Power conversion device, motor drive unit, and refrigeration cycle apparatus
A power conversion device includes a rectification unit that rectifies first alternating current (AC) power supplied from a commercial power supply, a capacitor connected to an output end of the rectification unit, an inverter that converts power output from the rectification unit and from the capacitor into second AC power, and outputs the second AC power to a load including a motor, which inverter is connected to both ends of the capacitor, and a control unit that controls operation of the inverter to output the second AC power from the inverter to the load to reduce current flowing to the capacitor, which second AC power includes a pulsation that depends on a pulsation of power flowing from the rectification unit to the capacitor. No discharge circuit and no overvoltage protection circuit are provided for the capacitor.
1. A power conversion device comprising:
a rectification unit that rectifies first alternating current power supplied from a commercial power supply;
a capacitor connected to an output end of the rectification unit;
an inverter that converts power output from the rectification unit and from the capacitor into second alternating current power, and outputs the second alternating current power, to a load including a motor, the inverter being connected to both ends of the capacitor; and
a control unit that controls operation of only the inverter to output the second alternating current power from the inverter to the load to reduce current flowing to the capacitor, the second alternating current power including a pulsation that depends on a pulsation of power flowing from the rectification unit to the capacitor, wherein
no discharge circuit and no overvoltage protection circuit are provided for the capacitor,
the capacitor has a capacitance greater than or equal to a capacitance of the capacitor that would be set when an overvoltage protection circuit is connected to the capacitor, and
the capacitance of the capacitor is determined by a value calculated using impedance of a reactor disposed in the power conversion device, system impedance, a maximum current value of the commercial power supply, an inductance component for one phase of the motor, a maximum current value of the motor, a withstand voltage of an element to which a voltage from the capacitor is applied, and a maximum voltage of the capacitor in a stationary state.
2. The power conversion device according to claim 1 , wherein
the capacitance of the capacitor is further limited by a system voltage of the commercial power supply when the inverter is out of operation, and by an induced voltage of the motor.
3. The power conversion device according to claim 1 , wherein
the capacitor has a capacitance less than a capacitance of the capacitor that would be set when first control is not performed, the first control being control of operation of the inverter performed by the control unit to output, from the inverter to the load, the second alternating current power including a pulsation that depends on a pulsation of power flowing from the rectification unit to the capacitor.
4. The power conversion device according to claim 3 , wherein
the capacitance of the capacitor is determined by a value calculated using a frequency of pulsation of current of the capacitor, the frequency being twice a frequency of the commercial power supply, an allowable ripple voltage of the capacitor at the frequency that is twice the frequency of the commercial power supply, a capacitor current of the capacitor at the frequency that is twice the frequency of the commercial power supply when the control unit does not perform the first control, and an input current pulsation of the inverter at the frequency that is twice the frequency of the commercial power supply when the control unit performs the first control.
5. The power conversion device according to claim 1 , wherein
a capacitor current at a frequency component that is twice the frequency of the commercial power supply, of the current flowing to the capacitor, is less than or equal to a first capacitor current at a frequency component that is twice a switching frequency of a switching element included in the inverter.
6. The power conversion device according to claim 5 , wherein
the first capacitor current includes a current component caused by rotation of the motor.
7. The power conversion device according to claim 5 , comprising:
a booster unit that boosts a voltage of the first alternating current power, or in place of the rectification unit, a rectification-boost unit that rectifies the first alternating current power supplied from the commercial power supply, and boosts the voltage of the first alternating current power, wherein
the capacitor current at the frequency component that is twice the frequency of the commercial power supply, of the current flowing to the capacitor, is less than or equal to a second capacitor current at a frequency component that is twice a switching frequency of a switching element included in the booster unit or in the rectification-boost unit.
8. The power conversion device according to claim 7 , wherein
the second capacitor current includes a current component caused by rotation of the motor.
9. The power conversion device according to claim 1 , wherein
the capacitor is an electrolytic capacitor or a film capacitor.
10. The power conversion device according to claim 1 , wherein
a voltage ripple appearing on the capacitor has a maximum value thereof less than twice a minimum value thereof.
11. The power conversion device according to claim 1 , wherein
the rectification unit performs full-wave rectification, and a voltage appearing on the capacitor does not have a waveform shape of a full-wave rectified voltage of the commercial power supply.
12. The power conversion device according to claim 1 , wherein
the discharge circuit includes an active element and a resistor, and switches a connect-disconnect status of the resistor for the capacitor by turning on or off of the active element.
13. The power conversion device according to claim 1 , wherein
the overvoltage protection circuit protects a device to prevent a voltage of the capacitor from increasing by a certain voltage or more, the overvoltage protection circuit not being a snubber circuit that protects a switching element from a surge voltage generated upon switching of a switching element.
14. A motor drive unit comprising the power conversion device according to claim 1 .
15. A refrigeration cycle apparatus comprising the power conversion device according to claim 1 .