Multilevel inverter and motor system

Abstract

To stabilize an output voltage.SOLUTION: A multilevel inverter 1 is a circuit having input terminals IN1 and IN2 to which a DC voltage is applied, capacitors CH and CL connected in series between the input terminals IN1 and IN2, transistors QHU, QHD, QLU, and QLD connected in series between the input terminals IN1 and IN2, and output terminals OUTu, OUTv, and OUTw. The multilevel inverter 1 includes switching circuits SWu, SWv, and SWw for generating potentials of a plurality of levels including a potential of a neutral point PO between the capacitor CH and the capacitor CL and output the potentials of the plurality of levels from the output terminals OUTu, OUTv, and OUTw, a cooling system 7H connected to both ends of the capacitor CH, and a control circuit 9 for controlling current consumption of the cooling system 7H. The control circuit 9 performs control to increase and decrease the current consumption of the cooling system 7H in accordance with a comparison result of a voltage of one of the capacitors CH and CL with a reference voltage.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a multilevel inverter and an electric motor system. [Background technology]

[0002] Conventionally, multilevel inverters have been used as inverters that convert a DC voltage into a voltage of multiple levels and output the voltage (see, for example, Patent Document 1 and Patent Document 2 below). By using such a multilevel inverter, it is possible to reduce voltage fluctuations, current ripples, and noise compared to a two-level inverter. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2008-161003 A [Patent Document 2] JP 2002-58256 A Summary of the Invention [Problem to be solved by the invention]

[0004] In conventional multilevel inverters, the neutral point potential may fluctuate depending on the load connected to the output terminal. When the neutral point potential fluctuates, the output voltage also fluctuates, making it impossible to drive the load stably.

[0005] Therefore, the present disclosure has been made in consideration of the above-mentioned problems, and has an object to provide a multilevel inverter and a motor system capable of stabilizing an output voltage. [Means for solving the problem]

[0006] In order to solve the above problems, a multilevel inverter according to one aspect of the present disclosure is a circuit having two input terminals to which a DC voltage is applied, a first capacitor and a second capacitor connected in series between the two input terminals, a plurality of switching elements connected in series between the two input terminals, and an output terminal, the multilevel inverter comprising: a switching circuit that generates a multilevel potential including a potential at a midpoint between the first capacitor and the second capacitor and outputs the multilevel potential from the output terminal; a load connected across the first capacitor; and a control circuit that controls the current consumption of the load to increase or decrease depending on a result of comparing either the voltage of the first capacitor or the voltage of the second capacitor with a reference voltage.

[0007] Alternatively, an electric motor system according to another aspect of the present disclosure includes the multilevel inverter according to the above aspect, and an electric motor connected to an output terminal of the multilevel inverter.

[0008] According to the one or other aspect, a change in the voltage of either the first capacitor or the second capacitor is detected, and the consumption current discharged from the first capacitor to the load can be increased or decreased according to the detection result. This makes it possible to stabilize the potential at the midpoint between the first capacitor and the second capacitor, and stabilize the multiple levels of potential output from the switching circuit. As a result, it becomes possible to stabilize the output voltage of the multilevel inverter.

[0009] In the above aspect, the multilevel inverter may further include another load connected to both ends of the second capacitor, and the control circuit may control to increase or decrease the current consumption of the other load according to a comparison result of either the voltage of the first capacitor or the voltage of the second capacitor with a reference voltage. With this configuration, a change in the voltage of either the first capacitor or the second capacitor is detected, and the current consumption discharged from the second capacitor to the other load can be increased or decreased according to the detection result. This makes it possible to further stabilize the potential at the midpoint between the first capacitor and the second capacitor, and further stabilize the multiple levels of potential output from the switching circuit. As a result, the output voltage of the multilevel inverter can be further stabilized.

[0010] In the above aspect, the control circuit may control the load to increase or decrease the current consumption according to a comparison result between the voltage of the first capacitor and the voltage of the second capacitor. In this case, the potential at the midpoint between the first capacitor and the second capacitor can be effectively stabilized, and the multiple levels of potentials output from the switching circuit can be effectively stabilized.

[0011] In the above aspect, the control circuit may also execute control to increase or decrease the output current from the output terminal according to the potential of the midpoint. In this case, the potential of the midpoint between the first capacitor and the second capacitor can be efficiently stabilized, and the multiple levels of potentials output from the switching circuit can be instantly stabilized.

