BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a motor controller, and more particular to a motor controller for accumulating electrical energy in a capacitor during deceleration of a motor and for supplying the electrical energy accumulated in the capacitor during acceleration of the motor.
2. Description of Related Art
[0002] A driving apparatus is known that includes a motor controller having a converter for receiving an AC voltage and performing AC-to-DC power conversion thereon, an inverter for receiving a DC voltage and performing DC-to-AC power conversion thereon, and a capacitor and a charging/discharging control circuit connected in parallel with a DC link between the converter and the inverter. Under the control of the charging/discharging control circuit, electrical energy is charged to the capacitor and the electrical energy accumulated in the capacitor is discharged.
[0003] In a driving apparatus of this type disclosed in
JP 2000-141440A , electrical energy is supplied from the capacitor to the DC link when a voltage across the capacitor is equal to or higher than a DC link voltage of the DC link. FIGS. 8a and 8b show a relation between motor speed, DC link voltage, and capacitor voltage in the prior art motor controller.
[0004] The above-described conventional motor controller operates to supply electrical energy from the capacitor to the DC link when the capacitor voltage is equal to or higher than the DC link voltage of the DC link. When the capacitor voltage is lower than the DC link voltage, electrical energy cannot be supplied from the capacitor to the DC link.
[0005] With the prior art motor controller, the capacitor can only be discharged up to the DC link voltage as shown in FIGS. 8a and 8b. Thus, energy able to be discharged by the capacitor is small. When a difference between the capacitor voltage and the DC link voltage becomes small, the response of current supply to load change is deteriorated.
[0006] US 5734258 discloses a bidirectional buck boost converter for regulating power flow between voltage sources.
SUMMARY OF THE INVENTION
[0007] The present invention provides a motor controller capable of effectively utilizing electrical energy accumulated in a capacitor and achieving a reduction in capacitance of the capacitor.
[0008] A motor controller of the present invention comprises: a converter that converts input AC power into DC power; an inverter that inverts the converted DC power into AC power; a DC link that connects the converter and the inverter; and a capacitor and a charging/discharging control circuit that are connected in parallel with the DC link so that electrical energy is supplied from the DC link to the capacitor and vice versa through the charging/discharging control circuit, wherein the charging/discharging control circuit includes a discharge circuit that performs a step-up operation of raising a voltage of the capacitor while the electrical energy charged in the capacitor is discharged, and wherein the step-down operation is switched to the step-up operation when a discharge current flowing from said capacitor during the step-down operation decreases to a given value such that the timing of switching from the step-down operation to the step-up operation is determined according to magnitude of the discharge current from the capacitor.
[0009] With the present invention, electrical energy can be supplied from the capacitor to the DC link even if the voltage of the capacitor is lower than the DC link voltage of the DC link, whereby electrical energy accumulated in the capacitor can effectively be utilized and the capacitance of the capacitor can be made small.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010]
FIG. 1 is a block diagram showing the essential part of an embodiment of a motor controller of this invention;
FIG. 2 is a view showing an example charging/discharging control circuit for use in the motor controller of this invention;
FIG. 3 is a view describing a first discharge operation of the charging/discharging control circuit in FIG. 2;
FIG. 4 is a view describing a second discharge operation of the charging/discharging control circuit in FIG. 2;
FIG. 5 is a view describing a charge operation of the charging/discharging control circuit in FIG. 2;
FIGS. 6a and 6b are views showing a relation between motor speed, DC link voltage, and capacitor voltage in the motor controller of this invention;
FIG. 7 is a view for describing an embodiment of this invention in which switching between step-down operation and step-up operation is automatically performed;
FIGS. 8a and 8b are views showing a relation between motor speed, DC link voltage, and capacitor voltage in a prior art motor controller.
DETAILED DESCRIPTION
[0011] FIG. 1 shows in block diagram the essential part of an embodiment of a motor controller of this invention. As shown in FIG. 1, the motor controller 1 is adapted to be supplied with AC voltage from a power source (three-phase power source). A converter 2 is for converting AC voltage into DC voltage. An inverter 3 is for converting the DC voltage into AC power of variable voltage and variable frequency and for supplying the AC power to a motor 4.
