><snip>>
>>The huge M26 had a 1,090 ci engine. 12 volt system.
>>Many heavy trucks with incredibly large engines had 6 or 12 volt systems.
>><snip>
>
>Agreed. Another argument for the higher voltage is the size of the cables
>and connectors. The lower the voltage, the more current is required for an
>equal amount of wattage (power). So, a 24v starter motor can be more
compact
>than a 6v one, simply because the wire in it can be smaller. But they can
>certainly be made to develop equal power.
>
Hi All,
Jim and the others are quite right and it may be helpful to look at this a
little more closely taking some other factors into account. The reduction
of current (amps) is the aim as this allows for reasonable sized cable and
sizes of starters, dynamos and alternators.
All batteries have _internal_ resistance too, it may be small but quite
significant when several hundred amps are needed to crank the starter
against the load of the engine, even with a modern system you will see your
car battery go to about 10V when cranking.
Power is a function of the applied voltage and current drawn, volts x amps =
watts, as an example suppose you need a 1200 watt starter for a large truck
engine, ignoring the internal battery volt drop then you need:
at 6V some 200 amps
at 12V, 100 amps
at 24v, 50 amps.
The 200A cable is huge and you need to keep all the connections absolutely
perfect, the length of the cable run is important as well to avoid some
losses, 50A cable by comparison is quite small. This applies to all the
other electrical circuits as well and becomes especially important as MV's
became more electrically complex over the years. This works the other way
up of course, for the same size 6V 1200W starter at 24V you get a 4800W
unit.
Looking at larger MV's, APC's, tracked vehicles and the like, you'll find
the engines and the batteries well separated out of necessity and
space/safety constraints, the military sensibly tends not to place the
batteries in the engine bay environment, there may be 20 ft or more of cable
between battery and starter, smaller low current devices with attendant
cable requirements are therefore essential. Once you get into the tank
sized diesel engines there is virtually no option in order to get enough
starter power in an acceptable size.
It would be entirely sensible for NATO to standardise on 24V systems, as is
normal here anyway for all vehicles over 5 tons or so, the British Army went
to 24V throughout in the 50's for fighting vehicles.
Richard
(Southampton UK)
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