Design of Variac transformers

MOST textbooks on transformers mention in a few concluding paragraphs that for certain applications special transformers can be used to good advantage. Foremost among these is the autotransformer which offers very important advantages if the ratio of input to output voltage is close to unity. Other special transformers mentioned are small instrument transformers, constant-current transformers, and voltage regulators. Voltage regulators are ordinarily divided into several groups: the automatic magnetic-flux-type regulator without moving parts, the induction regulator with one of its windings mounted so that it can be turned into different positions with respect to the other windings, and the tap-changing type, which requires special precautions if voltage has to be changed under load.