# Torque vs. Velocity



## eRev (Jan 10, 2009)

Kudos to all that have made this list a pleasurable read for the last several weeks - yeah, guilty as charged for being a lucker, but one question has been bugging me...

Using a typical commercial 500A H-bridge controller - what is being controlled voltage or current? 

In other words, approaching a grade from a steady lightly loaded speed - would one leave the throttle in a fixed position or would one invoke more entry throttle in order to maintain a steady speed?


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## david85 (Nov 12, 2007)

I think an H-bridge controller is a PMD controller (some one correct me if I'm wrong). PMD stands for pulse width modulation. Basically the controller allows current from the battery do "pulse" to the motor. The longer the pulse width, the more time the motor gets power compared to the off part of the cycle and this is how the controller varies power to the motor.

In theory, both current and voltage are at their maximum when the curcuit is turned on. In plactice, the voltage stays constant, and more or less amps are drawn to propel the vehicle. The cycling happens very quickly, so you don't normally feel any bumping of vibration, but if you have seen some Utube videos of EVs that employ the curtis controller, the whine you hear at light power settings when the car first starts moving is actually the sound of that rapid switching.

The simple answer to your question is: amps are the variable, and voltage is the constant.


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## Tesseract (Sep 27, 2008)

You can use either method with an electric motor, but controlling current will result in a throttle response more like an ICE. Keep in mind, no matter the control scheme some attention must be paid to the current if only to prevent frying the motor windings, the controller, the batteries or all of the above.

Anyway, it takes more torque to drive up a steeper incline so the throttle on a constant current controller would need to be depressed more. With a constant voltage scheme, amps will naturally increase as back emf from the motor decreases so the motor will tend to maintain a more constant speed (there is some variation due to IR drop and magnetic non-linearity).

My personal feeling is that the constant current scheme gives a more natural throttle response but that may not be a universally shared sentiment...


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## eRev (Jan 10, 2009)

Precisely my desire - driveability similar to an ICE powered vehicle and for clarity I think this is referred to as torque mode where the throttle position equates to a given allowable current output.

Well, I guess calling it constant current is valid too because the voltage (thus speed) will assume whatever value it can at any throttle position.

So, with that said - who are the commercial controller manufacturers that allow torque mode operation?

I appreciate the input and sorry for for the newbie questions.


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