# How does a controller work?



## spdas (Nov 28, 2009)

I notice my controller gives 900 or so amps and 70 volts or so to the motor and as I reach hi speed the amps drop and the motor voltage goes just under the battery voltage at sag. 

1:Why doesn't the controller keep the amps up to give greater speed? 

2: OK I hit 90 mph as a test and the motor voltage was at 119v and the battery was at 122v, roughly, does that mean that if I increased my battery volts to say 130v at sag my motor voltage will also climb and thereby get a higher speed? (so why doesn't the controller just supply more amps?)

thanks Francis


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## major (Apr 4, 2008)

spdas said:


> I notice my controller gives 900 or so amps and 70 volts or so to the motor and as I reach hi speed the amps drop and the motor voltage goes just under the battery voltage at sag.
> 
> 1:Why doesn't the controller keep the amps up to give greater speed?
> 
> ...


The motor determines the current draw according to the applied voltage and the RPM at that instant. The armature in the motor generates a voltage opposing the applied voltage. The current which the motor will draw is the difference between these potentials (Vm - Eg) divided by the motor's equivalent resistance (Rm) according to Ohm's law. So Im = (Vm - Eg) / Rm. Vm = voltage applied to the motor which cannot be greater than battery voltage (Vb) using standard PWM motor controllers which are buck converters. Eg = generated voltage in the armature which is proportional to RPM. 

At low RPM, Eg is much smaller than Vb, so the motor controller can set Vm such that (Vm - Eg) is large enough allow high current. As RPM increases, Eg increases and the controller must increase Vm so (Vm - Eg) stays high enough to draw the needed current. Motor torque is proportional to motor current. At some point along the way of increasing RPM, you reach a point called base speed. This is where Vm = Vb (minus a volt or two in the controller loss). This is where the PWM hit 100%. At higher RPM, the controller is essentially out of the picture and Vm = Vb. 

Once above base RPM, the motor behaves like it is directly connected to the battery and according to its characteristic curve, example here http://www.go-ev.com/images/003_16_T...rP_9_Graph.jpg As the RPM continues to increase, torque and motor current decrease until an equilibrium point is reached where motor torque equals opposing torque due to vehicle load. You can't go any faster because you can't apply higher voltage to the motor. You can't make it draw more current because you can't apply higher voltage to the motor. 

If you increase your battery voltage you will increase the base RPM and raise that motor characteristic curve RPM trace proportionally, so will increase the vehicle speed. You will see an increase in motor current at the top speed due to increased load. Another way to increase vehicle speed is to change the gear ratio. This will also increase motor current at top speed due to increased motor load. 

Hope that explains it for ya,

major


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## gor (Nov 25, 2009)

major said:


> The motor determines the current draw according to the applied voltage and the RPM at that instant. The armature in the motor generates a voltage opposing the applied voltage. The current which the motor will draw is the difference between these potentials (Vm - Eg) divided by the motor's equivalent resistance (Rm) according to Ohm's law. So Im = (Vm - Eg) / Rm. Vm = voltage applied to the motor which cannot be greater than battery voltage (Vb) using standard PWM motor controllers which are buck converters. Eg = generated voltage in the armature which is proportional to RPM.
> 
> At low RPM, Eg is much smaller than Vb, so the motor controller can set Vm such that (Vm - Eg) is large enough allow high current. As RPM increases, Eg increases and the controller must increase Vm so (Vm - Eg) stays high enough to draw the needed current. Motor torque is proportional to motor current. At some point along the way of increasing RPM, you reach a point called base speed. This is where Vm = Vb (minus a volt or two in the controller loss). This is where the PWM hit 100%. At higher RPM, the controller is essentially out of the picture and Vm = Vb.
> 
> ...


thank you, Major
considering above, how field weakening method affects back emf, mt amps, v, kw in, kw-out?

"Easy way to draw more current would be to do field weakening. Dennis Berube and http://www.poormansev.com have experimented with field weakening. Just make up some long segments of 1/0 cable, put them all in series and across the field terminals. Take out a segment at a time. You could risk motor damage if you weaken too far and work the motor too hard. You know your total current, you could put a clamp on Ammeter on the field weakening cable and see how much current is bypassing the field.
Seems like you'd want to weaken it until it just hit 1000 battery amps at the one point where it just drops out of current limit -- provided you don't hit the arcing limit first!"


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## major (Apr 4, 2008)

gor said:


> thank you, Major
> considering above, how field weakening method affects back emf, mt amps, v, kw in, kw-out?


Yes gor,

Field weakening is a valid control approach. BUT, I strongly recommend against it unless you really, really know what you are doing. I mean way beyond having to ask how the controller works. So this should not be discussed on this thread, please. 

Thank you,

major


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## spdas (Nov 28, 2009)

Thanks, Major for your explanation above. My Zilla/warp9/44x180calb in my light Yaris is powerful enough for me, but inquiring minds wanta know.

francis


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