Teams ask which current limit to set and usually get told a number. The number is not the useful part. CTRE and REV name and model their limits differently, which is most of the confusion, so this covers both.
Two limits, doing different jobs.
Stator current is the current in the motor windings. It is proportional to torque. A stator limit caps how hard the motor can push and how hot it gets.
Supply current is the current drawn from the battery. A supply limit protects the battery, the breakers and the voltage the whole robot sees.
The part that confuses people: a stator limit also limits supply current. Supply current is stator current times duty cycle, so it can never exceed the stator limit and is usually well below it. A motor at 50 percent output with 80 A of stator current is drawing 40 A from the battery. This is why CTRE says you often do not need both limits.
Find it by testing, not by guessing.
Anything above the slip point is current you cannot use, it just spins the wheels. Setting it too low costs acceleration, so do not go far below.
If the robot still browns out or still trips the main breaker after a sensible stator limit, then add a supply limit. Check the electrical first, because brownouts are more often a tired battery or a bad crimp than a current limit:
Then lower the supply limit on one mechanism at a time until performance suffers, and back off slightly. A drivetrain needs far more headroom than an intake.
CTRE's own worked example for a swerve robot:
| Mechanism | Stator | Supply |
|---|---|---|
| 4 drive motors | 120 A | 70 A |
| 4 steer motors | 60 A | none needed |
| Elevator | 80 A | 30 A |
| Intake | 20 A | none needed |
That is a theoretical peak near 570 A, which never happens, because the stator limits stop every motor from peaking at once. Four drive motors accelerating together is about 280 A for well under a second.
A 40 A breaker carries more than 40 A for a while before it opens. That is why a robot can pull 70 A per drive motor in short bursts and be fine, then trip the main breaker the first time it gets into a pushing match and holds. Check the trip curve in the breaker datasheet rather than assuming the rating is a hard ceiling. A binding swerve module makes this much easier to hit, so spin each module by hand before blaming the software.
REV does not split things into stator and supply. There is one smart current limit, measured in motor current, and it has three parts:
| Parameter | Default | What it does |
|---|---|---|
| Stall limit | 80 A | The limit at stall, and at any speed below the RPM parameter |
| Free limit | 20 A | The limit at free speed |
| RPM | 10000 | Where the limit starts ramping down from the stall value toward the free value. Set it above free speed to disable the ramp |
The stall limit is the one that matters for a main breaker. It defaults to 80 A, which is the REV equivalent of the Kraken 70 A trap: four drive motors in a pushing match sit near their stall limit at once, and 4 x 80 A is well past a 120 A main breaker.
Because the smart current limit is motor current, it behaves like a stator limit. It caps torque, which caps battery draw, and it is the right place to start. There is no separate supply limit to add afterwards, so if the stall limit alone does not fix it, look at the battery, the wiring or a binding mechanism.
Separately there is a current chop limit, default 115 A, maximum 125 A. That is hardware protection in the half bridge: it shuts the motor driver off for a set number of PWM cycles when current goes past it, and the bridge brakes during that time. It is blunt and it is not a substitute for setting the smart current limit. Leave it alone unless you know why you are changing it.
Use the REV Hardware Client, or configure it when the SPARK is set up in code. Either way, read the value back off the device afterwards, the same as with Tuner X. A limit that was put in a config object but never applied is a common way to think you have a limit and not have one.
The roboRIO disables all actuators to avoid a full reboot. roboRIO 1 trips at 6.3 V. roboRIO 2 trips at 6.75 V by default. Stator limits are the most effective fix, because supply current peaks hardest at the start of acceleration and that is exactly where a stator limit bites.