[0012] In the above aspect, the load may be a fluid machine.

[0013] In the above aspect, the fluid machine may be a machine that delivers a cooling refrigerant for cooling the switching circuit. In this case, the discharge current of the first capacitor can be utilized for cooling the switching circuit that generates heat due to the output current, and it is possible to achieve both stabilization of the output voltage of the multilevel inverter and stable operation.

[0014] In addition, in the switching circuit according to the above aspect, a first switching element, a second switching element, a third switching element, and a fourth switching element may be connected in series between two input terminals in this order, an output terminal may be connected between the second switching element and the third switching element, and the switching circuit may include a first clamp diode connected between the midpoint and the first switching element and the second switching element, and a second clamp diode connected between the midpoint and the third switching element and the fourth switching element. According to this configuration, the first switching element and the third switching element are complementarily switched, and the second switching element and the fourth switching element are complementarily switched, so that three levels of potential including the potential of the midpoint can be output from the switching circuit, and the three levels of potential can be stabilized. Effect of the Invention

[0015] According to the present disclosure, it is possible to provide a multilevel inverter and an electric motor system capable of stabilizing an output voltage. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram of a configuration of a multilevel inverter 1 according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a circuit diagram showing a configuration of a circuit including the multilevel inverter circuit 3 and the cooling systems 7H and 7L. [Diagram 3] FIG. 3 is a diagram showing an example of time waveforms of the phase currents, phase voltages, and line voltages generated by the multilevel inverter circuit 3. In FIG. [Figure 4] FIG. 4 is a waveform diagram for explaining the control state of the switching circuit SWu in the control circuit 9. As shown in FIG. [Diagram 5] FIG. 5 is a table for explaining the control state modes of the switching circuit SWu in the control circuit 9. In FIG. [Figure 6] FIG. 6 is a flowchart showing a control procedure performed by the control circuit 9 in this embodiment. [Figure 7] FIG. 7 is a schematic diagram showing the configuration of an electric motor system 100 according to another embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or corresponding elements are designated by the same reference numerals, and duplicated description will be omitted.

[0018] 1 is a schematic diagram of a multilevel inverter 1 according to an embodiment of the present disclosure. The multilevel inverter 1 shown in the figure is a power conversion system that converts DC power into AC power and drives an externally connected electric motor in a three-phase, three-wire system. The multilevel inverter 1 includes a multilevel inverter circuit 3, a heat sink 5 for dissipating heat generated from the multilevel inverter circuit 3, a cooling system 7H, a cooling system 7L, and a control circuit 9 that controls the operation of the multilevel inverter circuit 3 and the cooling systems 7H and 7L.

[0019] Here, a description will be given of the circuit configuration of the multilevel inverter circuit 3 and the cooling systems 7H and 7L. Fig. 2 is a circuit diagram showing the configuration of a circuit including the multilevel inverter circuit 3 and the cooling systems 7H and 7L.

[0020] The multilevel inverter circuit 3 has two input terminals to which a DC voltage is applied from the outside, namely, an input terminal IN1 to which a positive voltage is applied and an input terminal IN2 to which a negative voltage is applied, a capacitor (first capacitor) CH and a capacitor (second capacitor) CL connected in series in this order between the two input terminals IN1, IN2, three switching circuits SWu, SWv, SWw connected in parallel to the two capacitors CH, CL between the two input terminals IN1, IN2, and three output terminals OUTu, OUTv, OUTw connected to the three switching circuits SWu, SWv, SWw, respectively.

[0021] Next, the configuration of the switching circuits SWu, SWv, SWw will be described.

[0022] The switching circuit SWu includes a transistor QHU as a first switching element, a transistor QHD as a second switching element, a transistor QLU as a third switching element, and a transistor QLD as a fourth switching element, which are connected in series in this order between two input terminals IN1 and IN2, diodes DHU, DHD, DLU, and DLD connected in parallel to the transistors QHU, QHD, QLU, and QLD, respectively, and two clamp diodes DH and DL. The pair of the transistor QHU and diode DHU, the pair of the transistor QHD and diode DHD, the pair of the transistor QLU and diode DLU, and the pair of the transistor QLD and diode DLD each constitute an arm.

