Don't Use the Seated Ab Rotation Machine!
What's the worst possible situation for the longevity and health of the lumbar spine? Answer: Simultaneous flexion, rotation, and side bending, especially under loading. The ab rotation machine does all of these things. When cranking out rep after rep on it you're actually frying the neural system that would normally stabilize the spine properly. You then lose the dynamic stabilization your spinal segments and core need to protect themselves against the external load.
Do the Pallof Press Instead
Want to target the entire core, including the shoulder and hip complexes in addition to the abdominal wall? Give the Pallof press a shot. This anti-rotation movement will challenge your static and dynamic stability of the "pillar" all at once.
Think it looks too easy? Load it up and don't budge an inch except for controlling your arms in a smoothly coordinated manner. Press the band straight out from your chest, hold briefly, return to the starting position, and repeat. The internal tension you'll be able to achieve will sky-rocket your heart rate and truly train your core musculature.
You can use a cable machine as well. Brace your core tight and resist the rotation that the cable is trying to apply to you. Increase the challenge by either increasing the weight or bringing your feet closer together. Do three sets of thirty seconds.