On 22/12/10 06:02, John Green wrote:
> For a marine environment I see the weak point as being the controller. I would like to experiment on ways around this, mainly for cost reasons also, but also I have a suspicion that the use of controllers is a result of basing boat systems upon vehicle systems, where there is no natural slippage. This would also be driven by the manufacturers and sales sources for the controllers.
The modern electronic controllers are pretty bullet proof. I imagine
the only way to kill a properly rated one would be to consistently over
heat it.
The Alltrax I'm using, while intended for electric vehicle
applications is completely sealed. Water ingress won't do a lot to it.
It also wasn't that much more expensive to get one massively over rated
for my application. My motor should burn out before the controller does.
> I can remember years ago in a machine shop seeing an electric motor that required a lever to be slowly moved up a series of contacts, arranged in an arc, as the motor speed increased. Presumably each contact had a heavy-duty resistor in the circuit. Simple and safe, no electronics to burn out just when needed.
This is how 'traction' motors work - in a few hours i'll be off at a
museum driving trams (streetcars) that are operated by this very means -
series DC motors, which are started with a lot of resistance in circuit
which is steady removed as you 'cut notches'.
This system is not maintenance free - the controller 'fingers' do need
fairly precise maintenance to be reliable. Arcing of the fingers rapidly
damages them. (So the controllers must be handled properly or damage
will result). You also can't stay in a 'resistance' notch for long,
otherwise the resistors overheat. The resistances are for providing a
smooth start, not speed control. Our old trams really only have 2
running speeds, half and hull - either the motors in series pairs (1/2
line voltage each) or 'parallel' thus subject to full line voltage.
I would say in a high current situation, the 'old fashioned' way is
more likely to fail than a modern 'magic box', especially if the
contacts in the controller have a bit of moisture on them and are corroding.
Thus if you go the old fashioned way, you really only get one speed
that doesn't result in a cooked starting resistors (or if they are big
enough to 'run with', a lot of wasted energy).
You might find that in a marina or docking situation you will need to
use the resistances rather more than is desirable to get suitably low speed.
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