JP6721476B2 - Motor drive system and air conditioner - Google Patents

Description

The present disclosure relates to a motor drive system that drives a load such as a fan having a squared-down torque characteristic by a motor, and further to an air conditioner that uses such a motor drive system as a fan device.

A technique is known in which the connection method of the stator windings of a three-phase motor is switched to Y connection (also referred to as “star connection”) and Δ connection (also referred to as “delta connection”).

For example, in the motor drive device disclosed in Japanese Unexamined Patent Application Publication No. 2006-246674 (Patent Document 1), the method for connecting the windings of the motor that receives the drive voltage from the inverter is such that the operation range of the motor is low-speed operation below a specified value. In the case, it is switched to the star connection, and in the case of high-speed operation where the operation range is equal to or more than the specified value, it is switched to the delta connection. As a result, the motor can be operated with higher efficiency when operating at low speed.

The air conditioner disclosed in JP 2011-250692 A (Patent Document 2) is provided with means for switching the winding of the fan motor of the outdoor heat exchanger between the Y connection and the Δ connection. Then, the winding of the fan motor is switched to the Δ connection when the operation of the motor is stopped. As a result, the drive circuit can be protected from the electromotive force of the motor generated by the rotation of the fan due to the outside wind.

JP, 2006-246674, A JP, 2011-250692, A

By the way, when a load having a squared-down torque characteristic such as a blower fan is driven by a motor, the load torque is proportional to the square of the motor rotation speed. Since the motor current is proportional to the load torque, the motor current is also proportional to the square of the motor rotation speed. Therefore, when the blower fan is driven at a low rotation speed, the magnitude of the motor current becomes very small, and as a result, the motor current detected for motor control contains many errors.

Due to the properties of the fan motor as described above, it has been difficult to stably control the operation of the fan motor at a low rotation speed in the conventional technology. If the fan motor is forced to operate at a rotation speed below the stability limit, current control becomes unstable, and problems such as the rotor stopping due to step-out occur.

The present invention has been made in view of the above problems, and one of the objects thereof is to stably drive a motor having a rotation speed lower than a conventional one when a load having a squared-down torque characteristic is driven by the motor. It is to provide a motor drive system capable of driving a motor.

A motor drive system according to one aspect of the present disclosure includes a three-phase motor, an inverter circuit, and a switching circuit. The three-phase motor drives a load having a square-reduced torque characteristic. The inverter circuit drives the three-phase motor so as to rotate at a predetermined rotation speed. The switching circuit switches the connection method of the stator winding of the three-phase motor between Δ connection and Y connection. The switching circuit makes the connection of the stator winding Δ connection when rotating the three-phase motor at a rotation speed lower than the threshold, and the stator winding when rotating the three-phase motor at a rotation speed higher than the threshold. The wire connection is configured to be a Y connection.

Preferably, the three-phase motor is a permanent magnet synchronous motor using a rare earth magnet.
Preferably the load is a two blade fan.

An air conditioner according to an aspect of the present disclosure includes a compressor that compresses a refrigerant, a first heat exchanger that condenses the compressed refrigerant, and an expansion that adjusts a flow rate of the refrigerant that has passed through the first heat exchanger. And a second heat exchanger that evaporates the refrigerant that has passed through the expansion valve and returns the evaporated refrigerant to the compressor. The air conditioner further includes a fan for blowing air to the first heat exchanger or the second heat exchanger, a three-phase motor that drives the fan, and a three-phase motor that rotates at a predetermined rotation speed. An inverter circuit for driving and a switching circuit for switching the connection method of the stator windings of the three-phase motor between Δ connection and Y connection are provided. The switching circuit makes the connection of the stator winding Δ connection when rotating the three-phase motor at a rotation speed lower than the threshold, and the stator winding when rotating the three-phase motor at a rotation speed higher than the threshold. The wire connection is configured to be a Y connection.

Preferably, the rotation speed of the three-phase motor is set according to the rotation speed of the compressor.

According to the motor drive system described above, when the load having the squared-down torque characteristic is driven by the motor, the motor can be stably operated up to a lower rotation speed than conventional.

FIG. 3 is a block diagram showing an example of a configuration of a motor drive system according to the first embodiment. It is a figure for demonstrating the difference in the connection method of the stator winding of a three-phase motor. It is a figure which shows the relationship between a motor rotation speed and U-phase current amplitude at the time of driving the load which has a square reduction torque characteristic with a motor. 3 is a flowchart showing a control procedure of an inverter circuit and a switching circuit in the motor drive system of FIG. 1. FIG. 3 is a schematic diagram showing a relationship between a motor rotation speed and a U-phase current amplitude (a comparison between a 2-blade fan and a 3-blade fan and a comparison between a case where a ferrite magnet is used and a case where a neodymium magnet is used are shown). FIG. 9 is a block diagram showing an example of the overall configuration of an air conditioner according to a third embodiment. It is a figure which shows an example of the control procedure of the air conditioner by the refrigeration cycle control part of FIG.

Hereinafter, embodiments will be described in detail with reference to the drawings. Note that the same or corresponding parts will be denoted by the same reference symbols, and description thereof will not be repeated.

