Posted by Peter VanWylen at 1/7/2001 4:48 PM EST
Student on team #107, Team ROBOTICS, from Holland Christian High School and Metal Flow Corporation.
In Reply to: How do people get hot motors?
Posted by Elaine Owens on 1/7/2001 3:11 AM EST:
I'm not sure how many of you saw Woodie's presentation on DC motors at the kickoff yesterday. He did a real nize job of showing the linear relationship b/w torque and power, and then showed the power curve, which had its maximum value at half of stall torque, where the RPM was also half of its no-load RPM. This was really great, but what he forgot to show was the efficiency curve. Efficiency is a measurement of (power in)/(power out). Now, those with a physics background will know about the conservation of energy. We know that all the energy from the battery that goes into the motor goes somewhere. Guess what? That extra energy that doesn't go to the output shaft goes into good old heat. So, as you see energy is still conserved. Well, the point at which efficiency is greatest is also the point at which there is the least heat
ower ratio.
Basically, on a graph with the torque on the x-axis, we get a power curve which maximizes at half torque. But, the efficiency curve maxes out near 15% of max torque (don't hold me to this, its different for every motor). So, contrary to what Patrick said, the most heat is not put out at the same time as the most power. Talk about one big optimization problem. So, basically you want the torque to be somewhere between the place of max efficiency and the place of max power. On either side of this, both curves go down, and it is a lose-lose situation.
The real question for you is where to put it. If you use a motor very infrequently and need the extra power, then go for it! But remember that a frequently used motor that gets hot, only looses efficiency making things worse. So, for the drivetrain, you must carefully choose a gear reduction that provides enough power, but still stays cool enough for long practice sessions.