# Determining Correct Circuit Breaker

How can we determine the correct circuit breaker to use for a particular motor? I know the CIMS use 40 AMP breakers, but we are using a PG motor and 2 Snowblower motors. Would we need to look at the stall current to determine the breaker size? Ex: the PG has a 22 amp stall current, so we’d use a 30 amp breaker.

Certainly the motor is one consideration in selecting your breaker, but you may want to consider the load you expect placed upon the motor.

I expect that you would want 40 Amp breakers for motors being used for your drive base, and possibly for your climbing mechanism. Depending upon what you choose to lift cubes you may need 40 amps for that as well. Realize that you only have 8 40 Amp slots.

Chances are motors used for intake rollers can operate properly on 20 Amp breakers.

Consider how much load you are applying to the motor and make your selection on that basis.

The circuit breaker is related to the size of the wire uses to connect the speed controller to the motor and PDP.

You might want to use a breaker and wiring that exceed the stall current of the motor, but you are limited to 40 amps, so you can only do that with the low power motors.

If you are running the motor near stall, you might want to rethink your design!

We aren’t running near stall, but just wanted to be sure of the breaker size. When you wire a PG motor do you wire it to a 40 amp breaker or lower? Window motors/snowblower motors I assume can use 20 amp, but not sure about the PG

You need to look at the load you will put on that motor (and the duration of that load). The motor curves in the spec sheet will help you here. Loads can go above the breaker for short durations (hence the duration of the load statement)

Size the breaker for the load on that motor, then size the wires for the breaker used.

Personally, I would use a smaller circuit breaker, either 20 or 30 amp depending on what the motor is doing, and the odds of it having to work too hard, for long enough to pop a 20 amp breaker.

As an example, we put a BAG motor on the wrist on our robot, and ran 18 gauge wire to it, and powered it from a 20 amp breaker. If it has problems, we will consider rewiring it and using a larger breaker, but I don’t expect it to have problems…the control system for it is working well, and we designed it so the motor needs to use less than 20% of stall torque to operate the mechanism under load.

Our breakers are thermal breakers and are very slow to blow. I’ve see a stalled motor pull 90 amps, measured, through a 40 amp breaker with enough time for me to react.

Data sheet with graph at http://www.snapaction.net/pdf/vb3.pdf for our VB3 20A and 30A fuses.

From that page, a 150% overload (I hope that means 150% load and not 250% [100% load + 150% over]) takes 3.9 to 47 seconds to blow. Yes, fourty-seven seconds.

Same for our MX5 40A fuses: http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf.

Probably ought to go smaller rather than larger.

Ratings on breakers are typically misunderstood.
The rating is the continuous amperage where it will NOT trip.

90 amps is only 225% of rating for a 40 amp breaker, so I would expect ~10 seconds before it initially trips.

Breakers are also NOT designed to protect the devices on the wire. They are designed to prevent the wires themseves from catching fire, which is why FIRST requires specific wire sizes for the breaker rating.

Correct, breakers fall under the group of devices known as Circuit protection devices, they are not for protecting devices fed by the circuit. Their purpose is to prevent wires from heating to the point where the insulation is compromised, nothing more.

Huh, obviously I didn’t know that. One of the things I love about a FIRST and FRC is how much I learn from my students and the FIRST community. Thanks!