Breaker for Globe Motor?

[railerobotics]

Should I use a 30 amp or 40 amp breaker for a Globe Motor?

[Andrew Blair]

30 amp. It's stall torque is approximately 22 amps. Actually, I think that you should use a 20 amp breaker, to keep some protection for the motor.

[railerobotics]

Thanks

[Rick TYler]

I just want to expand a little on what Andrew wrote. If you used a 30- or 40-amp breaker there's a good chance you will see magic smoke before the breaker blows. I would recommend (like Andrew) a 20-amp breaker.

[Andrew Blair]

Exactly. Sorry that I wasn't clear. If you use a breaker that is rated for a higher current than the stall torque of the motor, the motor will stall, and eventually burn, before the breaker trips. So always use a breaker that is for a lower (but as close as possible) current than the stall torque of the motor you intend to use it on.

[railerobotics]

Ok, Thanks

[Al Skierkiewicz]

Gentlemen,
There is a little to consider before choosing the breaker size. If you think of the motor as in stall whenever it is not moving then you are hitting stall current every time you start the motor. If you are reversing direction, there may be even greater stress on the circuit breakers. If you are planning a critical function for the Globe and it will be in use often, I suggest a 30 amp breaker, which will allow you to use #14 or larger wire to feed the motor and speed controller. If you are planning on using a Spike for control, then you must use a 20 amp breaker and #18 or larger wire. Please remember that there is a 20 amp fuse in the Spike that is not auto-resetting. Please also refer to the First Robot Guidlines manual for additional info and be aware that the guidelines may conflict with this year's rules. The robot rules in section 5 as revised and any Q&A or Team update must be followed as well.

[Andrew Blair]

I always thought that the thermo-trip design of the breaker (auto-reset) allowed a little leeway, so that you can run the motor at currents higher than rated (stall current), and in this way, a stall underrated breaker, for starting motors, would still be adequate.

I also had thought that the breaker was to prevent motor damage mostly, in addition to kill a short to the frame. So wouldn't you actually want an underrated breaker? To protect your motor to an extent?

But more than anything, the rules are a great place to start. I should probably go back over them now that I think about it...

[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.