Calculating Torque of a V5 motor

[60883D-Noah]

Just an opening statement. I am aware that torque is provided by the V5 motor devices in the SDK. This post is about physical simulation that can be applied completely separate of any V5 hardware.

The above diagram is provided by VEX. I have two questions:

1. Does the x axis in the above diagram refer to current actual rpm of the motor. Or to the “desired rpm” you are trying to spin to. I imagine it is the first but I’m not sure.

2. Is the following model accurate for torque output of the motor, where power() is a function that takes the current rpm of the motor and returns a Watt value.
   Torque = power(current_rpm) * voltage

I am skeptical that my model in question 2 is correct. If the answer to that question is incredibly complex and anyone has the patience to explain it I would be incredibly appreciative.

[60883D-Noah]

After doing a little more digging I realize that this seems to be a much more complex problem than I initially believed.

SigBots wiki
The voltage provided to the motor will become a mix of torque and velocity. If the motor is stuck by some external force and cannot move at any velocity, then all of the provided voltage will go toward providing as much torque to try to get the motor moving.

How do you determine how much voltage will become torque and how much will become velocity? The concept of voltage “becoming velocity” is something I struggle with, as from my understanding of physics, different forces may act on a body which can be summed to determine a net force from which acceleration can be calculated, and then integrated to become velocity. How can we calculate voltage just becoming velocity?

[turbodog]

I’d probably go read some books on dc motor theory. Then you can move onto placing that motor inside a “vex motor” assembly and adding all the stuff they do (current limits, PWM control, etc).

That said…

If you apply a set voltage, say 10 vdc, with no load, the rpm will increase until the back emf (the motor acts as a generator, creating a voltage) counteracts the applied (battery) voltage, limiting the motor’s velocity.

If a load is applied, the rpms drop. The drops the back emf, which allows more current (amps) to flow through the circuit. This will increase until something limits it:
heat (plastic components melting)
heat (wire insulation melting, then wires shorting)
heat (battery insulation melting)
battery’s ability to deliver amps (voltage drop)
control circuitry (can you say ‘vex brain’?)
etc
This also determines, to answer your question, how much electrical power gets turned into torque and rpm. This is a reasonable question, but it muddies the water.

If the load applied is within the limits of motor, battery, wiring, etc then an equilibrium will be reached. Note, this can be short term… like when you motor functions normally then overheats 30 seconds later.

DC motor standing alone and a DC motor inside vex’s motor setup, fed power and controlled by their brain are two different things, the latter being MUCH more complex. Vex will increase the motor’s voltage to try and maintain the desired rpm… which is fine, but it’s also more complex than just a battery, wire, and motor.

That said… think of motors in terms of torque and rpm. These items, multiplied together, give your ‘power’ value.

If you ‘just become velocity’ and have no torque, then, by definition, you’ve done no work.

I think you will understand this more if you do some reading on less complex setups (battery, wire, and motor).