How to calculate RPM on paper?

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In summary, you can calculate the electrical power consumed by an electric motor by using the motor's motor constant and the rpm of the motor.
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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?
 
  • #3
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.
 
  • #4
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!
 
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  • #5
Good catch Russ.

njguy - how do you know the electrical power consumed?
 

1. How do I calculate RPM on paper?

To calculate RPM (revolutions per minute) on paper, you need to know the number of rotations or revolutions and the time it takes to complete those rotations. The formula for calculating RPM is RPM = (60 * Rotations) / Time. This will give you the RPM value in revolutions per minute.

2. How do I convert RPM to radians per second?

To convert RPM to radians per second, you can use the formula RPM * (2 * pi / 60). This will give you the value in radians per second. Remember to use the correct unit for pi, either radians or degrees, depending on your calculations.

3. Can I calculate RPM without a tachometer?

Yes, you can calculate RPM without a tachometer by using a stopwatch or timer to measure the time it takes for a complete rotation. You will also need to know the number of rotations. Using the formula RPM = (60 * Rotations) / Time, you can calculate the RPM value even without a tachometer.

4. How do I calculate the RPM of a motor or engine?

To calculate the RPM of a motor or engine, you will need to measure the number of rotations per minute. You can do this by using a tachometer or by counting the number of rotations in a specific time interval. Then, use the formula RPM = (60 * Rotations) / Time to calculate the RPM value.

5. How do I calculate RPM from speed and wheel diameter?

To calculate RPM from speed and wheel diameter, you will need to know the speed of the rotating object and the diameter of the wheel. Use the formula RPM = (Speed * 60) / (2 * pi * Diameter) to calculate the RPM value. Make sure to use the correct unit for pi, either radians or degrees, depending on your calculations.

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