Original poster: stanmoore@mindspring.com When reading posts on the sync spark gap, I see references to using a variac to control the supply voltage to the motor? Is this correct? After you modify a motor to make it synchronous, do you then also control the supply voltage to achieve sync speed? I am planning make an 1800 rpm sync motor. Also are variacs used on the supply to a NST, say a 15/30? Thanks, Stan