Effective Core Workouts (Machine vs Free Weights Ab Exercises)

I’ve decided to start training my core last week but I noticed sumakit lower back ko. Obviously, it was due to improper form. Ayoko nang i-risk ulit yung lower back ko to check what are its causes but I’m assuming the culprits are the crunches.

I have a few questions which may sound novice:

1. Do you have core workouts that have lesser chances to hit the back? (Note: My back did not hurt after I did standing marches variations)

2. Are machines effective? I see various answers din about dito. My current gym has no machines specifically for abs. While I love the environment, if I want more equipments, I need to switch na.

3. Do you train abs everyday?

Thank you!