# Field weakening == armature strengthening?



## major (Apr 4, 2008)

Stunt Driver said:


> I am looking if my budget conversion Fiero can do any facter on long stretches, like 5-7 mph more maybe, because other cars gather behind me.
> 
> I know in older days field weakening was used - a parallel resistor with field, to allow more current in the chain, and so more current thru Armature. Am I correct that goal is increased current thru armature, not decreased thu field?
> 
> ...


Interesting idea  I imagine the antiparallel diode in the main switch would conduct and you'd lock the wheels up big time. But that is just a quick look at your circuit. I doubt Alltrax will honor warranty 

And if you want to field weaken, why do you try to strengthen the field?

major


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## Stunt Driver (May 14, 2009)

major said:


> And if you want to field weaken, why do you try to strengthen the field?
> 
> major


Because I think bringing additinal energy into system is better than dissipating part of existing on a resistor


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## major (Apr 4, 2008)

Stunt Driver said:


> Because I think bringing additinal energy into system is better than dissipating part of existing on a resistor


The power in the diverter resistor is small in comparison to the load. But to field weaken a series motor, you need to have less field current than armature current.

Example:

Full field: Ia = 100A, If = 100A

Weakened field: Ia = 125A, If = 90A, diverted current = 35A. 

Diverter might be on the order of 5 -10 milliohms for this example. Wasting 6 to 12 watts. Big deal. 

In the example, you'll notice that the armature current (Ia) increases in the weak field mode. This is because you lose torque/amp with the weak field and also the load increases due to increased vehicle speed.

That was just an example, so don't take those numbers to the bank. And field weakening may or may not work well for your particular motor and application. A lot depends on the particulars. If the load point is too great, you may actually go slower in weak field mode while drawing more current. But if applicable to your system and done correctly, field weakening can provide like a 10 to 15% speed increase and negligible loss in the diverter resistor.

major


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## Stunt Driver (May 14, 2009)

Thank you, Major, I guess I should try first, before discarding resistor option. WHat would be best to use as such resistor?


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## major (Apr 4, 2008)

Stunt Driver said:


> WHat would be best to use as such resistor?


http://www.meci.com/index.php/braking-resistors-069-ohm.html 

Maybe part of this. The length and parallel strands of the element to be determined from your particular field and load. But the material in this braking resistor should be suitable (almost zero R change WRT temp). And it is cheap.


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## Stunt Driver (May 14, 2009)

Thank you, i should try.

At current I have advanced brushes, and I get extra 5mph above my 40, BUT current draw at 45 is 110A, while before at 40 it was 100A.
So I feel like consuption rose across all speeds, instead of just around 40MPH.

That is why I'm looking for something electrically switchable and remotely controllable to gain those 5pmh


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## major (Apr 4, 2008)

Stunt Driver said:


> At current I have advanced brushes, and I get extra 5mph above my 40, BUT current draw at 45 is 110A, while before at 40 it was 100A.


Odd as it might seem, advancing brushes will weaken the field 



> So I feel like consuption rose across all speeds, instead of just around 40MPH.


Likely this is just perception. Unless you've done something wrong, slight brush advance or field weakening (properly applied) will not make the motor noticeably less efficient. So battery current draw for equal vehicle speed before and after should not be noticeably different. Now because you have altered the RPM/torque/current characteristic of the motor by field weakening, at equal vehicle speed before and after, the motor current will be higher, but the motor voltage would be proportionally lower.

major


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## Stunt Driver (May 14, 2009)

Do you think additional field weakening will do some good?


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## EV-propulsion.com (Jun 1, 2009)

Many years ago we used field weakening on a 72 v system, when we used clunky contactors cutting in batteries for a speed control (1970's). We used a *.*8 ohm resistor similar to the one pointed out by major. The effect on speed was substantial-it was like having another gear in the transmission! Major's estimate of a 10-15 % is accurate, and maybe conservative. I say go for it and try it.
Mike
www.EV-propulsion.com


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## toddshotrods (Feb 10, 2009)

Interesting thread. I am planning to convert my SepEx to series for compatibility with a race-level controller. This would restore one of the main feaures of SepEx, if it's actually a viable concept.


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