Problems with design, or "KitBot's math seems wrong"

[Mr V]

Originally Posted by bardd

[*]Breakers=/=fuses. When a fuse breaks it's permanent and the fuse needs to be replaced, while a breaker will re-set some time after breaking (actually knew that before, but I use the word "fuse" for both out of habit so I'm taking this chance to clarify).

A little clarification on Circuit breakers. There are different types available. The ones that we use in the power distribution board on FIRST robots are of the self resetting type. After a rest period they will as the name implies reset themselves. The main breaker we use is a manual reset style. So once it trips you must reset it by moving the lever.

The majority of circuit breakers and all of the FIRST legal are a type of time delay style that will allow massive over current situations for a short time. There are also fast acting circuit breakers that will trip quicker or with less of an over current situation.

Fuses on the other hand are most commonly fast acting but you can get time delay fuses in some styles too.

Determining the amount of power that a motor will draw is a little more complicated than looking at the max current at a particular RPM. That is valid when it is supplied a constant power supply at the rated voltage. However with a motor controller that is providing pulsed power the average current through the motor can be lower than the max. Yes it might be pulling 80 amps at a particular rpm when power is applied but if the motor controller is commanded to be operating at 50% to achieve that rpm then the average current would only be 40 amps. The circuit breaker is then getting a "rest" period where it can cool between the times it is supplying that 80a so it will more or less act as if it was only carrying 40 amps and should be able to do so more or less indefinitely.