[0023] The transistors QHU, QHD, QLU, and QLD are, for example, N-type field effect transistors (FETs), with drain terminals connected to the input terminal IN1 (high voltage) side and source terminals connected to the input terminal IN2 (low voltage) side. A control voltage is applied to the gate terminals of these transistors QHU, QHD, QLU, and QLD from the control circuit 9, and these transistors QHU, QHD, QLU, and QLD are switched in response to the control voltage. The diodes DHU, DHD, DLU, and DLD are configured such that their cathode terminals are connected to the drain terminals of the respective transistors QHU, QHD, QLU, and QLD, and their anode terminals are connected to the source terminals of the respective transistors QHU, QHD, QLU, and QLD. These diodes DHU, DHD, DLU, and DLD are elements for forming a path for an output current that flows backward through the transistors QHU, QHD, QLU, and QLD.

[0024] The clamp diode (first clamp diode) DH is configured such that its anode terminal is connected to a neutral point (middle point) PO, which is a connection point between the two capacitors CH and CL, and its cathode terminal is connected to a connection point between the two transistors QHU and QHD. The clamp diode (second clamp diode) DL is configured such that its cathode terminal is connected to the neutral point PO, and its anode terminal is connected to a connection point between the two transistors QLU and QLD. The clamp diode DH has a role of clamping to the potential of the neutral point PO when the potential of the connection point between the two arms (hereinafter collectively referred to as the "upper arm") on the input terminal IN1 side, i.e., the connection point between the two transistors QHU and QHD, becomes equal to or lower than the potential of the neutral point PO. The clamp diode DL has a role of clamping to the potential of the neutral point PO when the potential of the connection point between the two arms (hereinafter collectively referred to as the "lower arm") on the input terminal IN2 side, i.e., the connection point between the two transistors QLU and QLD, becomes equal to or higher than the potential of the neutral point PO.

[0025] The configurations of the other two switching circuits SWv and SWw are the same as that of the switching circuit SWu. The three output terminals OUTu, OUTv, and OUTw are connected to the connection points between the upper and lower arms of the switching circuits SWu, SWv, and SWw, respectively, that is, the connection point between the two transistors QHD and QLU.

[0026] In the following description, among the components of the multilevel inverter circuit 3, the capacitor CH, the three clamp diodes DH, and the three upper arms are collectively referred to as upper arms 11H, and the capacitor CL, the three clamp diodes DL, and the three lower arms are collectively referred to as lower arms 11L.

[0027] The cooling system 7H is a load connected to both ends of a capacitor CH, including transistors Q1, Q2 and diodes D1, D2 constituting a two-level inverter, and a blower 13, which is a type of fluid machine. The transistors Q1, Q2 are, for example, N-type FETs, and are connected in series to both ends of the capacitor CH. The drain terminals of the transistors Q1, Q2 are connected to the high-voltage side, and their source terminals are connected to the low-voltage side. The diodes D1, D2 are connected in parallel to the transistors Q1, Q2, respectively, and their cathode terminals are connected to the drain terminals of the transistors Q1, Q2, and their anode terminals are connected to the source terminals of the transistors Q1, Q2. The connection point between the two transistors Q1, Q2 is connected to an input terminal for power supply of the blower 13. The blower 13 is a machine with a built-in fan that blows air as a refrigerant for cooling the multilevel inverter circuit 3, and has a function of adjusting the wind force of the air according to the current supplied from the input terminal. The blower 13 of the cooling system 7H is provided so as to blow air toward a portion capable of cooling the entire upper arm 11H on the heat sink 5 (see FIG. 1).

[0028] The cooling system 7L has the same configuration as the cooling system 7H, and is another load connected to both ends of the capacitor CL. The blower 13 of the cooling system 7L is provided to blow air toward a portion capable of cooling the entire lower arm 11L on the heat sink 5 (see FIG. 1).

[0029] The cooling systems 7H, 7L configured as described above have the function of adjusting the current (current consumption by the blower 13) supplied from each capacitor CH, CL to the blower 13 of the cooling systems 7H, 7L in accordance with the control voltage (control signal) supplied from the control circuit 9 to the gates of the transistors Q1, Q2.

