How to Calculate Power and Torque for a 15kg Load at 15 RPM?

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Discussion Overview

The discussion revolves around calculating the power and torque required for a motor to rotate a 15kg cylindrical load at 15 RPM within 2 seconds. Participants explore the calculations involved, including inertia, angular velocity, and the effects of friction on motor performance.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Patrick presents his calculations for inertia, angular velocity, torque, and power, expressing doubt about the adequacy of a 0.38W motor with 0.24Nm torque for the specified load.
  • One participant suggests that while the calculations may be correct for a frictionless motor, real motors require additional torque to account for friction proportional to the load's weight.
  • Another participant agrees with the need to consider friction and proposes adding 50% to the calculated power and torque to compensate for losses.
  • There is a mention of the importance of the motor's axis orientation relative to the gravitational field, which could affect the torque calculations.
  • Patrick indicates he is testing with a 10W motor and a gearbox ratio of 1:6, suggesting a practical approach to the problem.
  • A participant expresses concern that the negative torque due to friction might exceed the calculated 0.24Nm, indicating uncertainty about the adequacy of the initial calculations.

Areas of Agreement / Disagreement

Participants generally agree on the need to consider friction in the calculations, but there is no consensus on the exact impact of friction or the adequacy of the initial power and torque calculations.

Contextual Notes

Participants note that the calculations may not account for all real-world factors, such as friction and the orientation of the motor's axis, which could influence the required torque and power.

Who May Find This Useful

This discussion may be useful for individuals interested in motor calculations, mechanical engineering, and those exploring the effects of friction in rotational systems.

SuperPat
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Hi all
I am Belgian and I have difficulty with english formulas.
I'm trying to calculate the power and torque required of a motor to rotate a load.

Here is the problem:
I have a motor (axis up) on which is fixed a cylindrical load of 15Kg. The diameter of this load is 400mm
Calculate the power and torque of the motor to reach a rotation of 15RPM in 2 seconds?

Here is how I calculated:

I calculate the inertia of my load: J = 1/2 m r²
J = 1/2 x 15 x 0.2 x 0.2 = 0.3 Kg.m²

Angular velocity: omega = (pi x RPM) / 30
omega = (3.14 x 15) / 30 = 1.57 rad/s

acceleration: a = omega / t
a = 1.57 / 2 = 0.785 rad/s²

torque: c = J x a
c = 0.3 x 0.785 = 0.24 Nm

Power : P = c x omega
P = 0.24 x 1.57 = 0.38 watts

I don't believe a 0.38w motor with 0.24Nm torque
can rotate a mass of 15Kg at 15 rpm in 2s

I've searched, but I can't find the error.
Thanks for your help.

Patrick
 
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I think It can be done if the motor is frictionless, however real motors aren't and you should add in your result the torque due to the friction which is proportional to the weight of the load.
 
of course yes.
I would like first to calculate the motor characteristics and after add 50% power and torque to compensate the frictions and "yield"
But how to calculate ?
Is there an error in my calculation or should I use other formulas?
because 0.38W and 0.24Nm is not right for this heavy load.
Thanks
 
I can't find any mistake in your calculations. Note that 15RPM is relatively low RPM it is just 1/4 of full rotation in 1 second.
 
Ah it also matter the orientation of the axis of the motor, is the axis of rotation parallel to the direction of gravitational field? If yes then your calculations are correct (up to friction). If not then you have to calculate the torque of the weight of the load.
 
yes, the axis of rotation parallel to the direction of gravitational field.
I'am trying to test with a 10W motor and gearbox 1:6 ...
Thanks
Patrick
 
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BTW I am just a mathematician, I know about torque and power calculations but don't have engineering experience with motors, but something tells me that the negative torque due to friction might be a lot more than 0.24Nm.
I 'll page two more member which I think have more experience on this sector of technology
@berkeman @DaveE
 
Thank you very much
Patrick
 
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