Inverter

1. An inverter adapted for supplying alternating currents to an electric motor having coils forming a plurality of phases to drive the electric motor, the inverter comprising:

an inverter circuit having a plurality of switching elements in a bridge connection, wherein the inverter circuit has a pair of input terminals, which is configured to be connected to a direct-current power source, and a plurality of output terminals, which are configured to be connected to the coils of the respective phases of the electric motor;

a capacitor, which is connected to the input terminals of the inverter circuit;

a temperature detecting section, which detects a temperature of the capacitor;

a warm-up control section, wherein, in a state in which the electric motor is not operating, the warm-up control section controls the switching elements of the inverter circuit to perform warm-up control, in which direct currents are supplied to the coils of the respective phases of the electric motor, when the temperature of the capacitor detected by the temperature detecting section is lower than a prescribed temperature; and

a switching control section, wherein, when the warm-up control is switched to a normal operation control, in which alternating currents are supplied to the coils of the respective phases of the electric motor, the switching control section sets initial phases of the alternating currents such that the relationship between the values of the alternating currents supplied to the coils of the electric motor in the normal operation control matches with the relationship between the values of the direct currents that have been supplied to the corresponding coils of the electric motor in the warm-up control.

2. The inverter according to claim 1 , further comprising a normal operation control starting section, wherein, if a command for activating the electric motor is generated during the warm-up control, the normal operation control starting section stops the warm-up control and switches to the normal operation control.

3. The inverter according to claim 1 , further comprising:

a peripheral device temperature detecting section for detecting a temperature of a peripheral device other than the capacitor in the inverter; and

a warm-up control stopping section, wherein, when the temperature of the peripheral device of the capacitor detected by the peripheral device temperature detecting section exceeds a threshold value during the warm-up control, the warm-up control stopping section stops the warm-up control.

4. The inverter according to claim 1 , further comprising an output limiting section, wherein, if the temperature of the capacitor detected by the temperature detection section when the warm-up control is stopped has not reached a target temperature, the output limiting section limits output currents supplied to the coils of the respective phases of the electric motor in the subsequent normal operation control based on the temperature of the capacitor detected by the temperature detecting section.

5. The inverter according to claim 1 , wherein, when switching from the warm-up control to the normal operation control, the switching control section provides a suspended period, in which no current is supplied to the coils of the electric motor.

6. The inverter according to claim 1 , wherein, when supplying direct currents to the coils of the respective phases of the electric motor, the warm-up control section controls the switching elements of the inverter circuit such that direct currents that contain no torque current component but only contain an excitation current component are supplied to the electric motor.

7. The inverter according to claim 1 , wherein, at switching to the normal operation control, the values of the alternating currents supplied to the coils of the electric motor are equal to the values of the direct currents that have been supplied to the corresponding coils of the electric motor in the warm-up control.

8. The inverter according to claim 1 , wherein the relationship between the current values is a magnitude relationship.

9. An inverter adapted for supplying alternating currents to an electric motor having coils forming a plurality of phases to drive the electric motor, the inverter comprising:

an inverter circuit having a plurality of switching elements in a bridge connection, wherein the inverter circuit has a pair of input terminals, which is configured to be connected to a direct-current power source, and a plurality of output terminals, which are configured to be connected to the coils of the respective phases of the electric motor;

a capacitor, which is connected to the pair of input terminals of the inverter circuit;

a temperature detecting section, which detects a temperature of the capacitor;

a warm-up control section, wherein, in a state in which the electric motor is not operating, the warm-up control section controls the switching elements of the inverter circuit to perform warm-up control, in which direct currents are supplied to the coils of the respective phases of the electric motor, when the temperature of the capacitor detected by the temperature detecting section is lower than a prescribed temperature; and

a switching control section, wherein, when the warm-up control is switched to a normal operation control, in which alternating currents are supplied to the coils of the respective phases of the electric motor, the switching control section sets initial phases of the alternating currents supplied to the respective coils of the electrical motor in the normal operation control in line with the electrical degrees of the direct currents that have been supplied to the respective coils of the electrical motor in the warm-up control.