[0030] The control circuit 9 is an integrated circuit (IC) that controls the operation of the multilevel inverter circuit 3 and the cooling systems 7H and 7L. That is, the control circuit 9 generates a phase voltage that varies among three voltage levels including the potential of the neutral point PO in each of the three switching circuits SWu, SWv, and SWw, and controls the phase current that periodically increases and decreases to be output from the three output terminals OUTu, OUTv, and OUTw. FIG. 3 is a diagram showing an example of the time waveforms of the phase current, phase voltage, and line voltage generated by the multilevel inverter circuit 3, in which (a) shows the phase currents Iu, Iv, and Iw output from the output terminals OUTu, OUTv, and OUTw, respectively, (b) shows the phase voltage Vu at the output terminal OUTu, and (c) shows the line voltage Vuv between the output terminals OUTu and OUTv. Here, a case is shown in which a DC voltage Vdc of 400V is input and a three-phase motor is connected to the output terminals OUTu, OUTv, and OUTw, and a phase voltage Vu is generated so as to periodically vary among three voltage levels, namely, 0V, which is a neutral point potential, +Vdc / 2=+200V, and -Vdc / 2=-200V, and is output from the output terminal OUTu. As a result, phase voltages Vv and Vw having similar waveforms and phase-shifted from each other are generated at the output terminals OUTv and OUTw, and a voltage having a waveform that periodically varies among five voltage levels is generated as the line voltage Vuv. As a result, when an electric motor such as a three-phase motor is connected as a load to the output terminals OUTu, OUTv, and OUTw, the multilevel inverter circuit 3 can output phase currents Iu, Iv, and Iw having phase-shifted from each other from the output terminals OUTu, OUTv, and OUTw.

[0031] 4 is a waveform diagram for explaining the control state of the switching circuit SWu by the carrier level shift method in the control circuit 9, where (a) to (c) show the time changes of the control signal, and (d) shows the time change of the phase voltage Vu. Also, Fig. 5 is a chart for explaining the mode of the control state of the switching circuit SWu by the carrier level shift method in the control circuit 9.

[0032] When controlling the switching circuit SWu, the control circuit 9 refers to the carriers CA1 and CA2, which are two triangular waves whose signal levels are shifted up and down, and the modulated wave MS, which is a sine wave whose signal level varies periodically across the two triangular waves (FIG. 4(a)). Then, the control circuit 9 compares the levels of the carrier CA1 and the modulated wave MS, generates a pulse modulated wave (FIG. 4(b)) that changes between "0" and "1" depending on the comparison result, and compares the levels of the carrier CA2 and the modulated wave MS, generates a pulse modulated wave (FIG. 4(c)) that changes between "0" and "1" depending on the comparison result. Furthermore, the control circuit 9 classifies into operation modes "1" to "4" depending on the values ​​of the two pulse modulated waves, and controls the four transistors QHU, QHD, QLU, and QLD of the switching circuit SWu to switch in accordance with each of the operation modes "1" to "4".

[0033] For example, in the case of operation mode "1" where the comparison result of carrier CA1 is "1" and the comparison result of carrier CA2 is "1", the transistors QHU and QHD are turned on and the transistors QLU and QLD are turned off, and as a result, the output voltage of the switching circuit SWu is set to +Vdc / 2. In the case of operation mode "2" or "3" where the comparison result of carrier CA1 is "0" and the comparison result of carrier CA2 is "1", the transistors QHD and QLU are turned on and the transistors QHU and QLD are turned off, and as a result, the output voltage of the switching circuit SWu is set to 0V, which is the neutral point potential. Note that operation mode "2" is an operation mode in which the output current of the switching circuit SWu is set to the "positive direction" flowing to the motor side, and operation mode "3" is an operation mode in which the output current of the switching circuit SWu is set to the "negative direction" flowing from the motor side. In addition, in the case of operating mode "4" in which the comparison result of carrier CA1 is "0" and the comparison result of carrier CA2 is "0", transistors QHU and QHD are turned off and transistors QLU and QLD are turned on, resulting in the output voltage of switching circuit SWu being set to -Vdc / 2.

[0034] The control circuit 9 controls the current consumption of the cooling systems 7H and 7L while controlling the operation of the multilevel inverter circuit 3. That is, the control circuit 9 detects at least one of the voltage VCH across the capacitor CH and the voltage VCL across the capacitor CL during the operation control of the multilevel inverter circuit 3. Then, the control circuit 9 compares the detected voltage with a reference voltage and controls the cooling system 7H or the cooling system 7L to increase the current consumption according to the comparison result. In this embodiment, the control circuit 9 detects both the voltage VCH and the voltage VCL, compares the voltage VCL with the voltage VCL, and increases the current consumption of either the cooling system 7H or the cooling system 7L according to the comparison result. Instead of such control, the control circuit 9 may increase the current consumption of the cooling system 7H when the voltage VCH exceeds the reference voltage or the voltage VCL falls below the reference voltage, and increase the current consumption of the cooling system 7L when the voltage VCL exceeds the reference voltage or the voltage VCH falls below the reference voltage.

