Cryocooler and control method of cryocooler

A cryocooler comprising:

• a compressor;
• an expander that includes a motor and is driven by the motor;
• an inverter that controls an operation frequency of the motor;
• a high pressure line that connects the compressor to the expander such that a high pressure working gas is supplied from the compressor to the expander;
• a low pressure line that connects the compressor to the expander such that a low pressure working gas is collected from the expander to the compressor;
• a pressure sensor that is configured to measure a pressure of the high pressure line and a pressure of the low pressure line or to measure a differential pressure between the high pressure line and the low pressure line;
• a controller that compares the differential pressure between the high pressure line and the low pressure line to a target pressure based on an output from the pressure sensor, and the controller controls the inverter such that the operation frequency of the motor is increased when the differential pressure exceeds the target pressure,
• a bypass line that connects the high pressure line to the low pressure line;
• and
• relief valve that is provided in a bypass line and opens when the differential pressure, which is equal to or higher than a set pressure, acts between an inlet and an outlet of the relief valve,
• wherein the target pressure is smaller than the set pressure.