Dear XMOD Community,
I have been working for some time on something everyone has wanted a way to answer the question – “What is the best motor for my XMOD?” I built a home-made motor dyno to systematically test each motor by placing them under a constant load and measuring the pertinent characteristic parameters. The dyno is constructed by using a load absorbing motor coupled to the drive motor. The drive motor is supplied power by a constant voltage (5V) power supply capable of handling the current draw without a large voltage drop (a battery pack will not work due to the voltage drop under high current loads). The necessary instrumentation is wired to the system so that all information is collected at the same time. This requires the use of two multi-meters and a laser tachometer. The first multi-meter is placed in series with the drive motor to measure the current draw. The second multi-meter is connected to the motor tabs to measure the drive voltage. The laser tachometer is set-up to read the reflection signal off of the motor coupling to give a real-time reading of motor speed. Using the supply voltage, current draw and motor speed at various conditions the motor characteristics can be found (explanation of motor characteristics would be too lengthy for this post – can be found on many websites like
www.micromo.com/library/docs/notes&tutorials/ Motor%20Calculations.pdf ). By using DC motor theory and the experimentally determined characteristics I was able to calculate the speed vs. torque, current draw vs. torque, power vs. torque and efficiency vs. torque curves for some of the motors I currently have. I have attached a zip file that contains the results of these tests as well as a spreadsheet file for everyone to use. The functions are already in place and only require two data points to complete the calculations. The first point is the no-load speed and current (measured without the motor coupled to the absorbing motor), and the second point can be at any appreciable constant load placed on the motor. When testing I typically triple check the results by measuring four or five data points and making sure the resultant characteristics are consistent. The two data points are used to find the characteristics and then the DC motor model uses these characteristics and a desired supply voltage to generate the motor curves. You might be saying to yourself, “This is a load of garbage, there is no way this model can be right.” Well I wouldn’t put it out here for everyone to see if I didn’t verify the results by some method. I started by applying the DC motor model to the specifications given by Mabuchi on their website. Using the no-load condition and the maximum efficiency point I was able to accurately determine the motor characteristics. I could then use the model to predict the performance at a different voltage – what do you know – It worked! The one thing I noticed is Mabuchi has a small correction factor in some of their data (the voltage constant is not equal to the torque constant) to make the results match the theory. In most cases this correction factor is around 0.9. I have included this factor into the calculations. I hope this will be a way for everyone to compare motor A to motor B without being so subjective about it. As soon as I get any more data I will make it available as well.
Edit: The Results and Analysis file has been updated 02/04/04