How to Calculate RPM for an Electric Motor on Paper?

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SUMMARY

This discussion focuses on calculating RPM for electric motors, specifically in the context of designing a motor for applications like model aircraft. The key formula for power is established as Power = Torque * Angular Velocity, emphasizing the trade-off between torque and RPM. The conversation highlights the importance of characterizing the load to determine the necessary torque and RPM. Additionally, it introduces the concept of motor constants, such as rpm/volt, which are crucial for selecting appropriate motors for specific applications.

PREREQUISITES
  • Understanding of electric motor design principles
  • Familiarity with torque and angular velocity concepts
  • Knowledge of DC permanent magnet motors
  • Basic grasp of power calculations in electrical systems
NEXT STEPS
  • Research the calculation of torque and RPM for electric motors
  • Explore the concept of motor constants and their applications
  • Learn about gear reduction systems and their impact on motor performance
  • Investigate the efficiency factors affecting power calculations in electric motors
USEFUL FOR

Engineers, hobbyists, and designers involved in electric motor applications, particularly those focused on model aircraft and similar projects, will benefit from this discussion.

njguy
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I am trying to design an electric motor and calculating how much electrical power it consumes is easy. Trying to figure out the horsepower it produces on paper is not. I can calculate the torque of the motor, but in order to calculate horsepower I need the RPM. Does anyone know how I can figure out the RPM on paper? I imagine that I would need to come up with an imaginary load..? I am confused by this one. I can't find the math for it.
 
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What is the purpose of the motor?
 
Power = torque * angular velocity

What that's saying is that you can trade torque and rpm to achieve the same power. Within reason the rpm is something you as a designer choose to suit the application and the availability of suitable motors. If you are designing a motor you can choose to make it a high rpm low torque motor or a low rpm high torque motor.

Your first job is to characterise the load. What torque and angular velocity (rpm) does it need?

Suppose we're looking at DC permanant magnet motors for a model aircraft. You might take a look at the motor constant which has units rpm/volt. Let's say you want to drive a model aircraft propellor at 10,000 rpm using a 10V battery then you might look at a motor that has a 1000 rpm/volt motor constant or perhaps a 3000 rpm/volt constant and a 3:1 reduction gear box. There are reasons why one might be a better choice then the other but that's for another day.
 
njguy said:
I am trying to design an electric motor and calculating how much electrical power it consumes is easy. Trying to figure out the horsepower it produces on paper is not.
That doesn't make much sense: minus inefficiencies, they should be the same number!
 
Last edited:
Good catch Russ.

njguy - how do you know the electrical power consumed?
 

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