Maxi Block Issue

[drewjones13]

We were wondering if we were able to use a single slot 40 Amp maxi block for a van door motor that we are using. We would rather not use one, due to space and weight.

[AJ R]

I believe the van door motor only needs a 20 amp breaker, which can be powered off of the ATC fuse panel.

[cdennisxlx2]

it was our understanding in 06 (the last time we used one, lol) that since the stall current of that motor was 40amps we had to have it on a 40amp breaker, we had a van door motor on our 06 bot through a 40amp and had no issues, i could be wrong about it needing to be on a 40amp tho....

[Paul Copioli]

You can use a 40, 30, or 20 amp breaker for the van door motor (or Fisher-Price, or CIM). The breaker is meant to protect so as long as the breaker is BELOW the stall current, you are legal per the rules. For example, you can't use a 40 amp breaker on a motor with a 20 amp stall current, but you can use a 20 amp breaker on a motor with a 40 amp stall current.

The main reason you want to do this is weight. Per the FRC electrical wiring rules, you must use certain gauge wire for certain breaker sizes. If you are using a Fisher-Price motor, but design your mechanism to only need a continuous current of 24 amps (for example), then you only need a 30 amp breaker instead of a 40 amp breaker. In this case you save weight in two ways: (1) you do not have to use the maxi block (it is heavy) and (2) you can down size your wire diameter (more weight). This is a key weight savings opportunity that many teams miss. We take advantage of the fuse sizing in as many places as we can.

-Paul

[Mike Betts]

Originally Posted by Paul Copioli

You can use a 40, 30, or 20 amp breaker for the van door motor (or Fisher-Price, or CIM). The breaker is meant to protect so as long as the breaker is BELOW the stall current, you are legal per the rules...

Paul,

Your advise, while sound engineering design policy, does not accurately reflect the rules.

<R47> defines CB size based on wire size. Stall current is mot mentioned in the rules and is irrelevant. According to the rules, you can select a motor with very large stall currents (example: a CIM), wire it with 18 AWG wire and protect the circuit with a 20A CB. It is legal. You will repeatedly trip the breaker under almost all conditions (making it practically useless) but it is legal.

Regards,

Mike

[Paul Copioli]

Mike,

You said what I said. As long as the trip point of the breaker is BELOW the stall current of the motor, then you are good. What am I missing? The rules do state that you can't use a 40 amp breaker for the window motors, but I can't seam to find it right now. The reason you can't is that the breaker will not protect you as it is above the stall current of the motor.

-PAul

[Al Skierkiewicz]

Paul,
The wire is protected by the breaker sizing. If the motor stalls, the breaker must trip to prevent the wire from going incandescent.
The breakers realistically have a trip charachteristic that allows them to pass more than 600% over current for a short period of time and can pass almost 150% current indefinetely. As strange as it sounds, you can put a 40 amp breaker on a motor that has a 20 amp stall current as long as the wire is #12 AWG or larger. I wouldn't do it, but you could.
BTW, In looking at spec sheets for both the Kayang and Taigene motors, there appears to be an internal, temperature controlled, breaker on both motors.