[Al Skierkiewicz]

Andrew,
Strictly speaking, any circuit protection device is to prevent the circuitry from catching on fire. In the case of the robot, the breakers are chosen to prevent flaming the wiring, hence the rule book references to breaker size and minimum wire size. A room temperature breaker in any size that we use has as much as a 600% overload before trip. All of the breakers work on a temperature rise principle, (unlike house breakers) so the warmer a breaker gets in operation the closer to trip it will get. Remember that stall current is not a point that kills the motor, it is the repeated and lengthy time at stall that will build up internal heat. In the case of the Globe motor, if you were to use a 20 amp breaker and run the motor at 19 amps with occasional stalling, the temperature in the breaker would rise to the point where the trip point would occur almost constantly. In the same case, a 30 amp breaker would remain cool until the motor was at stall for 20-30 seconds. As the 30 became hot it would likely trip as well. The Globe motor with the transmission attached (unmodified) is a strong little package. It can run for years if it is used as designed. I have two of these motors in service on a steerable satellite dish where I work. One is used to position the feedhorn vertically and the other to rotate the feedhorn. The feedhorn assy. weighs in at about 20 pounds and these little motors can keep it moving in any weather and are very repeatable.