# Shared ground for motors on turntable

Hey all. My team is looking at our bot and trying to figure out what we could have done better. We have a turntable on our bot so we have 4 12v lines and 4 ground lines. 2 for flywheel 1 for hood 1 for limelight. I know you can’t run all of that on one 12v line but could we share a ground on all of them get power from the pdh and have there grounds there too. If anybody know if this would be legal or not on an actual robot please add your thoughts below. Thanks

Edit: we have all of our speed controllers up there so it is power from the pdh not speed controllers

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R625 would prevent this. Robot rules are usually pretty exhaustive.

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Unlike power wiring in your house, the power wiring on our robots don’t actually have ground wires. Current will actively flow in either direction between your motor controllers and motors, depending on which way you want your motors to spin. It flows one way to spin your motor clockwise, and the other to spin it counterclockwise.

So not only would this be against the rules, it wouldn’t allow for independent control of your motors.

R622 forbids this. Proper electrical practice and convention would also forbid it. The electrical rules are derived from common electrical practice and convention.

You are required to use certain wire sizes to accommodate the current flowing in each circuit.

R622 refers to “circuits”. An electric circuit is defined as “a complete and closed path around which a circulating electric current can flow”. In this context, each single circuit starts at your PDP/PDH, goes to your motor controller (red wire for +12V) and returns to the PDP/PDH (black wire for “ground”).

If you have 4 circuits, you must have 4 circuits worth of current capacity in your wiring. Each circuit must have the same current carrying capacity along THE WHOLE LENGTH of that particular circuit.

Thus, for each 12V wire you must have a corresponding return/ground wire of equivalent size.

I’m not sure what the people going on about “proper electrical practice” are referring to. You’re 100% in the clear to combine the supply lines anywhere but on an FRC robot for low voltage DC systems. You’d need to do the math on cross-section area, bundling, insulation type, appropriate energy loss, etc obviously. Doing that is fundamentally the same as an electrical subpanel in a house or vehicle and incredibly commonplace.

That said, I don’t think you’d gain much regardless. Even if you had a system where only one of several motors were used at a time (and sized the wire for a single motor), 12 awg zipcord is decently small on almost all scales of construction.

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While one can do the calculations to get the appropriate conductor size for a single return path, it will become awkward to prove those calculations to an RI or LRI at a competition.

If the total current is sufficiently high, one would also have to transition back to multiple conductors to fit the connectors on the PDP/PDH.

If one is using the current sense function of the PDP/PDH, going to a single return conductor will cause the reading to be for the combined current if the sensing is done on the low-side.

Basically, there is no free lunch. If one wants to use a motor, one has to deal with controlling it and managing the wiring in appropriate ways.

Proper electrical practice includes following rules or code for your application. For FRC that is the rule book. For buildings in the US that is the NEC electrical code for the most part.

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I’m pretty sure they mean sharing the 0V (black wire) coming from the PDH to the motor controllers, not sharing the “M-” wire coming out of the motor controllers to the motors.

The reason you can’t do this (aside from the robot construction rules forbidding it) is because your worst case current in that single wire is the sum of all currents in all motors. So if you have four 40A channels all sharing the same black wire, then you’d have to size the black wire to handle 160A, which would probably mean connecting it directly to the negative main power (battery) terminal.

I suppose if you had four 10A channels, you would be safe to use a single 12AWG black wire coming back to the PDH, but it’s still against the rules.

Even apart from the rules, you would have to run the same total cross-section of copper, so you would not really save anything in this case. You would likely wind up with less flex in the overall wiring.

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Even when you combine wires, you still need to have the same number of ground/return leads. Current flows out through the supply voltage wire and back through the return wire, so they should be the same size.

Robot rules require 4 pairs though, no changing that.

I agree with @troy_dietz that outside FRC combining ground returns back to a battery or DC generating system is a very common practice. In your car, the chassis of the car itself is the return path for the 12V DC current (unless it is the high voltage system in a hybrid or electric car). In small aircraft, the fuselage and other metallic structure is used as the ground return (typically 28V DC systems). This is generally done to cut down on wire bundle size, cost and weight. It also helps reliability as it cuts down on the number of connections within a given circuit and removes the possibility that a power wire could short circuit to the return wire due to chafing within the wire bundle.

However, in FRC, this is not allowed. And even combining them into a single return power wire (separate from the chassis) is not allowed. As others have said the rules for FRC are very clear on this point.

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Proper electrical practice is what you do when there aren’t rules or code for your application.

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