I give a short introduction to the differences between autotransformers, variable autotransformers and variable transformers. I show my collection of 7 variac transformer / variotransformer in the range from 1x260V @ 2A to 3x400V @ 25A along with a 3x400V 10kVA isolation transformer.
3x400V @ 13A Variac demonstration with data monitor: https://www.youtube.com/watch?v=PqeLLLZTBb8
You might wonder what is a variac, it is a brand name of autotransformers. Autotransformers are not galvanic isolating between a primary and secondary coil. The autotransformer is a single coil transformer, where the midpoint can be adjusted and thus vary the output voltage. Some known brands are Powerstat variable autotransformer and autotransformador.