What are PTC compressor start relays?
PTC compressor refrigerator relay
Back in the good ol' days, compressor start relays were the mechanical type. This was a metal plunger inside a tube with a heavy gauge copper coil wrapped around it. As current flows thru the copper coil, it creates a magnetic field that moved the metal slug up and took the compressor start winding out of the circuit thata way.
Grand Master Funk kdog has put together some examples showing common PTC compressor relays and a good ol' mechanical, non-PTC type relay; click the links above the pics for a larger view or to order the part:
Some common examples:
This is what a NON-PTC starter looks like:
Source: Whirlpool acronym