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Old 04-12-2015, 03:56 PM   #7028 (permalink)
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Originally Posted by z_power View Post
Well, parallel connection of motors can lead to large imbalaces in currents due to smallest differences in their resistances (or should I say impedances?). Google for CroDriver's BMW, he's had this unlucky experience. It would be more safe to connect them in series if their shafts are coupled direct or indirect trough pavement but you don't need to modify controller to do this - single "output channel" would do.
Imbalance in current should be imbalance in power to the wheels as well? The wheel(s) that get more current should be doing more of the work?

I did a couple of simple searches - saw Crodriver's EValbum entry, a couple of videos, and a build log on DIYelectriccars ... but a search for "Crodriver", "BMW" and "imbalance" did not get any results about motors - just batteries. I'll try again later with perhaps a better set of search terms but I'd appreciate a link if you have one.

Crodriver is running a sub-10 second racer ... I think he stresses a bunch of components on each run!

I don't like the idea of putting DC motors in series. Maybe it's because I don't understand how they work together. As I understand it, they develop the same torque since they draw the same current. But if one is slipping and the other has traction, both get the same current so one will spin like mad (likely overspeed and possibly destroyed) and the other will do it's normal drive. In that case, if your controller is using twice the voltage that one motor should get (so that you can get rated voltage and rated current to each motor), the voltage drop across one motor is lower and the other is higher, so the motor doing the work will .. possibly .. have damage done to the brushes or armature due to arcing.

Motors in parallel get the same voltage instead, so as I understand it the motor without traction draws less current (it still spins like crazy because it has no traction, and so no load, and it may still damage itself with overspeed. Not sure on the overspeed stuff) but the one with traction can take more current and get you down the road.

Maybe I need to do more reading ...
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