A centrifugal compressor adapted to be used in a chiller, the centrifugal compressor comprising: a casing having an inlet portion and an outlet portion; an inlet guide vane disposed in the inlet portion; an impeller disposed downstream of the inlet guide vane, the impeller being attached to a shaft rotatable about a rotation axis; a motor arranged and configured to rotate the shaft in order to rotate the impeller; a liquid injection passage arranged and configured to inject liquid refrigerant; a diffuser disposed in the outlet portion downstream from the impeller with an outlet port of the liquid injection passage being disposed between the impeller and the diffuser such that the liquid injection passage injects liquid refrigerant into an area between the impeller and the diffuser; and a controller programmed to control an amount of liquid refrigerant injected into the area between the impeller and the diffuser, wherein the liquid injection passage includes an at least one valve disposed therein, the valve being controlled by the controller to control the amount of liquid refrigerant injected into the area between the impeller and the diffuser, wherein the controller is further programmed to control the valve so as to inject liquid refrigerant into the area between the impeller and the diffuser when the centrifugal compressor runs with less than predetermined capacity, wherein the controller is further programmed to determine that the centrifugal compressor runs with less than predetermined capacity based on a rotational speed of the motor and a position of the inlet guide vane, wherein the controller is further programmed to stop injecting liquid refrigerant into the area between the impeller and the diffuser when the position of the inlet guide vane moves beyond a predetermined position value and a predetermined amount of time passes, or the controller is further programmed to stop injecting liquid refrigerant into the area between the impeller and the diffuser when the rotational speed of the motor exceeds a predetermined value. a casing having an inlet portion and an outlet portion; an inlet guide vane disposed in the inlet portion; an impeller disposed downstream of the inlet guide vane, the impeller being attached to a shaft rotatable about a rotation axis; a motor arranged and configured to rotate the shaft in order to rotate the impeller; a liquid injection passage arranged and configured to inject liquid refrigerant; a diffuser disposed in the outlet portion downstream from the impeller with an outlet port of the liquid injection passage being disposed between the impeller and the diffuser such that the liquid injection passage injects liquid refrigerant into an area between the impeller and the diffuser; and a controller programmed to control an amount of liquid refrigerant injected into the area between the impeller and the diffuser, wherein the liquid injection passage includes an at least one valve disposed therein, the valve being controlled by the controller to control the amount of liquid refrigerant injected into the area between the impeller and the diffuser, wherein the controller is further programmed to control the valve so as to inject liquid refrigerant into the area between the impeller and the diffuser when the centrifugal compressor runs with less than predetermined capacity, wherein the controller is further programmed to determine that the centrifugal compressor runs with less than predetermined capacity based on a rotational speed of the motor and a position of the inlet guide vane, wherein the controller is further programmed to stop injecting liquid refrigerant into the area between the impeller and the diffuser when the position of the inlet guide vane moves beyond a predetermined position value and a predetermined amount of time passes, or the controller is further programmed to stop injecting liquid refrigerant into the area between the impeller and the diffuser when the rotational speed of the motor exceeds a predetermined value.