At 06:25 AM 25/05/2010, adam wrote:
>hi everyone, im new to the forum.
G'day Adam, and welcome from Tasmania.
>i live in new zealand and i am about to start building a 35' sailing
>catamaran.
><snip>
>
>where i get most confused is it the many possible ways of doing
>things.. i am on a budget and do not want to spend more $ than i
>have to. the main reason for having electric drives is just to use a
>single engine.. i am not worried about having a long battery range
>or anything just decent efficiency and reliability. i would quite
>happily just run the motors straight off a generator and start it
>whenever i want to move.. what would i require for such a setup? is
>ac or dc the way to go? to be honest i still cant get my head around
>the difference!! what would be the best speed controller to use?
>props? as you can probably tell im pretty lost..
Firstly, you're in a good place for help, so just ask. Remember, the
only stupid question is the one you were stupid enough not to ask,
the others are all to do with learning. Yes, bucketloads of ways to
do things, so it's cheap to kick ideas around and look for options,
and see what you can find that fits the situation.
AC vs DC, the difference is in the complexity and availability of
parts. Most of the AC stuff readily available is either high voltage
or a bit specialised, so DC is probably the better way to go, unless
you are buying everything new. If buying everything new, you have the
possibility of AC golf cart motors and controllers, which run from
typically 48 volt DC (so a battery or DC generator).
The most efficient way of generating low voltage from an engine-drive
is with a DC generator that is already correct for the voltage. If
you can find a suitable engine, then a permanent magnet DC motor
being run a little faster than what its' voltage-speed is will
generate (If the motor is permanent magnet and it says 2000RPM 48V,
then 2200RPM will have it generating a bit over 48V).
OK, the simplest ICE(diesel)/electric drive is to use large
permanent-magnet (DC) motors, one as a generator (since if you spin a
permanent magnet motor, it generates), two identical ones as motors.
The generator one has to be capable of generating at least twice the
power as the two motors use (but not necessarily twice as powerful,
if the ones in the hulls are not going to be easy to cool, you may
want oversized motors anyway to keep the heat down). Connect the
motors in parallel, directly off the output of the generator.
As you rev up the engine, the motor-used-as-a-generator increases
its' output voltage proportional to speed. The two motors follow this
speed. Reverse the polarity of the connections and you reverse the
drive direction, so a pair of contactors or "big-as" switches (or two
pairs if you want to be able to control the motors seperately for
increased manoeverability.
The down side of this is that they want to run at precisely the same
speed, all the time, so you if one needs to run a fraction faster
than the other then that becomes a bit tricky.
Next option, similar setup, but run the diesel at constant speed, so
the generator is making (for example) 48 volts. Now you add two
golf-cart style controllers between the 48 volts and the motors,
giving you full control of each motor individually. You can also add
a 48 volt battery bank, and charge it from the diesel, allowing hybrid use.
You may be able to start with just the diesel and three motors and
build it up as you get the funds available and have the need.
Regards
[Technik] James
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