Battery charging circuit and charging method
A charging circuit is provided that charges a battery for supplying power to a motor drive system that includes a three-phase motor and a three-phase inverter for controlling the three-phase motor. The three-phase inverter includes first to third sets of switching elements. Each set corresponds to one of the three phases. The charging circuit includes a single-phase output transformer, a rectifier circuit, a line, and a controller. The single-phase output transformer includes a secondary side output section having a first terminal and a second terminal. The rectifier circuit is connected in parallel with the three-phase inverter and the battery. The rectifier circuit is also connected to the first terminal of the secondary side output section. The line connects a connecting point between the first set of the switching elements in the three-phase inverter with the second terminal of the secondary side output section. The controller performs on-off control of the first to third sets of the switching elements. During charging of the battery, the controller maintains the first set of the switching elements in an OFF state, and performs the on-off control of at least one set of the second and third sets of the switching elements.
1. A charging circuit for charging a battery that supplies power to a motor drive system including a three-phase motor and a three-phase inverter for controlling the three-phase motor, the three-phase inverter having first to third sets of switching elements, each set corresponding to one of the three phases, the charging circuit comprising:
a single-phase output transformer having a secondary side output section that has a first terminal and a second terminal;
a rectifier circuit that is connected in parallel with the three-phase inverter and the battery, the rectifier circuit being connected to the first terminal of the secondary side output section;
a line connecting a connecting point between the first set of the switching elements in the three-phase inverter with the second terminal of the secondary side output section; and
a controller that performs on-off control of the first to third sets of the switching elements,
wherein, during charging of the battery, the controller maintains the first set of the switching elements in an OFF state, and performs the on-off control of at least one set of the second and third sets of the switching elements.
2. The charging circuit according to claim 1 , wherein, during charging of the battery, the controller performs the on-off control of at least one set of the second and third sets of the switching elements, so as to match the current waveform at the secondary side output section with the voltage waveform at the secondary side output section.
3. The charging circuit according to claim 1 , wherein the rectifier circuit includes a series circuit and switching elements, the series circuit being divided into two sections and including a plurality of diodes that are connected in series with each other such that the cathodes are arranged on the positive side of the rectifier circuit, and each switching element being connected in parallel with one of the two sections of the series circuit, and
wherein the controller performs the on-off control of one set of the switching elements that corresponds to one phase of the three-phase inverter and the switching elements of the rectifier circuit, thereby supplying the power of the battery to the primary side of the single-phase output transformer.
4. The charging circuit according to claim 1 , wherein: the motor drive system is a motor drive system of a forklift that includes a vehicle-driving motor and a cargo-handling motor; the single-phase output transformer is a Scott connection transformer that includes first and second single-phase output sections; a vehicle-driving inverter is connected to the first single-phase output section; and a cargo-handling inverter is connected to the second phase output section.
5. The charging circuit according to claim 1 , wherein the single-phase output transformer is configured to input a single-phase alternating current.
6. The charging circuit according to claim 1 , wherein each switching element is an insulated-gate bipolar transistor, a diode being connected in antiparallel with each transistor.
7. A method for charging a battery that supplies power to a motor drive system including a three-phase motor and a three-phase inverter for controlling the three-phase motor, the three-phase inverter having first to third sets of switching elements, each set corresponding to one of the three phases, the method comprising:
providing a single-phase output transformer having a secondary side output section that has a first terminal and a second terminal;
connecting a rectifier circuit in parallel with the three-phase inverter and the battery;
connecting the rectifier circuit with the first terminal of the secondary side output section;
connecting a connecting point between the first set of the switching elements in the three-phase inverter with the second terminal of the secondary side output section; and
during charging of the battery, maintaining the first set of the switching elements in an OFF state, and performing the on-off control of at least one set of the second and third sets of the switching elements.