It's working! I finally got the field controller installed.
A while back I took delivery of my new Kelly 72100 - 72V/100A controller. I know, I know...it's a Kelly, but I am only using it to half capacity, so it should be good, right?
The LEM amp transducer that I am measuring armature current with has a resting signal of 2.5V. Current going one way causes the signal to go higher and current going the other way causes the signal to go lower. At 450A in the direction I am measuring, the signal is 3.9V. I tested this with a whole bunch of batteries and a carbon pile to load them with.
You can see in the attached Kelly screenshot how I dealt with these odd voltages by moving the throttle effective starting position and throttle effective ending position.
I fine tuned it so that at key on ("idle"), the field is getting 9A. When the armature is at full current, the field is getting 50A.
I was going to employ my idle microswitch to turn the field off altogether, but what I am finding is that when I am coasting down a hill at high motor RPM, if the field is zero and you turn it on, it confuses the Alltrax controller by suddenly generating voltage on the armature and the Alltrax goes brain dead for a few seconds. So I am leaving the field on at idle for now. 81W of power isn't much anyway.
So there you have it. I have only gone for one test drive (with my remote inductive ammeter installed on the field wire) and it worked brilliantly.
The throttle acts completely different now now that the field current is changing with armature current. Where is used to act like a farm tractor (where 50% throttle = 50% rpm), it now acts like a normal car throttle, where 50% throttle = 50% torque).
Tomorrow's commute to work should a better test. No more flicking a toggle switch back and forth on the gearshift lever!
To quote James May: "It's a brilliant solution to a problem that shouldn't have existed in the first place."