How to Calculate RPM for an Electric Motor on Paper?

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    Paper Rpm
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Discussion Overview

The discussion revolves around calculating the RPM of an electric motor on paper, particularly in the context of designing a motor and understanding its performance characteristics, such as horsepower and torque. The scope includes theoretical considerations and practical applications in motor design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about calculating RPM for an electric motor, noting that while electrical power consumption is straightforward, determining horsepower requires knowing the RPM.
  • Another participant questions the purpose of the motor, suggesting that the application may influence the design choices.
  • A third participant explains the relationship between power, torque, and angular velocity, indicating that RPM can be chosen based on the application and load characteristics.
  • This participant provides an example involving DC permanent magnet motors for model aircraft, discussing motor constants and potential gear reductions to achieve desired RPM.
  • One participant challenges the initial claim about horsepower, suggesting that, minus inefficiencies, the electrical power consumed and horsepower should be equivalent.
  • Another participant acknowledges the challenge and asks for clarification on how the electrical power consumed is determined.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to calculate RPM or the relationship between electrical power and horsepower, indicating multiple competing views and unresolved questions regarding the motor design process.

Contextual Notes

Participants express uncertainty regarding the assumptions needed for calculating RPM and the implications of different motor designs and load characteristics. There are also unresolved aspects related to the efficiency of the motor and how it affects power calculations.

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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