vectorges@aol.
> Do resistors just turn current into heat, and therefore waste it?
Yes.
This is why on the 'big stuff', eg electric trains, the controller
handle must not be left in the 'start' position too long - as they are
feeding the full motor current through a big bank of resistors (Or did
before they started fitting electronic controls). Leave in 'notch 1' or
'start' too long and smoke starts appear under the car.
A tram museum I work for has a SF 'PCC' car. It's been said that the
only efficient way to drive a PCC is flat out. (We can't on the museum
line). The car draws nearly as much power going slow as it does going
fast. Huge blower fans keep the resistor banks cool(ish). You can hear
it coming, where as the stuff with the 'traditional' GE or English
Electric controls can creep up on you.
> I am thinking of my old slot car controllers. They were barrel coiled resistors with a wiper to give you an infinitely variable? resistor. One which you would not want to touch.
Which is why big high current stuff uses more complex systems to
control speed.
> The relay idea seems simple.
Not really - the number of relays need to reconfigure the battery string
to give your speed steps, and then the relay logic you would have to
implement to make it actually usable as a throttle, it would no longer
be simple any more. Unless you WANT to remember what combination of
relays to trip to get a given speed! And also remember that some relay
combinations may result in short circuits in the high current traction
path - so you DON'T want to get it wrong!.
That's a lot of moving parts. Unlike a modern electronic controller
that has no moving parts other the actual low power throttle pot and
then the main power 'contactor' and possibly a reversing contactor - you
are replacing 2 relays and a 'pot' and a 'magic box' with may be a dozen
more relays all with moving parts that have to move 'just right' ?.
Relays are only simple if you are happy with 1 or 2 fixed speeds, so
you have a 3 way switch with off - half - full and that's it.
Might be appropriate for an electric canoe or something similarly small
and one or 2 person sized, bigger than that, and you probably want more
fine grained control.
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