<Embodiment 1>
[Hardware configuration of motor drive system]
FIG. 1 is a block diagram showing an example of the configuration of a motor drive system according to the first embodiment. Referring to FIG. 1, motor drive system 10 includes a three-phase motor 14, a converter circuit 11, an inverter circuit 12, a switching circuit 13, and a control unit 20.

The three-phase motor 14 is, for example, a permanent magnet synchronous motor or a brushless DC motor. In a permanent magnet synchronous motor (brushless DC motor), a rotor is provided with a permanent magnet, and a stator is provided with windings SU, SV, SW. In the case of this embodiment, the stator windings SU, SV, SW are three-phase windings.

The three-phase motor 14 drives a load 15 such as a fan and a pump that has a characteristic that the load torque is proportional to the square of the motor rotation speed (referred to as “reduced squared torque characteristic”).

The converter circuit 11 converts the AC voltage supplied from the external AC power supply 5 into a DC voltage. The converter circuit 11 is, for example, a diode bridge rectifier circuit.

The inverter circuit 12 converts the DC voltage output from the converter circuit 11 into a three-phase AC voltage. By controlling the switching timing of the switching elements Q1 to Q6 included in the inverter circuit 12, it is possible to generate a three-phase AC voltage having a desired frequency.

Specifically, the inverter circuit 12 includes a positive power supply node Np, a negative power supply node Nn, output nodes N1, N2, N3, upper arm side switching elements Q1, Q2, Q3, and lower arm side switching element Q4. , Q5, Q6, diode elements D1 to D6, and shunt resistors R1, R2, R3.

The DC voltage output from converter circuit 11 is applied between positive power supply node Np and negative power supply node Nn. The output nodes N1, N2 and N3 are respectively connected to one ends of the U-phase, V-phase and W-phase stator windings SU, SV and SW of the three-phase motor 14. The upper arm side switching elements Q1, Q2, Q3 are connected between the positive side power supply node Np and the output nodes N1, N2, N3, respectively. Lower arm side switching elements Q4, Q5, Q6 are respectively connected between negative side power supply node Nn and output nodes N1, N2, N3. The diode elements D1 to D6 for flowing the return current are respectively connected in antiparallel with the switching elements Q1 to Q6 (that is, in parallel and in the reverse bias direction). The shunt resistors R1 to R3 are respectively connected in series with the switching elements Q4 to Q6 on the lower arm side.

As the switching elements Q1 to Q6, an example of a bipolar power transistor is shown in FIG. 1, but the switching elements are not limited to this. For example, the switching elements Q1 to Q6 may be power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or IGBTs (Insulated Gate Bipolar Transistors).

The switching circuit 13 includes a switch group SWY including three switches SW1, SW2 and SW3 and a switch group SWΔ including three switches SW4, SW5 and SW6. Although the three-phase motor 14 and the switching circuit 13 are shown as separate configurations in FIG. 1, in practice, the switching circuit 13 is built in the three-phase motor 14 so that they are integrated. It may be in the form.

The connection of the switches SW1 to SW6 will be described below.
As described above, one ends of the stator windings SU, SV, SW are connected to the output nodes N1, N2, N3, respectively, while the other ends of the stator windings SU, SV, SW are respectively the switch SW1. , SW2, SW3 are connected to one end. The other ends of the switches SW1, SW2 and SW3 are connected to the common node NC. The switch SW4 is connected between one end of the stator winding SU and the other end of the stator winding SW. The switch SW5 is connected between one end of the stator winding SV and the other end of the stator winding SU. The switch SW6 is connected between one end of the stator winding SW and the other end of the stator winding SV.

According to the configuration of the switching circuit 13 described above, all the switches SW1, SW2 and SW3 forming the switch group SWY are made conductive, and all the switches SW4, SW5 and SW6 forming the switch group SWΔ are made non-conductive. For example, the connection method of the stator windings SU, SV, SW is Y connection. On the contrary, if all the switches SW1, SW2, SW3 forming the switch group SWY are set to the non-conductive state and all the switches SW4, SW5, SW6 forming the switch group SWΔ are set to the conductive state, the stator winding SU, The SV, SW connection method is Δ connection.

The control unit 20 controls on/off of the switching elements Q1 to Q6 of the inverter circuit 12 based on the motor current detected by the shunt resistors R1 and R2, and also controls the switching circuit 13. Specifically, the control unit 20 includes a current detection circuit 22, a microcomputer 21, a PWM (Pulse Width Modulation) signal generation circuit 23, and a switching signal generation circuit 24.

The current detection circuit 22 detects the motor current Im1 flowing when the switching element Q4 is in the ON state as the voltage V1s generated in the resistance element R1, and detects the motor current Im2 flowing when the switching element Q5 is in the ON state. It is detected as the voltage V2s generated at. The currents supplied from the output nodes N1, N2, N3 to the three-phase motor 14 are motor currents Im1, Im2, Im3, respectively. The current detection circuit 22 amplifies these voltages V1s and V2s by an amplifier (not shown), and converts the amplified signal into a digital signal by an A/D (Analog to Digital) converter (not shown).

