Hello David and All,
David Dymaxion wrote:
> Great description! Were you able to adjust the field strength
> independently of the armature? In other words, was it 12V-24V-48V
> across both the field and armature together, or did the armature have
> a different voltage than the field?
Yes, I was able to independently control the field. I used a fairly simple control that consisted of a beefy TO3 package transistor on a heat sink and a potentiometer that varied base current. I could ‘dial in’ anything from about 12 volts to 48 volts. I also had a second way of getting regen without playing with the field at all, and this was simply a byproduct of my 3 step contactor controller.
The controller worked this way. At the first depression of the accelerator pedal, a micro switch would engage 1st speed. The car would start off with the eight 6 volt batteries wired in a series-parallel 24V setup through a forklift nichrome resistor that helped reduce the initial jerk of torque. The resistor ‘had’ to be cooled off, or it would try to glow orange-hot. Though resistor control is considered primitive by today’s standards, it was only used for that initial take-off period of a 1-5 seconds, so not too much power was dumped away in heat. Speaking of that heat, I used an innovative way of cooling the resistor off. I morphed a 120 vac fan from a desktop type small space heater with a 12 vdc tape drive motor from an 8 track tape player to make a 12 vdc fan (brushless DC muffin fans were not commonly available yet). The input wires to the motor went across the resistor, as I measured anything from 10 volts to 16 volts or so being dropped across it. Thus, the 12V accessory power from the car was not used to run the fan (that draw would have had a minimal effect on the 12V system, but hey, I was still learning stuff). The hotter the resistor tried to get, the faster the fan blew. The very hot exhausted air from this crude affair was channeled into the car’s weirdo heating system (the aforementioned wasted motor exhaust air) to give extra heat for warming the cabin in the Winter months.
I digress…..after the 1st resistor/24V stage, as you depressed the accelerator farther down, a second micro switch fired another contactor that would short out the resistor….now the car is accelerating harder at a straight 24 volts.
The last 3rd speed was triggered at full throttle, when the 24V series/parallel contactors would let go and the pack was reconfigured to a straight 48V. Now, for the regen part….if you were flying down the road at 48 volts in the shunt motor mode, then let up a little to trigger the 24V mode, you’d get BIG regen. If you at the same time, twisted the handy dash controlled pot. and dialed up full field current, it would almost lock up the rear tires.
> Thanks. I’m wondering if the
> shunt acceleration might have been the same as the series with sepex
No, it wouldn’t have been. The shunt field windings were very fine wire. Though I had actually turned the motor into a sepex type, to make a really ‘effective’ sepex motor, the field windings would have needed to be made of fewer turns of thicker wire. The Sepex motors used in the more recent electric forklifts do have considerably smaller field wire size than their series-wound brothers that use thick flat ribbon wire for the fields that like the armature windings have to pass 500 amps of juice, but the field wires are still fairly robust. The field windings of a true sepex type are generally sized somewhere in between a regular shunt-wound motor’s fine strands of windings and a series-wound motor’s lower number of beefy ribbon wires.
As Lee said, our crude conversions based on these compound-wound starter-generators served as our rolling classrooms.
See Ya…..John Wayland