Since adding gears, axles, shaft collars, etc., takes a fair amount of time (especially for novices), each student team could modify their frame in a different way (different simple & complex gear arrangements), and then the students then rotate around the room to the different “stations” to run the experiment under each gear configuration. Mentors: If you are ALSO interested in teaching your team about gear ratios for simple and compound gears, this setup can be modified by adding more axles to the frame on the table and gearing up or down, and running the experiment again. When the PTC cools sufficiently, the circuit will re-connect, once again allowing current to flow to the motor.
Once the PTC exceeds a certain temperature, it breaks the circuit/stops the flow of electricity into the motor, and the motor stops working. The PTC is actually a safety device that prevents the motor itself from overheating and becoming damaged.
This creates a vicious cycle: less and less current and voltage become available to the motor itself. If the heat generated exceeds the heat lost to the surrounding environment, the PTC keeps heating up, causing its resistance to increase, which in turn causes MORE heating. Current flows through it on the way to the motor, and heats the PTC as it does. (If a broken motor is available, teams should open one up and completely disassemble it-the gears from both sides, and this DC motor.) The PTC is the little yellow tab on top of the motor in the photo at right. The photo at right 3) shows the actual DC motor that's inside the standard black-and-green VEX motor this is the part that's actually driving the robot. The circuit-breaker that's inside the VEX 2-wire 393 motor is a combination thermostat-and-resistor called a PTC (Positive Temperature Coefficient) component. Each motor (and motor controller) has its own built-in “circuit breaker” at which point it stops working before this stall point, because the stall point is when internal damage occurs.
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