# Controller basics and motor torque



## Phil_P (Jun 16, 2010)

I wondered if someone here could answer a question I have concerning the advantages or not of higher voltage controllers for series DC motors. As I understand it controlling a DC series motor with PWM, involves the rapid switching of the supply (battery pack) and varying the pulse width (up to 100% on) to control output power. The biggest advantage of a higher voltage controller/battery pack (other than reduced I*IR losses) would be greater power at higher RPM because you could push the current against the greater back EMF generated at higher motor speeds.... In my very rudimentary understanding of series DC motors, the torque is proportional to the current so what happens at lower RPMs where a lower voltage controller could supply the same current? If we had a 120 volt controller supplying an average of 1000 amps from a 120 V pack at 500 rpm, how would it compare in torque production to the same motor receiving an average of 1000 amps from a 300 V controller coupled to a 300 V pack at 500 rpm... would it be the same (and hence of lower efficiency), or would it be greater reflecting the increased wattage supplied (in which case the torque would not be proportional to the average current), or am I missing something???
Thanks Phil


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## major (Apr 4, 2008)

Phil_P said:


> If we had a 120 volt controller supplying an average of 1000 amps from a 120 V pack at 500 rpm, how would it compare in torque production to the same motor receiving an average of 1000 amps from a 300 V controller coupled to a 300 V pack at 500 rpm... would it be the same


Hi Phil,

Yes, it would be the same motor torque. But different PWM duty cycle.



> (and hence of lower efficiency), or would it be greater reflecting the increased wattage supplied (in which case the torque would not be proportional to the average current),


No, efficiency would be the same, well maybe a percent different. But essentially the same efficiency for either case.

Remember that motor torque is proportional to motor current, which is not the same as battery current. So if you have a 1000A of motor current, you get XX.X lb.ft. of torque regardless of motor voltage, battery voltage, battery current or RPM.

Regards,

major


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## Phil_P (Jun 16, 2010)

Firstly, Major, thanks for your prompt reply.



major said:


> Remember that motor torque is proportional to motor current, which is not the same as battery current. So if you have a 1000A of motor current, you get XX.X lb.ft. of torque regardless of motor voltage, battery voltage, battery current or RPM.


Another question; if motor current and battery current are not the same, are modern controllers somewhat like DC DC converters, and can give an increase in current for an inversely proportional change in voltage (minus losses).
Thanks Phil.


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## major (Apr 4, 2008)

Phil_P said:


> if motor current and battery current are not the same, are modern controllers somewhat like DC DC converters, and can give an increase in current for an inversely proportional change in voltage (minus losses).


Yeah,

That gets explained here quite often. Just the other day, post #8 http://www.diyelectriccar.com/forums/showthread.php?t=46017 

major


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## alvin (Jul 26, 2008)

This may not be what you are talking about but... On my conversion I started out with 120 volt system and as I used the power the voltage would sag. Now with 144 volts it still sags but the sag is at a higher voltage and the rpm's stay up to where I can use more of the battery
for more range with out the vehicle going slower.
Alvin


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