If I have a motor with a nominal torque t at some current attempting to spin a disk of some known mass m, what other information do I need to know in order to determine whether this is well within the motor’s capability?
Unfortunately my only intuition about torque comes from the notion of applying a force on a lever arm, not from driving at an ideal center point. Since the center of mass of a perfect disk is located on the motor shaft, my first approximation intuition tells me that any motor can rotate any mass and, clearly, that’s wrong.
How could I work with this calculation? I started wading through wikipedia’s entries surrounding torque but they went from “I know that” to “what the fuck” very rapidly.
Any application of torque, no matter how small, can rotate any sized disk, as long as friction is equal to zero. How is the disk supported? How quickly do you want it to accelerate? What’s the required final speed (RPMs) the disk is turning? What is the radius of the disk(the bigger the radius, the harder it’s going to be to start turning)? Does the disk have an even mass distribution?
For a first approximation it is a perfect disk with a uniform distribution of mass and a radius of 25 mm. (I haven’t weighed the disk, but I could. It’s also not perfect, but whatever.) Actually, part of the question is about accelleration, as I can control accelleration. One question I am having is whether this motor can accellerate my load, given its nominal torque. I think the question is further complicated by the fact that while there’s no friction worth approximating there is a detent torque because it is a permanent magnet stepper motor.
My problem is that I can imagine applying some force along some axis, but it’s never occurred to me to figure out what this amounts to in terms of angular rotation. If I want to accellerate this load in some rad/sec/sec, what sort of torque will I need? I feel the answer is simple and I’m just missing something obvious, but maybe that’s not true.
Just like in linear mechanics ** F = M * a **, in angular mechanics the torque equals the moment of inertia times the angular acceleration. Angular acceleration - Wikipedia
Think in terms of work, power, etc. and you can find corresponding equations between linear and angular mechanics.
Electric motors draw more current when they’re coming up to speed than when they’ve reached full speed. Many motors aren’t designed to handle the starting current for very long - their windings can burn out if they stall or take a long time speeding up. So even if there is little or no friction a motor can burn out if it is overloaded.
YamotoTwinkie is steering you true. You are right to think you misunderstand where the mass in the rotor acts. For the purpose of angular momentum it has an average lever arm that is not zero, and you are doing the equivalent of accelerating its mass at that lever arm.
A stepper motor? Then it doesn’t have a detent torque, at least not when you are running it. When you don’t power it up, and you turn the shaft by hand, THEN you have a detent torque. Or, perhaps more precisely, an operating stepper motor has a detent torque that is referenced to where the motor is supposed to be, not to the rest of the world. In this sense, overcoming that detent torque amounts to stalling the motor (not starting it).
Stepper motors can have a more difficult time accelerating a flywheel. There are speeds at which it will be resonant, and at which you can’t apply much torque. And stepper motor makers usually have a dog’s breakfast of proprietary methods they say deal well this this situation.