Heat pump devices exist that supply a high-frequency low voltage to a compressor during a shutdown during heating in order to improve the rising speed of the air conditioner when heating is started (for example, see Patent Literature 1). A similar technique is used in a heat pump device that supplies a single-phase AC voltage having a higher frequency than that at the time of a normal operation to a compressor when it is detected that the temperature of the air conditioner's surroundings becomes low (for example, see Patent Literature 2).
Moreover, in order to prevent the refrigerant stagnation phenomenon from occurring, a heat pump device exists that generates, as driving signals for a compressor motor, signals to be output with a predetermined static phase angle in the PWM (Pulse Width Modulation) output in a two-phase modulation system during the locked energization for preheating the compressor (for example, see Patent Literature 3).
In general, as a stagnation prevention measure for a compressor, during a shutdown of the compressor, the inside of the compressor is heated by a heater or locked energization (a voltage with which a compressor motor is not driven) is applied to the winding of the compressor motor by an inverter to heat the inside of the compressor. However, there is a problem in that electric power is always consumed for heating the inside of the compressor during a shutdown and the standby power increases. Therefore, in heating control for the compressor in the past, the outside air temperature is detected by an outside air temperature detector and, when the detected outside air temperature is equal to or higher than a predetermined value, the heating by the locked energization or the heater is stopped to reduce the power consumption (for example, see Patent Literature 4).
The system explained above is a system for predicting the refrigerant state in the compressor by detecting the outside air temperature or temperatures in other parts, and there is a system for directly detecting the refrigerant state by setting, in the compressor, a sensor that detects the refrigerant state. This system includes a heater that heats the compressor and an insulation resistance sensor that detects an electric resistance of a refrigerant and refrigerant oil. When the insulation resistance value detected by the sensor is equal to or smaller than a predetermined value, the heater is energized and the refrigerant oil is heated to prevent two-phase separation of the refrigerant. When the insulation resistance value is equal to or larger than the predetermined value, the energization to the heater is stopped to attain a reduction in power consumption (for example, see Patent Literature 5).