 7, Jun 20 2014

//Sp. "eight-track cartridge" // No, this was Britain.
-- MaxwellBuchanan, Jun 20 2014

Ahh, the shellac 78 ... we shall not see its like again ... thankfully.
-- 8th of 7, Jun 20 2014

Many aircraft had 3 phase electrical systems, until they figured out that it sucked around 1940.
-- DIYMatt, Jun 21 2014

//Many aircraft had 3 phase electrical systems

How many times was it really necessary to run a milling machine in an aircraft?
-- not_morrison_rm, Jun 22 2014

3 phase is efficient only if running continuously and at a steady and usually mandatory speed.
-- pashute, Jun 23 2014

For a directly-wired squirrel-cage, yes, but if you're prepared to pay for an inverter drive then you can have anyy speed you like.
-- 8th of 7, Jun 23 2014

Cars have enough problems packaging the wiring in them as it is.
-- RayfordSteele, Jun 24 2014

random, halfbakery