# Silly question about a motor and finding the required torque.

1. Oct 2, 2012

### tectactoe

I say "silly" because I'm actually baffled that I can't figure this out. I'm just forgetting the basics and am stumped at how to correctly approach this.

Let's say I want an AC motor that is connected to a shaft which is, in turn, connected to a metal plate. I know the weight of the shaft and the plate, as well as the diameter of the plate.

How would I use that information to find out the torque required by the motor to turn the plate at the rated RPM without binding or slowing down?

And then, let's say I place objects of known weight on top of this plate, that will obviously add weight on the plate, but will not necessarily move WITH the plate. How could this be added into the equation?

Thank you for the help.

2. Oct 3, 2012

### tiny-tim

hi tectactoe!

at constant rpm (zero acceleration), the applied torque only has to equal the opposing torque, and that's just the friction from the axle

3. Oct 12, 2012

### tygerdawg

I posted this answer to another question, it generally applies to your question.

Your motors should be sized for peak torque. Account for all translational & rotational masses and determine the amount of torque required of the motor to accelerate those masses to the desired speed in the required amount of time. Sum everything, and that's peak torque. Your motor must provide at least that much.

This is basic 2nd year dynamics calculations. Websearch for a PDF document called "Smart Motion Cheat Sheet", it is summarized in there.