Find a toroidal transformer of sufficient VA rating to handle your burden. Loop a few turns of heavy cable through the core, and leave the regular secondary open-circuit. Connect a Variac to the primary winding.
This basically what Bill has described, other than a toroidal transformer has a primary of known characteristics whereas a CT will require some guesswork and/or calculation to enable you to use it effectively.
I thought in use a variac with welding machine transformer, because I need more VA. The secondary transformer will be short circuit. Will I have current precision, just moving variac?
Some variac based relay test sets have provisions for inserting a series resistance in the primary. The result is more precision in setting the desired current.
