How to Select an SSR for Motor Switching

If it is a motor, AC-51 would not apply, that is for resistive loads. It would be AC-53.

So the difference is that AC-3 would typically be associated with electro-mechanical contactors and AC-53 would be specific to Solid State switching devices. You can select an SSR device based on the AC-3 ratings, but the AC-53 ratings are more comprehensive because they take overload and switching duty into account, i.e. on-time vs off-time, which AC-3 makes no provisions for in testing (I believe).