Quote:
Originally Posted by anuragh
just a small question
we have a dc motor connected to the flywheel, for max torque should we worry about the mass of the flywheel or the both mass and the radius of the flywheel.
plz quick reply
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Both the mass, the radius, and the distribution of the mass in the flywheel are important, as well as the gear ratio. If you know how the mass/radius of a flywheel, and how the mass is distributed, you can determine the moment of inertia, which measures how hard it is to accelerate the wheel. If your flywheel has a lot of mass near the center, it has a smaller moment of inertia. If all the mass is located on the outside, then it has a larger moment of inertia.
I'll assume that your flywheel is a circular disc with mass distributed evenly throughout. If it isn't, you can find how to calculate the moment of inertia here.
http://en.wikipedia.org/wiki/List_of_moments_of_inertia
The full calculation is difficult because your motor's torque decreases as it speeds up. Your torque is inversely proportional to angular velocity, so as you speed up, your acceleration decreases. To make it easier we can just look for an average torque and an average angular acceleration. You only need to know three things, your desired average angular acceleration, your motor's torque, and your required moment of inertia for the flywheel.
The moment of inertia (I) for a disc is equal to 0.5 * M * r^2, where m is the mass of the disc, and r is the disc's radius. Make sure you use kg and m for the moment of inertia calculation. Your units for I should be kg(m)^2.
Next, you'll need to know your desired angular acceleration, which is alpha, and is measured in radians per second squared (rad/s^2). To find this, you'll need to know your desired maximum speed for the wheel (it should be less than the free speed of your motor+gear reduction if you have one), and the amount of time you want it to take for the flywheel to get up to speed. If your desired rotational velocity is in rpm, you'll need to multiply by (pi/30) to get to rad/sec, then divide by the desired time to get the angular acceleration, in rad/sec^2.
You've given your motor torque as 8 kg cm, but you'll only get that much torque when the motor is stalled. The kg-force cm unit is a silly, horrible, and sometimes confusing unit, and should be converted to Nm. 1 kg cm = 0.09807 Nm. As it speeds up, it will decrease, so I would select an average motor torque of a little less than half of that, so that the motor will get to speed in a reasonable amount of time. The closer your desired rpm is to the motor's free speed, the harder it is to get there. It would be better to have a heavier flywheel and go slower that to have a really light flywheel going quickly for acceleration, as you'll spend more time toward the bottom of the motor curve, where there's lots of torque. This works because it increases the average torque, and therefore your alpha. If you're planning on controlling the velocity of the flywheel with a feedback loop, I'd set the desired rpm to half the free speed, and the average torque to a little bit under half.
Finally, you can plug your values into the equation T = I (alpha), and adjust I, and alpha as necessary. Make sure your units work out - kg m^2 (1/sec^2) = kg m^2 = kg (m/sec^2) * m = Nm.