# How to calculate RPM on paper?

1. May 4, 2013

### njguy

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.

2. May 6, 2013

### Jano L.

What is the purpose of the motor?

3. May 6, 2013

### CWatters

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. Lets 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. May 6, 2013

### Staff: Mentor

That doesn't make much sense: minus inefficiencies, they should be the same number!

Last edited: May 6, 2013
5. May 6, 2013

### CWatters

Good catch Russ.

njguy - how do you know the electrical power consumed?