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The current/torque relationship for series wound motor is not quite linear. In theory, it would actually be a square function. For example, if you double the current you (since that same current flows through both field and armature windings) have done two things - you magnetized the motor twice as hard (because current in the field winding doubled) AND you doubled the armature winding current which doubles the strength of its interaction with magnetic field. So, doubling the current would quadruple the torque .... if theory was what actually happens. In practice level of motor magnetization is not proportional to field current due to magnetic core saturation as well as effects of armature current on (killing/skewing some of) the magnetic field. At the end of the day the relationship between torque and current is somewhere between proportional and square function. The higher the current (the more saturated the core is) the closer to proportional. Also, the faster the motor is spinning the less torque increase happens with same current increase.
In general, SepEx motors are really not that different - they only have field winding made with many more turns of thinner wire, so that the winding ends up needing a lot less amps (but with higher voltage needed for it) to create same magnetic field in the motor. The byproduct is that field current can be controlled independently. Since the two currents can be controlled independently then their direction can also be controlled independently and that allows for electronic reversing (changing direction of only field current) and regenerative braking (allowing armature current to change direction while field is kept in "fwd").
I realize that this probably doesn't answer your question/wonder - which motor should you use. Specific models/brands of motors can have all kinds of other differences and their specification/rating style was probably different so there is no good way to say which motor would give you better performance and/or reliability/longevity. I am afraid that your biggest factor would become availability of the controller. For on-road vehicles offering is not great for either motor type, but it is still a whole lot better for series than it is for SepEx. In fact, if you want to get decent power you may not really have the choice - SepEx controller may not even exist, so ...
 
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