# HELP! rpm and g.cm of motor

#### Paul McD

Joined Feb 9, 2012
10
I'm building a solar car and have been given a particular motor to use and need to work out if my design and gear ratios are sufficent. (I intend to use a gear ratio of 5:1 and a mass of 200g, with 2 wheel drive)The motor has a zero load rpm of 16'400 and a loaded speed of 13'100, with a load of 8 g.cm.
But the motor is going to be under more load than that, so does the graph of the rpm and load (g.cm) follow a linear pattern or a curve? This will help me determine how fast the motor will be spinning with my estimated load of 25 g.cm

Many Thanks

#### Ron H

Joined Apr 14, 2005
7,014

#### John P

Joined Oct 14, 2008
2,000
Well, it's theoretically linear. But what about the power source? To do this properly, I think you have to plot out the power output curve of the solar cell, i.e. what current it can deliver for a given voltage. Volts will certainly drop as current rises. Then compare that with the matching curve for the motor. Ideally, you'd want to get the best power match you can.

Motor listings often show a curve of efficiency at the same time as torque and speed, and the maximum occurs when the motor is running at something like 80% of no-load speed. They also show power out, and the maximum is at half no-load speed.

I think maybe if the solar cell is small compared to the motor, you'd want to use it as efficiently as you can, so you'd want to operate near the maximum efficiency point. If the solar cell is large, you'd have power to burn, so get the most out of the motor and run near the maximum power point. But you would have to figure out how much torque the motor needs to run the car at a particular speed, and that might be tricky to measure.

#### Ron H

Joined Apr 14, 2005
7,014
Well, it's theoretically linear. But what about the power source? To do this properly, I think you have to plot out the power output curve of the solar cell, i.e. what current it can deliver for a given voltage. Volts will certainly drop as current rises. Then compare that with the matching curve for the motor. Ideally, you'd want to get the best power match you can.

Motor listings often show a curve of efficiency at the same time as torque and speed, and the maximum occurs when the motor is running at something like 80% of no-load speed. They also show power out, and the maximum is at half no-load speed.

I think maybe if the solar cell is small compared to the motor, you'd want to use it as efficiently as you can, so you'd want to operate near the maximum efficiency point. If the solar cell is large, you'd have power to burn, so get the most out of the motor and run near the maximum power point. But you would have to figure out how much torque the motor needs to run the car at a particular speed, and that might be tricky to measure.
It's even more complicated. I'm pretty sure this is related to his other thread.
Paul, it is a good idea to keep related questions in a single thread. I think you can see why.

• Paul McD