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What I meant by the linear relationship (maybe not the best choice of words) of amps/torque in a series motor, according to a post I read by major in another thread, is that a given amperage (e.g. 500 amps) will yield the same amount of torque, regardless of rpm. So, if a graph shows 100 lb-ft. of torque at 500 amps, then it won't change whether it's at 5 rpm or 5000 rpm. I assume the same would be the case for a sepex motor if the relationship between the field and armature currents remain constant. E.g. the Kostov motor graph shows 28.5 field amps 500 armature amps and produces 85 lb-ft of torque. The controller I have limits armature current to 500 amps but has 50 amps available for the field. I'm wondering if higher field amps would increase the torque.
I know it would reduce the rpm and probably wouldn't be a good idea to overload the field continuously as I suspect they would overheat, but having a little extra would improve acceleration.
I suspect that the effect of field weakening is to reduce the voltage required to get a specific current - so increasing the field coil current will increase the torque PROVIDED that
(1) You don't immediately lose armature current
(2) The field coil magnet is not already saturated
DC series motors normally run above the saturation level - this is why torque is proportional to current and not current squared
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