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Understanding Motor and Gearbox Design 2

Step 3: Motor Curves

A motor’s speed, current draw, power, and efficiency are often plotted against the output torque to make their values easier to visualize. The equations for these curves are all derived from the four specifications discussed above using equations 1 through 4 of the previous few pages.
The graph on this page shows the motor curves for a CIM motor, one that is very common in FRC.
Step 4: Choosing a Motor and Gear Ratio

Now that you understand the specifications that distinguish motors, you can work on choosing a motor and gear ratio for your application. Which motor is most appropriate for a given job is entirely dependent on the application’s requirements. This means that you must determine end results such as how big of a load are you moving and how fast do you want it to move, and then translate these into requirements such as output torque and speed.

Start by looking at the specifications of the available motors. The Motor Spec Sheet for the 2012 FRC season is included on this page. There are many factors to consider when choosing a motor and gear ratio, including:
?How gearing will affect the motor’s output torque and speed. Usually, gears will be used to decrease speed and increase torque.
?Inefficiency in power transmission – each stage of gearing or chain run is approximately 90% efficient.
?Differences between theoretical and actual performance. Because theoretical performance is usually better than actual performance, even after accounting for inefficiency, it is important to choose motors and gear ratios with a healthy safety factor. That is, make sure that they will be able to handle more than the expected load at a faster than required speed.
?The amount of current that a single motor can draw is limited by the circuit breakers on the power distribution board. When using 40 amp breaker, your current draw is limited to a maximum of 40 amps, meaning you should design the motors to draw less than 40 amps under the expected load. In addition, the robot can draw a maximum of 120 amps at a time, as limited by the main circuit breaker.
?Running motors at or near stall load, the maximum amount of torque they can output, will cause them to burn up because much of the energy supplied to the motor will be turned into heat. The amount of heat that a motor can handle is directly related to its total mass. For this reason, heavy motors like CIMs are much less likely to burn out than smaller ones like the Fisher Price motors.
?If no single motor will fulfill your requirements, consider pairing motors. When combining two motors, the output torque and current draw are additive, while the output speed does not change. If two different motors are matched together, their free speeds must be matched through a gear reduction. For example, combining a Fisher Price and CIM motor would require an extra 3:1 gear reduction for the Fisher Price motor because its output speed is approximately 3 times faster than a CIM’s. If the output speeds are not matched, it will cause added resistance in the gearbox and negate any benefits of having multiple motors.

By accounting for all of these factors in your calculations when choosing a motor and gear ratio, you will ensure that your robot works as you intend the first time around. The example problem at the end of this tutorial will demonstrate how to go through the process of making these calculations.

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