David Nelson wrote:
You might want to consider two contactors. One that engages first
through a starting resister and the second to short out the resistor
for full power. This will save the contacts in the contactor.
Michael K Johnson wrote:
Thought about that (with the starter contactor and resister together
in parallel with the main contactor) but it started to get complex,
because I want to not energize both coils during normal operation,
as it seems like a waste when I want to eke out as much range as
possible.
First, a 48v contactor coil will draw 0.5 amp or less. This is
negligible compared to the motor current.
Second, you can wire the two contactor coils in series. Wire your
"start" switch (that pulls in only the start contactor) so it applies
power to both coils, *and* shorts the coil of the "run" contactor. That
means it applies the full 48v to just the "start" contactor coil. Full
power, so it pulls in quickly.
Then a moment later when the motor has started, open the switch that
shorts the "main" contactor coil. An interesting thing happens at that
point. The contactor coils are inductors. The "start" coil insists on
keeping the same current flowing; so it *forces* the current in the
"main" coil to the same current. Thus *both* contactors turn on at full
current.
A moment later, the inductive energy in the "start" coil is used up.
Ohm's law then divides the voltage according to the resistance of the
two coils. Assuming they are the same, each coil gets 1/2 the pack
voltage, or 24v each. This is plenty enough to keep them pulled in (they
won't drop out until the voltage falls to 10-20%). And at half voltage,
they draw half current; thus you're holding two contactors on with only
half the current. :-)
My solution was that I'll probably eventually need to buy a
new contactor
Write me off-list if you need one. I have piles of surplus contactors at
good prices.
I'll have an emergency disconnect (SB350 with pull handle installed
An SB350 takes a heck of a strong pull to separate it. You probably
don't need one nearly that big anyway, but if you have it, it's fine.
But you may need a foot switch rather than a finger switch to pull it apart.
--
If you're not stubborn, you'll give up on experiments too soon.
And if you're not flexible, you'll pound your head against the wall
and not see a different solution to a problem you're trying to solve.
-- Jeff Bezos (founder of Amazon.com)
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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