# DC Series vs SepEx Motor. Is performance Comparable?



## piotrsko (Dec 9, 2007)

The only comment I can make is that the Kostov can't be abused as much as the Hitachi that Duncan uses. Imho, the Kostov will outlast the Hitachi, but only within parameters which aren't conducive to sporty performance.


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## puddleglum (Oct 22, 2008)

Thank you, you are probably right. I think it's capable of more than the controller can put out but it looks much less robust. It would probably take higher voltage and rpm's better, but at less torque. It has some impressive features though. A higher power controller would help, but I don't have the skills to do it and it's pretty much obsolete tech now.

I still hope someone can answer my question about whether or not the current/torque relationship can be modified with sepex though.


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## zubmio (May 19, 2020)

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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## puddleglum (Oct 22, 2008)

Thank you zubmio, that's helpful. I really wasn't asking for a recommendation as to which motor was best even though it may have come across that way. I'm just weighing out pros and cons to make the best choice, and trying to understand the sepex motor characteristics better as well. To do that I wanted to determine if the lower torque of the sepex motor ( con) could be improved by manipulating the field current a bit. You are right that sepex controllers are rare and building one would be very complicated. (a really big con) This motor is definitely limited by the controller it has now. At least it has a controller and does regen (pros). I don't have one for the series motor.


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## puddleglum (Oct 22, 2008)

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.


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## Duncan (Dec 8, 2008)

puddleglum said:


> 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.


Hi
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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## puddleglum (Oct 22, 2008)

Duncan said:


> increasing the field coil current will increase the torque PROVIDED that
> The field coil magnet is not already saturated


Thanks Duncan. Yes, this is why I'm wondering if it would improve torque much at the upper end of the current limit. 
As I understand it, field weakening reduces the magnetic flux and BEMF in the field allowing higher speed and reduced torque for a given armature current. Increasing the field current has the opposite affect and should increase torque and lower speed. But, saturation is probably close to nominal field current ( 14-15 amps on this motor), so if the field is already well beyond saturation I don't know if increasing current will still increase torque, or just increase heat in the field windings and shorten the life of the motor. 
The sepex motor is a much more refined unit with some cool features compared to the Hitachi motor, but I don't know how to make a controller that can unlock it's full potential. I think it would still work well with the current controller, if it was going into a smaller lighter car. I like the idea of having regen braking.
The Hitachi has a proven track record though (your car), and even though I don't have a controller for it, there are a few inexpensive options out there still. I'm leaning toward using it instead, for this car, if I can ever complete it. I wish I could find the plans for the controller you have. I got the coding files from someone, but not the plans and parts list for the boards.


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