1. A failure determining apparatus for a motor driven compressor system having a compressor and a motor for driving the compressor, comprising: mass flow detecting means adapted to be coupled to said compressor for sensing fluid mass flow related to said compressor; compression ratio detecting means adapted to be coupled to said compressor for detecting a compression ratio of said compressor; reference value generating means coupled to at least one of said mass flow detecting means and said compression ratio detecting means, said reference value generating means generating at least one of variable upper and lower current reference values in response to an output of said at least one of mass flow detecting means and compression ratio detecting means, whereby said variable upper and lower current reference values are responsive to variable operating conditions of said compressor; and failure determining means coupled to said reference value generating means and adapted to be coupled to said motor for detecting motor current value above or below said variable upper or lower current reference value, whereby a failure in the compressor system is detected.