# Field Weakening



## DavidDymaxion (Dec 1, 2008)

In my 48V car, field weakening raised the top speed from 39 mph to 55 mph... But there's a big but! My Kostov motor is a sepex and designed for it. A typical series motor is not really designed for field weakening. You'd want to try just a little bit and watch out for arcing! (A video camera can help looking for arcing.)

Check out http://www.poormansev.com/ he does some field weakening on the cheap.


Roy Von Rogers said:


> Hi all.
> 
> I'm, trying to get a better understanding of field weakening a series motor. I understand how its implemented and when it should be applied, what I like to understand is how one arives at the values to make it work.
> 
> ...


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## major (Apr 4, 2008)

Roy Von Rogers said:


> I'm, trying to get a better understanding of field weakening a series motor. I understand how its implemented and when it should be applied, what I like to understand is how one arives at the values to make it work.


Hi Roy,

I just wrote a post about FW (post #2 on this thread) http://www.diyelectriccar.com/forums/showthread.php?t=59366 It was a series motor intended to have FW. The motor you have may not be. Only the motor designer knows for sure and you would need to verify that with the motor company or supplier. If the particular series wound motor is not suitable for FW, weakening the field can have the adverse effect or damage the motor.

Any gains you could get with FW will be obtainable with other means, like gear ratio, wheel size or voltage. Or get a different motor. If you insist on trying FW, measure the field resistance and figure your diverter circuit to bypass about 10% of the current around the field coils. Test it while watching commutation and motor current as well as speed. If all looks good, then maybe try 20% diverted current (20% FW). For each increment, you'll increase motor current, sparking and motor heat for your maximum speed. At some point you'll either see the limit of tolerable sparking, heat, current or you'll see the top speed actually decline. 

Never bring in the FW while your controller is chopping and never bring in FW at a point where the increase in current will send it into current limit. Always drop out FW when you let up on the throttle.

This diverter circuit resistance value must include the contactor, cables and resistor. I imagine you will be on the order of 10 to 20 milliohms. You will need the instruments to measure this accurately. Guesswork can zorch your motor or blow your controller in a heartbeat.

So good luck 

major

edit: Those tests must be done under load. No load testing will tell you little or nothing about FW behavior at load.


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