[0012] Smoothing capacitors 6 connected in parallel with and between the converter 2 and the inverter 3 are for smoothing DC voltage obtained by conversion by the converter 2 and for inputting the smoothened DC voltage to the inverter 3. A connection part by which the converter 2 is electrically connected with the inverter is called a DC link.
[0013] The converter 2 includes a bridge circuit comprised of power elements (such as for example, transistors) and diodes which are connected in inverse-parallel with the power elements. With the converter 2, three-phase AC power is full-wave rectified by the six diodes at power running, and the six power elements operate to divert regenerative power to the power source at power regeneration.
[0014] The inverter 3 includes a bridge circuit comprised of power elements (such as for example, transistors) and diodes which are connected in inverse-parallel with the power elements. The power elements are ON/OFF controlled by an inverter control circuit 5 to thereby convert the DC voltage produced by conversion by the converter 2 into AC voltage, and the resultant AC power is supplied to the motor 4.
[0015] A first electric current sensor, CT1 (9), is for measuring a value of electric current flowing into the inverter 3.
[0016] A capacitor 7 is connected in parallel with the smoothing capacitors 6 of the DC link via the charging/discharging control circuit 8. Under the control of the charging/discharging control circuit 8, the capacitor 7 supplies electrical energy to the inverter 3 and is charged with regenerative electrical energy supplied from the motor 4. The details of the charging/discharging control circuit 8 are described with reference to FIG. 2.
[0017] FIG. 2 shows in a block diagram the essential part of the example charging/discharging control circuit 8 according to the embodiment of this invention. As described in the embodiment of the motor controller in FIG. 1, the charging/discharging control circuit 8 is connected in parallel with the DC link of the motor controller 1. The charging/discharging control circuit 8 is connected with the capacitor 7 adapted to be charged with regenerative electrical energy supplied from the motor 4. The charging/discharging control circuit 8 includes a CT2 (20) as a second electric current sensor, voltage sensors 25a and 25b, a DC reactor 21, a SW1 (22) as a discharge control switch, a SW2 (23) as a charge control switch, a SW4 (24) as a step-up switch, and diodes 26a, 26b, 26c and 27. The switches SW1 (22), SW2 (23), and SW4 (24) are each implemented, for example, by an IGBT or the like. The voltage sensor 25a detects a DC link voltage Vdc as a voltage of the DC link, and the voltage sensor 25b detects a capacitor voltage Vc of the capacitor 7. The CT2 (20) or second current sensor detects charge and discharge currents of the charging/discharging control circuit 8.
[0018] Next, with reference to FIGS. 3 and 4, a discharge operation of the capacitor 7 performed under the control of the charging/discharging control circuit 8 in FIG. 2 is described, with the discharge operation divided into first and second discharge operations. The first discharge operation is shown in FIG. 3 , and the second discharge operation is shown in FIG. 4.
[0019] The first discharge operation is implemented in a case that electric current flows from the capacitor 7 into the DC link and the voltage Vc of the capacitor 7 is higher than the DC link voltage Vdc of the DC link. The first discharge operation is therefore a step-down operation in which the voltage of the capacitor 7 is lowered while the electrical energy charged in the capacitor 7 is discharged.
[0020] The second discharge operation is implemented in a case that electric current flows from the capacitor 7 into the DC link and the voltage Vc of the capacitor 7 is lower than the DC link voltage Vdc of the DC link. The second discharge operation is therefore a step-up operation in which the voltage of the capacitor 7 is raised while the electrical energy charged in the capacitor 7 is discharged.
[0021] The following is a description of the first discharge operation (step-down operation) (see FIG. 3).
[0022] When the SW1 (22) is made ON and the SW2 (23) and the SW4 (24) are kept in an OFF state, electric current flows from the capacitor 7 to the DC link along a path shown at W1. At that time, the discharge current increases.
[0023] When the SW1 (22) is made OFF, electric current flows into the diode 27 due to the presence of magnetic energy (electrical energy) accumulated in the DC reactor 21. At that time, the discharge current decreases.
[0024] When the SW1 (22) is made ON, the discharge current flowing from the capacitor 7 increases. When the SW1 (22) is made OFF, the discharge current decreases. Thus, the discharge current flowing from the capacitor 7 can be controlled by controlling the ON and OFF of the discharge control switch SW1 (22).
