[email protected] wrote:
This 43 year old tractor is better than my old gasser tractor was on day one!
That's for sure! EV "traction motors" are ideally suited for pulling. Why do you think they are used in railroad engines? :-)
The next big frontier after drag racing is probably in EV tractor pulls.
My "controller" is just basically a contactor switch... I had thought to do Lee's suggestion with a capacitor and resistor RC circuit, but my fear was the relay might pull in or release slowly, causing bounces or arcing in the switch (any comments on that, Lee?).
I just posted a bit on it. Here's more for engineering drivel for "contact geeks" that want to know. :-)
First, there are big differences in design between relays. Some types work well with ramping coil voltages, others work poorly. Older, open frame, high quality industrial relays seem to work better than cheap modern relays. But in general...
If you slowly ramp up the voltage on a relay coil, the armature (the moving part) typically stays motionless until a critical voltage is reached (somewhere around half of the rated coil voltage). At this point, the magnetic force equals the spring force.
As the voltage increases further, the pressure on the normally closed contact begins to fall. This can increase the NC contact's on-resistance. When the pressure on the NC contact falls to zero, the armature begins to move. The NC contact opens slowly (not the usual "click" you're used to). Thus, a relay being used in this fashion needs a large safety factor in its NC contact rating. Like, only use it at 1/10th of its normal current rating.
As the armature moves, the magnetic gap closes. The smaller gap increases the force, so the armature accelerates as it moves. There's a little "drag race" going on, where the farther it moves, the faster it's going. Thus, the coil voltage doesn't make much difference from here on. The normally open contact closes fast, regardless of the coil voltage.
When the coil voltage is reduced, again nothing happens until the magnetic force equals the spring force. At that point, the armature begins to move. The opening gap decreases the magnetic force, so the armature quickly accelerates as it moves. The NO contact opens relatively quickly, regardless of the coil voltage. So you don't need much derating here -- 1/2 of rated current is usually enough.
When you look closely at a relay or contactor's construction, you can kind of tell if the designer was thinking about all this. A cheap relay just has one spring, and the moving contacts themselves are leaf springs that do part of the work to move the armature. A high quality relay has much more elaborate mechanisms, so the armature can start to move without any change in the contact pressure. That way, the armature gets "up to speed" before it actually trips the contact open or closed.
-- "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 _______________________________________________ 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)
