Refrigerant circulation system

A refrigerant circulation system configured to circulate refrigerant in which CO2 contains oil, the system comprising:

• a compressor configured to compress the refrigerant;
• a motor provided with a rotor and a stator, a rotation shaft coupled to the rotor, and slide bearings configured to be lubricated using liquefied refrigerant compressed by the compressor and support the rotation shaft;
• an oil tank configured to store the oil;
• a first passage through which the refrigerant supplied from the compressor flows, a second passage through which the oil supplied from the oil tank flows, and a third passage communicating the first passage with the second passage and configured to supply to the slide bearings of the motor the refrigerant from the first passage mixed with the oil from the second passage;
• at least one flow regulating valve provided to at least one of the first passage and the second passage;
• a motor rotational speed sensor configured to detect a rotational speed of the motor;
• and
• a controller configured to control the at least one flow regulating valve,
• wherein the controller controls the at least one flow regulating valve according to the rotational speed detected by the motor rotational speed sensor to change a proportion of the oil in the refrigerant supplied from the third passage to the slide bearings.