Michael K Johnson wrote:
Clever! I'm too used to thinking in binary...
I'm going to wire some safety switches in. Including the seat sensor.
Good idea! Every so often, you read about someone who was injured or
killed by falling off his tractor, and it ran over him.
Note that if you SHORT a PM motor (like the one with this kit), it stops
almost instantly. If you simply break power to it, it will freewheel and
coast for a considerable length of time.
So I'm trying to understand whether there is a simple circuit that
will accomplish the start sequence merely by power being supplied.
In that case, as I sit back down on the seat, power is applied and
the sequence you describe happens. It's designing that simple
circuit that I am not confident I would do right.
The simplest way to get a short motor starting delay is to add a small
relay with a 12v coil, a capacitor across the coil, and a series
resistor to the pack voltage. When you first connect it to the pack, the
relay won't pull in until the RC time constant has elapsed. A second or
two should be enough for the motor to start.
So, you wire things so when you hit the "go" switch:
1. The small relay is initially off. Its contact shorts the coil of the
"main" contactor.
2. So the "go" switch powers the "start" contactor immediately.
3. When the capacitor across the small relay charges, it pulls in.
Its contact opens, which removes the short across the "main"
contactor coil. So it pulls in. Now the motor is running at
full power.
If you momentarily open the "go" switch (or seat switch, or whatever):
4. Both contactors, and the small relay all lose power. The contactors
drop out, cutting all power to the motor. The small relay stays
on, due to the capacitor across it.
5. If you re-close the "go" switch quickly, the motor is still turning,
the small relay is still on, so it goes right back to "run" without
needing the "start" position.
6. If the "go" switch is off for more than a couple seconds, the small
relay's coil discharges the capacitor, so it turns off, so you will
get a "start", then "run" sequence again to re-start the motor.
Make sense?
I suppose the right answer is to debounce the safety system with
a capacitor of sufficient size. If I do that, even with a single contactor
I'll have many fewer cycles on the contactor and prolong its life.
Write me off-list if you need one. I have piles of surplus contactors at
good prices.
Thanks! I have one contactor already; I'd guess that to make this work
I'd need one with a similarly-rated coil; otherwise they won't balance
that way. I'll think about that if I find that the simple circuit is
available...
If you have two contactors whose coils don't quite match, you can add a
resistor across one so they measure the same resistance.
An SB350 takes a heck of a strong pull to separate it.
The things I don't know... I was worried about an SB350 behind a 400A
fuse being a little too small
The SB350 is rated for 350 amps continuously. The fuse is only good for
seconds at its rated current. I can't imagine drawing enough current for
long enough through an SB350 to hurt it. Your fuse, motor, or batteries
will blow up first!
--
"Obsolete" means nothing more than "the salesmen would prefer you buy
something else". -- Dave McGuire
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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