On Sat, Nov 2, 2013 at 9:23 PM, Lee Hart <[email protected]> wrote: > 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. > :-)
Clever! I'm too used to thinking in binary... I'm going to wire some safety switches in. Including the seat sensor. If I register as momentarily off the seat, both contactors would then disengage and the mowing would stop. And I'd have to go through a whole start sequence to get the system going again. 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. 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... > 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. The things I don't know... I was worried about an SB350 behind a 400A fuse being a little too small (all I saw was 350A), but I wanted to make sure I had an emergency disconnect. I now found the product data at http://www.andersonpower.com/products/multipole-sb.html SB120 isn't available above 2awg, which won't work with my 2x4awg. SB175 can handle inrush of about 8 seconds at 400A with 1 awg. All my anderson disconnect experience is with UPSes which I suspect are all SB50. According to their datasheets, that should be 10-15 lbf; the SB175 25 lbf, and the SB350 30 lbf. _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
