Quote:
Originally Posted by Ninja_Bait
I think I have been told that speed and torque on a motor aren't necessarily linked in voltage control... but I don't understand the science behind that.
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Say you have a small DC brush motor mounted on a workbench and you connect it to a fixed voltage. The motor will spin up to a certain speed and then stay there. (That's called the "free speed" or the "no load speed").
The reason the motor reaches a certain speed and then stops increasing is because as the motor speeds up it generates more and more "back emf". This back emf is a voltage that opposes the applied voltage. At free speed, the back emf is almost equal to the applied voltage. The small difference in voltage creates just enough current in the motor to keep it spinning at the free speed (some current is required at free speed to supply the torque necessary to balance the friction and other losses in the motor). This current is called the "free current" or the "no load current".
DON'T DO THE FOLLOWING, YOU MIGHT HURT YOUR HAND, IT IS A THOUGHT EXPERIMENT: Let's say Superman walks in and holds the motor shaft and gently squeezes it. What happens? The motor slows down. It slows down because it has a load on it. It also draws more current in order to generate the torque necessary to balance that load. If Superman squeezes harder, the motor slows down more. If he squeezes hard enough, the motor will stop turning. This is called "stall". The torque at stall is called the "stall torque". The current is called the "stall current".
The important take-away from all this is that, for a fixed applied voltage, the torque output and the speed of the motor are inversely related. Put more load on the motor and it slows down. Put less load on the motor and it speeds up.
Now consider what happens if you have a fixed load on the motor. If you increase the voltage applied to the motor, the motor will speed up and reach a new equilibrium speed. If you decrease the voltage applied to the motor, the motor will slow down and reach a new equilibrium speed. At the equilibrium speed, the applied voltage minus the back emf provides just enough voltage to push the current through the motor required to balance the applied load.
If you want to run some "virtual" experiments to explore these concepts, I have written a small motor calculator which is available for download
here. Grab the most recent revision, at the bottom. It has all the FRC 2011 motors in it. You can change voltage, load, speed and see how the motor reacts.