[0025] The following is a description of the second discharge operation (step-up operation) (see FIG. 4).
[0026] When the SW1 (22) and the SW4 (24) are made ON and the SW2 (23) is kept in an OFF state, electric current flows from the capacitor 7 to the DC reactor 21 along a path shown at W2. At that time, the discharge current increases.
[0027] When the SW4 is made OFF with the SW1 kept ON, electric current flows to the DC link along a path shown at W3 due to the presence of magnetic energy (electrical energy) accumulated in the DC reactor 21. At that time, the discharge current decreases.
[0028] When the SW4 (24) is made ON with the SW1 (22) kept ON, the discharge current from the capacitor 7 increases. When the SW4 (24) is made OFF, the discharge current decreases. In other words, the discharge current from the capacitor 7 can be controlled by controlling the ON and OFF of the step-up switch SW4 (24) with the discharge control switch SW1 (22) kept ON.
[0029] Next, a charge operation is described.
[0030] With reference to FIG. 5, an operation for charging the capacitor 7 by the charging/discharging control circuit 8 is described. In the following, the charge operation is described for a case where electric current flows from the DC link into the capacitor 7, i.e., the voltage Vc of the capacitor 7 is lower than the DC link voltage Vdc of the DC link.
[0031] When the SW2 (23) is made ON with the SW1 (22) and the SW4 (24) kept in an OFF state, the charge current flows into the capacitor 7 along a path shown at W4 (see FIG. 5). At that time, the charge current increases.
[0032] When the SW2 (23) is made OFF, the current flows into the diode 26b along a path W5 (see FIG. 5) due to the presence of magnetic energy (electrical energy) accumulated in the DC reactor 21. At that time, the charge current decreases.
[0033] The charge current supplied to the capacitor 7 can be controlled by controlling the ON and OFF of the charge control switch SW2 (23). The SW1 and the SW4 are in the OFF state at that time.
[0034] With reference to FIGS. 6a and 6b, a description is given of an operation by which electrical energy is able to be supplied from the capacitor 7 to the DC link by the charging/discharging control circuit 8 in FIG. 2 irrespective of whether the capacitor voltage VC of the capacitor 7 is equal to or higher than the DC link voltage Vdc of the DC link or lower than the DC link voltage Vdc.
[0035] FIGS. 6a and 6b show a relation between motor speed, DC link voltage Vdc, capacitor voltage Vc, step-down operation (first discharge operation), and step-up operation (second discharge operation) in the motor controller 1. FIG. 6a shows acceleration and deceleration of the motor drivingly controlled by the motor controller 1. FIG. 6b shows a relation between DC link voltage Vdc and capacitor voltage Vc at the motor acceleration and deceleration in FIG. 6a .
[0036] When the voltage Vc of the capacitor 7 is higher than the voltage Vdc of the DC link, the discharge from the capacitor 7 to the DC link is performed by the step-down operation (first discharge operation).
[0037] The electrical energy supply from the capacitor 7 to the DC link is continued by the step-up operation (second discharge operation) as shown in FIG. 6b , even when the capacitor voltage Vc of the capacitor 7 becomes equal to or lower than the DC link voltage Vdc of the DC link. The energy supply is thus made greater than in the prior art by an amount of electrical energy supply from the capacitor 7 to the DC link by the step-up operation.
[0038] In other words, the capacitance of the capacitor required to supply the same regenerative electrical energy can be decreased as compared to the prior art.
[0039] In FIG. 6b, Q1 and Q2 represent discharge energy but do not directly indicate electrical energy since FIG. 6b is a graph simply showing a relation between voltage and time. An amount of electrical energy can however be calculated in accordance with the relation between voltage and time shown in the graph.
[0040] FIG. 7 is for describing an embodiment of this invention in which switching between the step-down operation and the step-up operation (first and second discharge operations) is automatically carried out. When the discharge current flowing from the capacitor 7 becomes equal to or less than a given value during the step-down operation, switching to the step-up operation is automatically carried out to avoid a reduction in discharge current and deteriorated response in current control caused when a difference therebetween becomes small in the step-down operation. As shown in FIG. 7, a timing P3 of switching from the step-down operation to the step-up operation is determined according to magnitude of the discharge current from the capacitor 7.