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Here's a very interesting suggestion Duncan fro Cor van de Water:
BTW, you *can* change (weaken, but also increase) the field on a series motor
because most motors have external contacts for field and armature (which are
usually connected together with a strap to send the same current through both)
The simple solution to field weakening is to add a contactor and a low value,
high power resistor like a piece of steel rod and wire this across the field
contacts. When yo uwant field weakening, close that contactor and part of
the current bypasses the field.
The original issue of the 130V drag racer that was maxing out at 59MPH
can easily be solved this way: closing the field weakening is like engaging
overdrive on the series motor, it will run faster at the same voltage
(note that it can’t produce the same torque due to the weaker field,
so it is best to make it controllable and you only enagage it at a certain speed)
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