# speed control versus torque control



## major (Apr 4, 2008)

John Caldwell said:


> When designing the control system for an electric car, should I use speed control or torque control?


Torque control.


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## EVfun (Mar 14, 2010)

I believe most current series DC motor controllers are more or less speed control. That is, they vary the target voltage based on the throttle position with a secondary feedback that limits torque so 25% of throttle from a stop doesn't end up being maximum motor current.


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## John Caldwell (Nov 22, 2011)

Why not use speed control?


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## major (Apr 4, 2008)

EVfun said:


> I believe most current series DC motor controllers are more or less speed control.


With the series DC motor, it is kinda the same thing. All you really have control over is voltage to the motor (via PWM which means really all you can do switch the electrical source on and off). When you increase the voltage on the motor it will attempt to increase RPM but cannot do so instantaneously so really increase current and torque at the same RPM. Unless you have a shaft sensor and a control loop, I fail to see it as speed control. But everybody calls them speed controllers 

It is voltage control  With current limit. And ramps sometimes.


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## major (Apr 4, 2008)

John Caldwell said:


> Why not use speed control?


When you go beyond the series DC motor and PWM controller into systems where you can choose, select torque control, or you'll be sorry. Imagine driving your car only able to use your cruise control. That is speed control. That pedal on the floor is torque control


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## Qer (May 7, 2008)

major said:


> It is voltage control  With current limit. And ramps sometimes.


In the Soliton case, nope.

Or rather, yes, in the most inner control loop of the software the PWM duration, and thus the voltage, is of course controlled since it's a buck converter, but it's so completely abstracted in the software that everything else works with current and thus acts as torque control.

That's why, for example, logger reports throttle position as a current rather than, say, percent.


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## DavidDymaxion (Dec 1, 2008)

Another reason is that is not what ICE drivers expect. They expect to have to push harder on the gas pedal to maintain speed up hills, and less hard downhill.

Yet another reason is strict speed control could be dangerous. Do you really want to initiate wheel skidding amounts of regen when the pedal is lifted, or just reduce the torque to a zero or low braking value that steadily slows the car? Imagine driving on ice, lifting the pedal, and the controller is working very hard to slow the car to zero speed!

Having said that, I did have speed control on my sepex car for a while. With only 48V it worked pretty well, the power was low enough that commanding a new speed didn't burn rubber.


John Caldwell said:


> Why not use speed control?


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