[0035] When attempting to increase the current consumption of the cooling system 7H or the cooling system 7L, the control circuit 9 increases the current consumption by changing the control value of the control signal for controlling the switching of the transistors Q1 and Q2 to change the on / off duty ratio of the transistor Q1 (transistor Q2). In addition, the control circuit 9 may reduce the current consumption of the cooling system 7L when the voltage VCH exceeds the reference voltage or when the voltage VCL falls below the reference voltage, or may reduce the current consumption of the cooling system 7H when the voltage VCL exceeds the reference voltage or when the voltage VCH falls below the reference voltage.

[0036] Furthermore, the control circuit 9 may also execute a known control (neutral point potential stabilization control) for increasing or decreasing the phase currents Iu, Iv, Iw from the output terminals OUTu, OUTv, OUTw according to the neutral point potential when controlling the operation of the multilevel inverter circuit 3. By using this type of control in combination, the neutral point potential can be quickly stabilized.

[0037] The following describes the control procedure in the control circuit 9. FIG 6 is a flowchart showing the control flow of the control circuit 9.

[0038] First, the control circuit 9 executes control based on the carrier level shift method to start driving the motor connected to the multilevel inverter circuit 3 (step S101). After that, the voltage VCH of the capacitor CH of the upper arm 11H and the voltage VCL of the capacitor CL of the lower arm 11L are detected (step S102). Then, the voltage VCH and the voltage VCL are compared (step S103). As a result of the comparison, if the voltage VCH exceeds VCL (step S103; YES), the control value of the control signal for controlling the cooling systems 7H and 7L is set so as to increase the current consumption of the cooling system 7H (step S104). On the other hand, if the voltage VCH falls below VCL (step S103; NO), the control value of the control signal for controlling the cooling systems 7H and 7L is set so as to increase the current consumption of the cooling system 7L (step S105).

[0039] Next, the generated control signal is output from the control circuit 9 to the cooling systems 7H, 7L to control the operation of the cooling systems 7H, 7L (step S106). Furthermore, if the driving of the motor is to be continued (step S107; YES), the control cycle of the above steps S102 to S106 is repeated. On the other hand, if the driving of the motor is not to be continued (step S107; NO), the control value of the control signal for controlling the cooling systems 7H, 7L is reset to an initial value (step S108), and then the motor driving control is terminated.

[0040] The configuration of an electric motor system according to another embodiment of the present disclosure will be described. Fig. 7 is a schematic diagram showing the configuration of an electric motor system 100. The electric motor system 100 includes the above-mentioned multilevel inverter 1 and a three-phase motor system 30 which is an electric motor connected to the output terminals OUTu, OUTv, and OUTw of a multilevel inverter circuit 3. The three-phase motor system 30 includes an inductor and a three-phase motor, and is configured so that the three-phase motor is connected to the output terminals OUTu, OUTv, and OUTw via the inductor. However, a generator is used as the electric motor connected to the multilevel inverter 1, and the electric motor system 100 may be used for power regeneration purposes.

[0041] According to the multilevel inverter 1 described above, a change in the voltage of either the capacitor CH or the capacitor CL is detected, and the consumption current discharged from the capacitor CH to the cooling system 7H can be increased or decreased according to the detection result. This makes it possible to stabilize the neutral point potential between the capacitor CH and the capacitor CL, and stabilize the three-level potentials output from the switching circuits SWu, SWv, and SWw. As a result, it becomes possible to stabilize the output voltage of the multilevel inverter.

[0042] In this embodiment, a change in the voltage of either the capacitor CH or the capacitor CL is detected, and the consumption current discharged from the capacitor CL to the cooling system 7L can be increased or decreased according to the detection result. This makes it possible to further stabilize the neutral point potential between the capacitors CH and CL, and further stabilize the three-level potentials output from the switching circuits SWu, SWv, and SWw. As a result, the output voltage of the multilevel inverter can be further stabilized.

