The purpose of this paper is an overview of the history of protection systems for hermetic motor-compressor in the integral horsepower sizes, touching on motor developments, fault conditions found in operating systems, and action of protection system. The intent is to familiarize the engineer who has not had an opportunity to work closely with this aspect with the complexity of operation and protection of such a system. The experience described in this paper is with the low side case, internally spring mounted, and reciprocating motor-compressor. This is not intended to be a cookbook method of system design or application.

The term "protection system" can be thought of as an over-all system using several components acting independently or a single integrated system. Protection for as many or as few fault conditions as can possibly exist can be considered in a design depending on economics, safety, specifications, approval agency requirements, etc. For the purpose of discussion, some important fault conditions which may be found in air source heat pump operation are listed in this paper.