Since many real-life missions are limited by the amount of fuel they carry to perform station-keeping, it's pretty common to use magnetic torque rods to bleed off the momentum that the wheels build up. The wheels are like a bucket for momentum, the torque rods are like a tiny little drain. As long as you're not putting too much in the bucket.... it never overflows. They system operates by: 1.) Spacecraft checks reaction wheel speeds. Of note, many spacecraft have more than 3 wheels and they're not necessarily aligned to the body principal axes. 2.) Spacecraft calculates total stored momentum in each axis. Depending on the specific design, it decides to "unload" the momentum based on being above thresholds for a specific wheel or above the stored momentum in that axis. 3.) Spacecraft calculates the current to apply to each torque rod based on the momentum to unload, the local vector value of the earth's magnetic field, and the spacecraft orientation in the field. IRL, your spacecraft accumulates momentum through silly things like solar radiation pressure and gravity gradients across the body of the spacecraft.