[0043] Furthermore, according to this embodiment, the control circuit 9 controls the current consumption of the cooling systems 7H and 7L to increase or decrease depending on the comparison result between the voltage VCH of the capacitor CH and the voltage VCL of the capacitor CL. In this case, the neutral point potential can be effectively stabilized, and the multiple levels of potentials output from the switching circuits SWu, SWv, and SWw can be effectively stabilized. In particular, in this embodiment, the discharge current of the capacitors CH and CL is consumed for cooling the upper arm 11H and the lower arm 11L. Therefore, the discharge current can be effectively utilized, and the neutral point potential can be stabilized.

[0044] In the above aspect, the control circuit 9 also executes control (neutral point potential stabilization control) to increase or decrease the output current from the output terminals OUTu, OUTv, and OUTw according to the neutral point potential. In this case, the neutral point potential can be efficiently stabilized, and the three-level potentials output from the switching circuits SWu, SWv, and SWw can be instantly stabilized. In addition, when the neutral point potential stabilization control is executed, if the voltage of the capacitor CH becomes higher than the voltage of the capacitor CL, the phase current is controlled to shift in the positive direction, and as a result, the amount of heat generated in the upper arm 11H increases relatively. In this case, the control of this embodiment can increase the current consumption of the cooling system 7H to efficiently cool the upper arm 11H, thereby stabilizing the operation of the multilevel inverter circuit 3. Similarly, if the voltage of the capacitor CL becomes higher than the voltage of the capacitor CH, the amount of heat generated in the lower arm 11L increases relatively, but the operation of the multilevel inverter circuit 3 can be stabilized by efficiently cooling the lower arm 11L. Furthermore, by using two control methods for stabilizing the neutral point potential in combination, the amount of phase current shift when neutral point potential stabilization control is executed can be suppressed, and the voltage utilization rate in the multilevel inverter can be increased.

[0045] In this embodiment, the cooling systems 7H and 7L are fluid machines that send out a cooling refrigerant for cooling the switching circuits SWu, SWv, and SWw. In this case, the discharge current of the capacitors CH and CL can be utilized for cooling the switching circuits SWu, SWv, and SWw that generate heat due to the phase current, and it is possible to achieve both stabilization of the output voltage of the multilevel inverter 1 and stable operation.

[0046] Moreover, each of the switching circuits SWu, SWv, and SWw according to the above aspect includes four arms and two clamp diodes. With this configuration, a pair of arms is complementarily switched and another pair of arms is complementarily switched, so that three levels of potential including a neutral point potential can be output from the switching circuits SWu, SWv, and SWw, and the three levels of potential can be stabilized.

[0047] Conventionally, when attempting to utilize the neutral point potential stabilization control method, only a specific modulation method (triangle wave comparison PWM method, etc.) could be used, and the voltage utilization rate (effective value of line voltage) tended to decrease. In this embodiment, since it is not necessarily necessary to utilize the neutral point potential stabilization control method, the degree of freedom of the modulation method increases, and as a result, it becomes possible to adopt the space vector PWM method, and the voltage utilization rate can be increased.

[0048] Various embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and may be modified or applied to other things without departing from the spirit of the invention as described in each claim.

[0049] The multilevel inverter 1 of the above embodiment is not limited to a configuration that outputs three levels of potential as a phase voltage, and may be configured to output three or more levels of potential, such as five levels, as a phase voltage. For example, in the case of a configuration that outputs five levels of potential, three capacitors connected in series between two input terminals IN1 and IN2 are provided, and a load, which is a fluid machine, is connected to each of them.

[0050] Furthermore, the cooling systems 7H and 7L may be any machine that delivers a cooling refrigerant, and may include a pump that circulates cooling water for cooling the multilevel inverter circuit 3 instead of a blower. In addition, instead of the cooling systems 7H and 7L, a known load may be used as long as it is a load that can increase the current consumption based on a comparison of at least one of the voltage VCH across the capacitor CH and the voltage VCL across the capacitor CL with a reference voltage.

[0051] The multilevel inverter of the present invention is [1] "a multilevel inverter having two input terminals to which a DC voltage is applied, a first capacitor and a second capacitor connected in series between the two input terminals, a plurality of switching elements connected in series between the two input terminals, and an output terminal, the multilevel inverter comprising: a switching circuit that generates a multilevel potential including a potential at a midpoint between the first capacitor and the second capacitor and outputs the multilevel potential from the output terminal; a load connected to both ends of the first capacitor; and a control circuit that controls the current consumption of the load to increase or decrease depending on a comparison result of either the voltage of the first capacitor or the voltage of the second capacitor with a reference voltage."

