G'day Paul, All
At 05:43 PM 29/03/2010, Paul wrote:
><snip> From what I have read it would appear that the recommendation
>is an electric motor that has 50% the rating of the original, so I
>have been looking at a Mars ME0709 motor which appears to provide
>the required power output, looking at powering it the only real
>option appears to be to use 48v.
48V is a nice compromise voltage of safety, amp draw, availability of
surplus bits-and-pieces, with reasonable power available when needed.
><snip> assuming 5 hours of actual motoring time.
I don't know if you are aware of pukerts' (I think I mis-spelled
that) effect where the faster you draw power from a battery, the less
total energy you get. Also the temperature effect, lower temperature
is also less energy.
Pukerts' effect first: 220Ah is a common size, 220Ah at 20 hour rate
(11 amps) at 5 hour rate is likely to give 150Ah/30 amps at 20
degrees C. Depending on your temperature, you may only be getting 2/3
of that (say 10 degrees C) so 100Ah so only about 3 hours run time,
plus you don't want to pull them that flat, so call it 2.5 hours. So
*very roughly* your minimum battery capacity probably should be
440Ah, if your power calculation is correct.
>Looking at the performance data for the motor,<snip> I'm not going
>to be running at full power on the UK canals, normal cruising speed
>is around 3-4 knots.
Firstly, up to a point, *you cannot have too big a motor*. A motor
that is over-sized for the job has lower resistance, so is slightly
more efficient than one that is working on its' limit. Next, heat is
the enemy of motors, and the closer you are running to the amperage
limit, the closer to the thermal limit you are, and the more critical
the cooling requirements. Ideally you should be operating a motor
about 2/3 of its' designed maximum speed in its' typical use, unless
you need the higher top end, in which case you have to size and gear
the motor for that max power/speed.
>Would I be better going for a larger battery bank and using a
>reduction gear and if so what would be the advantages?
Ah, now then, larger battery bank is more cruising hours and nothing
to do with the maximum power available. You will need to gear down
the motor, and the easiest and simplest way is to use a toothed belt
and pulleys. That way if you have the ratio wrong, it is easy to fix.
You can readily add another string of batteries for a bigger 'fuel
tank' if needed.
If you try and run the motor too slowly (not enough reduction ratio)
you will be needing to deliver lots of amps into the motor (at
reduced voltage) to get the power needed, and amps = heat = cooling issues...
>I'm planning on using a 2KVa silent generator on board to top the
>batteries up while I am out cruising, if I can put 50% back into the
>bank while out cruising then it would mean I would be able to have
>three full days of cruising without having to worry about being
>connected to shore power for a full recharge.
Well that reduces any concern about too small battery pack. Looks
like you need to pick a motor and get that grafted in - you may be
able to graft in the electric motor before pulling the ICE, allowing
you to do a 'hack' wire up and find out what speed/power is needed,
then size the battery pack, etc to suit.
>I've done a lot of car audio electrics using very high power amps
>etc so the actual wiring etc isn't concerning me, its just working
>out if the motor is the right one and what power I need in the battery bank.
>
>I would also like to use a motor controller with a reversing
>function on it as I plan to remote control the boat when its in
>locks if I'm cruising alone, has anyone got any recommendations as
>to the best controller?
There are lots of controller options out there, most use contactors
to do the reverse, some do it straight off the electronics (which is
less efficient). Here is a suggestion (if your pockets are a bit
shallow): find a 'dead' stand-up or larger walk behind forklift that
still has good drive motor and controls, and canibalise that (the
power size will be in the ball-park and will have all the
bits-and-pieces in one go).
Hope this helps
Regards
[Technik] James
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