Al, Thanks!
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Originally Posted by Al Skierkiewicz
Keep in mind that a lower R is translated into a greater demand on torque input. Electric trains use just such a system for braking but the motors are designed for this duty. The control system just switches in more resistance in parallel (lower R) to reduce speed.
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My intent was to use one of the FIRST motors in the Kit of Parts as a "controlled brake" where we'd hang a variable resistance across the leads of the motor. By varying the resistance, we could adjust the torque/speed on the DC motor (generator) shaft.
It is difficult for me to explain the configuration that I have an interest in but another example might be illustrative. Consider a pully type configuration where a heavy weight is on one end of the pully and a DC motor (generator) is on the other end.
If the heavy weight is allowed to fall due to gravity, could I control the resistance across the leads of the motor to gradually bring the falling weight to a controlled stop?
(A) I realize other methods may be suitable for this task, but again, I'm only using the task to illustrate the concept. It's not the actual task that I'm interested in. If this makes sense, what motor in the FIRST kit of parts would make sense to use?
(B) For heavy loads, I'm thinking that the variable resistance controlling the torque/speed of the DC motor (generator) will see a very high current. That resistance control will need to absorb a lot of power. Any ideas on how to set up a "variable resistance" that could handle the power? One thought I had is to use a power transistor and run it in an ohmic region. (I really don't think I could design this control because I'm not that smart.)
(C) As an extension of (B), I would like to avoid a potentiometer (if one even exists to handle the power). I would like to control the resistance through an output of the FIRST robot controller either an analog or a couple of digital outputs. Any thoughts on a design here?
I know this seems like a crazy thing to be thinking about, but the primary objective here is to use this "controlled break" to avoid any current drain from the battery. In others words, if you just reverse everything above and run the DC motor as a motor and use it slow the fall of the weight, then you have to pull a certain amount of power from the battery that has a limit relative to the fuses (30 amps?).
But, as generator, your concern (which is probably a big concern) is if you'll smoke the motors. I'm not so worried at this point about smoking the motors, I'm more worried about drawing more than 30 amps.
Does this all make sense?