It is a bit dangerous to use a variac as a supply, It is an auto-transformer so it is not isolated from the incoming service, therefore it is possible to have both conductors going to the bridge have a potential referenced to ground, depending on wether the 120v socket/plug is wired correctly.
If you then ground one of the conductors you are using for the drive, the results are unpredictable.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.