[0052] The multilevel inverter of the present invention may be [2] "the multilevel inverter described in [1] above, further comprising another load connected across the second capacitor, wherein the control circuit controls the current consumption of the other load to increase or decrease depending on a comparison result of either the voltage of the first capacitor or the voltage of the second capacitor with a reference voltage."

[0053] The multilevel inverter of the present invention may be [3] "the multilevel inverter described in [1] or [2] above, wherein the control circuit controls the load to increase or decrease its current consumption depending on a comparison result between the voltage of the first capacitor and the voltage of the second capacitor."

[0054] The multilevel inverter of the present invention may be [4] "the multilevel inverter according to any one of [1] to [3] above, wherein the control circuit also executes control to increase or decrease the output current from the output terminal according to the potential of the midpoint."

[0055] The multilevel inverter of the present invention may be [5] "the multilevel inverter according to any one of the above [1] to [4], wherein the load is a fluid machine."

[0056] The multilevel inverter of the present invention may be [6] "the multilevel inverter according to the above [5], wherein the fluid machine is a machine that sends out a cooling refrigerant for cooling the switching circuit."

[0057] The multilevel inverter of the present invention may be [7] "the multilevel inverter according to any one of the above [1] to [6], wherein in the switching circuit, a first switching element, a second switching element, a third switching element, and a fourth switching element are connected in series between the two input terminals in that order, the output terminal is connected between the second switching element and the third switching element, and the switching circuit includes a first clamp diode connected between the intermediate point and the first switching element and the second switching element, and a second clamp diode connected between the intermediate point and the third switching element and the fourth switching element."

[0058] The electric motor system of the present invention is [8] "an electric motor system including a multilevel inverter according to any one of [1] to [7] above, and an electric motor connected to the output terminal of the multilevel inverter." [Explanation of symbols]

[0059] 1. Multilevel inverter 7H, 7L Cooling system (load) 9 Control Circuit 30 Three-phase motor system (electric motor) 100 Electric Motor System CH Capacitor (First Capacitor) CL Capacitor (second capacitor) QHD, QHU, QLD, QLU Transistors (1st to 4th switching elements) DH clamp diode (first clamp diode) DL Clamp diode (second clamp diode) IN1, IN2 input terminals OUTu, OUTv, OUTw output terminals PO Neutral point (midpoint) SWu, SWv, SWw Switching circuit VCH, VCL voltage Vdc DC voltage

Claims

1. Two input terminals to which a DC voltage is applied; a first capacitor and a second capacitor connected in series between the two input terminals; a switching circuit having a plurality of switching elements connected in series between the two input terminals and an output terminal, the switching circuit generating a multi-level potential including a potential at a midpoint between the first capacitor and the second capacitor and outputting the multi-level potential from the output terminal; a load connected across the first capacitor; a control circuit for controlling a current consumption of the load, the control circuit controls the load to increase or decrease a current consumption in response to a comparison result of either the voltage of the first capacitor or the voltage of the second capacitor with a reference voltage. Multilevel inverter.

2. a second load connected across the second capacitor; the control circuit controls the current consumption of the other load to be increased or decreased depending on a comparison result of either the voltage of the first capacitor or the voltage of the second capacitor with a reference voltage.

2. The multilevel inverter according to claim 1.

3. the control circuit controls the load so as to increase or decrease a current consumption in accordance with a comparison result between the voltage of the first capacitor and the voltage of the second capacitor. The multilevel inverter according to claim 1 or 2.

4. The control circuit also executes control to increase or decrease an output current from the output terminal in response to the potential of the midpoint. The multilevel inverter according to claim 1 or 2.

5. The load is a fluid machine.

3. The multilevel inverter according to claim 1 or 2.

6. The fluid machine is a machine that delivers a cooling refrigerant for cooling the switching circuit. The multilevel inverter according to claim 5 .

7. In the switching circuit, a first switching element, a second switching element, a third switching element, and a fourth switching element are connected in series between the two input terminals in this order; the output terminal is connected between the second switching element and the third switching element; a first clamp diode connected between the midpoint and the first and second switching elements; a second clamp diode connected between the midpoint and the third switching element and the fourth switching element. The multilevel inverter according to claim 1 or 2.

8. A multilevel inverter according to claim 1 or 2; an electric motor connected to the output terminal of the multilevel inverter; An electric motor system comprising:

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