I know that torque is proportional to RPM and vice versa, but id like to know why we cant just decrease the circumference of a generator shaft in order to compensate for increases in torque.

In other words, if the power input is held constant, will decreasing the circumference of a generator shaft increase RPM without decreasing torque? If not, why not and what happens? Im looking for reasons other than the laws of thermodynamics.

  • $\begingroup$ By what mechanism does decreasing diameter increase RPM? I don't see the direct relation here. $\endgroup$ – JMac Apr 6 '17 at 17:15
  • $\begingroup$ I can rotate a standard pencil by pushing it half an inch with my finger, but id have to push a fat pencil a bit farther for it to rotate, same goes for most anything. If it helps, imagine the generator shaft is moved by a rack. $\endgroup$ – Lgnttsrm Apr 6 '17 at 17:32
  • $\begingroup$ So essentially you're saying the required energy should decrease because of the decrease in mass? The thing is that the shaft is designed to not break from the torque. Decreasing the diameter means you have to decrease torque, or your shaft was over-designed in the first place. $\endgroup$ – JMac Apr 6 '17 at 17:36
  • $\begingroup$ Welcome to Engineering.SE! This seems a bit like a case of unknown assumptions - what is holding the power input constant? A generator's torque will increase in response to higher RPM, but an AC motor turning the generator will have nearly constant RPM regardless of Torque. A DC motor would have decreasing torque with higher RPM. A mechanical turbine will be somewhere in between, depending on it's driving fluid. (And turbine v. multi-cylinder piston engine will also be different). $\endgroup$ – Mark Apr 6 '17 at 17:37
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    $\begingroup$ The input is a huge factor. Connecting an AC motor to the generator, and both will run at the same RPM, stabilizing at a torque demand where the RPM*Torque will equal the motor's power. In this case, shaving down the diameter would do nothing, it all spins at the same speed regardless, because the torque is independent of RPM. $\endgroup$ – Mark Apr 6 '17 at 18:42

Just like gear ratios, if you change the diameter you change the torque. The power will be the same, but if you decrease the primary gear the RPM will go up and the torque will increase.

I always remember it from riding a bicycle.When you change from the larger sprocket on the pedal gear to the smaller it is easier for you to pedal but you go slower.

In industrial application the shaft diameter is not often changed except to compensate for strength or weight. What is changed is the output shaft gear or pulley. To increase torque you decrease the drive pulley / gear or increase the loaded pulley / gear.

This is often done with constant speed motors to change the speed and torque of the output. If you look at the top of a drill press you will often see a bunch of stacked pulleys connected by a drive belt. Scroll down about half way on the page I linked below to see images of a drill press pulley system. They use 3 sets of pulleys and 2 belts to give more control over the speed, it works kinda like changing gears in a car.



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