# Can we use 30 AMP PDP ports for our 775 motors?

We’re not sure if the motors will burn out or if the breakers will trip. We are very set and are working under limited conditions so the amount of motors cannot change.

I’m not the best person to answer this but I’m pretty sure that using 20, 30, 40 AMP breakers won’t affect whether or not your motor burns out. Depending on what the sim does, you may need more power, but that really shouldn’t affect whether or not your motor burns out.

Yes, you can run Sims on 30 amp breakers. The only rule you have to follow is using the correct gauge wire. I think 40 amp requires 12 gauge and 20/30 requires 14.

Oops. I was misinformed. We need them for 775 motors. I don’t think that makes any difference but I would like your input.

The question isn’t really that simple. 775pros will draw 134A at stall at 12V. But they won’t draw anywhere near as much (~1A) at free speed. When a load is applied but the motor is still spinning, the current draw will scale linearly with the torque applied. So the steady-state current that the motor draws is really going to be based on the load applied on the motor.

I highly suggest using the mechanism calculator on my design spreadsheet to help calculate the current the motor will draw under your specific conditions. You can then compare that to the trip-time curve on the breaker’s datasheet to see whether the breaker can sustain that current for the required time. If not you’ll have to change your mechanism to lower the current draw.

As far as burning the motor, that isn’t really affected by the breaker size. If anything, a smaller breaker will make the motor last longer because it can draw less current therefore heat up slower. You can check how long different motors last under different conditions at motors.vex.com, and use that information to predict whether the motor will survive the conditions you’re putting it under.

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This isn’t exactly true. A smaller breaker itself won’t make the motor output less torque or draw less current (or make it less likely to self-destruct). It will just lower the current threshold before the motor gets cut off by the breaker tripping. Teams will design their mechanisms to draw less current when using smaller breakers to avoid tripping them, but they could design similarly with a larger breaker and it would have the same effect.

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A common misconception is that breakers and fuses can actually limit current flow to some level. They do not. They are protection devices. They cut off all current flow based on the recent overload history i.e. the current goes to zero immediately after the breaker or fuse opens.

The self-resetting breakers will close the circuit after it has cooled sufficiently so current will flow again. While it is possible to calculate an “average current”, this value is not very useful since one would have to average over many seconds. The cycling on and off will be very noticeable and it is unlikely that your mechanisms will function in a satisfactory way.

The rating of the breaker or fuse must be chosen based on the overload tolerance of the circuit to be protected. A 40 amp breaker will not save a low power throttle motor from frying if it is stalled continuously.

The proper way to limit motor current is to use a controller like the Talon SRX.

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From our testing it seems that 775s are fine with stalling at a limit of 25amps, above that they can burn out if stalled for long durations.
Again, that’s just our tests, but it seems consistent with what Vexpro has released.

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For the 775pro a stall current of 25A would be equivalent to a stall voltage of 2.25V. So that matches with the locked rotor testing data posted on motors.vex.com

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Yeah that’s what I meant

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