# Power curve of Generator

1. Mar 3, 2008

### woodicw

I'm designing a power assisted bicycle for my senior project in mechanical engineering and I have a few questions about the electric side of the design. I want to attach a small motor to be driven by the rear wheel to generate electricity to recharge the battery. The specs of the motor I was looking at are as follows
American Bosch Motor/Generator
-1/3 HP
-24 volt DC
-Motor body: 2.5" diameter x 4.5" long
-Shaft: .25” diameter x .75”
-flatted on two sides.
-Two 10-32 mounting studs on front, .7” long, on 1.94” centers.
-self-resetting bi-metal thermal protection.

Now my question is, is there a way for me to calculate what kind of current is being generated at a certain rpm? I'm hoping there is a generic formula and its not totally dependent on the motor.

2. Mar 3, 2008

### Mech_Engineer

Manufacturers often times have a torque vs. rpm graph to describe the motor's performance. If you are looking for a power vs. rpm graph for the motor, it can be calculated using a torque vs. rpm graph.

$$P = \tau * \omega$$

$$P$$ is power (Watts)
$$\tau$$ is the torque (N-m)
$$\omega$$ is the angular speed (rad / second)

So say a motor is developing 20 N-m of torque, and is spinning at 20 rad/s; that means it is developing 400W of power.