Line choke or input choke is nothing but like a transformer. Its basic function is to protect drive from surge voltage or over voltage spikes in the Mains input supply to VFD. Ac drive works on switching principle hence it produces harmonics and disturbs mains Line. Line choke helps in controlling the harmonics produced by AC drives or VFD. Line chokes are recommended to installed right before VFD input. It is recommended to use line choke or input choke for every ac drive installation. This not only protect your VFD but also increase its life.