The microcomputer 21 has a known configuration including a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), a non-volatile memory, and the like. By executing the programs stored in the ROM and the non-volatile memory, the CPU controls the start-up, stop, rotation speed, etc. of the three-phase motor 14, and also controls the switching circuit 13.

More specifically, the CPU of the microcomputer 21 calculates the motor current Im1 and the motor current Im2 based on the digital signals representing the voltages V1s and V2s received from the current detection circuit 22. The motor current Im3 is calculated by Im3=-(Im1+Im2). The CPU generates a command value of the motor voltage based on the calculated motor currents Im1 to Im3 and outputs it to the PWM signal generation circuit 23. Further, the CPU outputs a control signal for controlling the switch groups SWY and SWΔ of the switching circuit 13 via the switching signal generating circuit 24 according to the target motor rotation speed.

The PWM signal generation circuit 23 generates a pulse signal according to the command value of the motor voltage received from the microcomputer 21 and outputs it to the gates of the switching elements Q1 to Q6. For example, in the case of the triangular wave comparison method, the PWM signal generation circuit 23 compares the command value of the motor voltage received from the microcomputer 21 with the triangular wave signal (also referred to as the carrier signal) to determine the command value of the motor voltage. A pulse signal of the duty ratio is generated.

[About the difference between Y connection and Δ connection]
FIG. 2 is a diagram for explaining the difference in the connection method of the stator windings of the three-phase motor. 2A shows a circuit diagram of the Y connection, FIG. 2B shows a circuit diagram of the Δ connection, and FIG. 2C shows a difference in characteristics depending on the connection method in a tabular form.

Referring to FIG. 2, the same magnitude E voltage is applied to the stator windings SU, SV, and SW of each phase regardless of the difference in connection method, and the same magnitude I current flows. Be present. Here, the magnitude means an amplitude value or an effective value. Since the magnitude of the current and voltage of each stator winding is the same regardless of the connection method, the copper loss of the three-phase motor, that is, the resistance loss of the stator windings SU, SV, SW is different in the connection method. It will be the same regardless of Specifically, assuming that the resistance value of each stator winding is R, the copper loss of the three-phase motor is given by 3RI ² .

On the other hand, the voltage Em applied between the terminals of the three-phase motor (that is, between the nodes N1 and N2, between the nodes N2 and N3, and between the nodes N3 and N1) and each terminal (that is, each node N1, N2, N3). The current Ic flowing through) differs depending on the connection method. In the case of Y connection, the magnitude of the current flowing through each terminal of the three-phase motor is I, while the magnitude of the terminal voltage is √3×E. In the case of Δ connection, the magnitude of the current flowing through each terminal of the three-phase motor is √3×I, while the magnitude of the inter-terminal voltage is E. Therefore, by changing the Y connection to the Δ connection, the magnitude of the inter-terminal voltage becomes 1/√3, while the magnitude of each terminal current becomes √3 times.

Since the magnitude of each terminal current varies depending on the wiring system as described above, the resistance loss of the switching element in the inverter circuit (that is, the IPM (Intelligent Power Module) varies depending on the wiring system of the motor. When the ON resistance of the switching element is R _c , the resistance loss for the three phases is 3R _C I ² in the case of the Y connection, whereas it is 9R _C I ^{2 in} the case of the Δ connection, which increases.

[Relationship between current flowing through stator winding of fan motor and rotation speed]
FIG. 3 is a diagram showing a relationship between a motor rotation speed and a U-phase current amplitude when a load having a squared torque characteristic is driven by a motor. In FIG. 3, a case where the stator winding of the three-phase motor is Y-connected and a case where the stator winding is Δ-connected are shown in comparison.

When a load having a square-reduced torque characteristic such as a blower fan is driven by a motor, the load torque exhibits a so-called “square-reduced torque characteristic” that is proportional to the square of the motor rotation speed. In this case, the air volume is proportional to the motor rotation speed and the power consumption is proportional to the cube of the motor rotation speed. Further, since the load torque is proportional to the motor current, the relationship between the motor current amplitude and the motor rotation speed is such that the motor current amplitude is proportional to the square of the motor rotation speed as shown in FIG.

Therefore, in the case of the load having the gradual decrease torque characteristic, the magnitude of the motor current becomes significantly smaller as the motor rotation speed becomes smaller, as shown in FIG. This tendency becomes more remarkable in the case of the Y connection. Then, as described above, the inverter circuit 12 is PWM-controlled based on the detected value of the motor current, so that if the magnitude of the motor current becomes significantly small, the detection error becomes large, and as a result, the control of the inverter circuit 12 becomes difficult. become.

Therefore, in the motor drive system 10 of the present embodiment, when the three-phase motor is operated at a motor rotation speed lower than the threshold value, the connection method of the stator winding is switched from Y connection to Δ connection. As a result, the magnitude of the motor current is increased by √3, so that the detection accuracy of the motor current is increased, and as a result, the range of rotational speed at which the three-phase motor can be stably operated can be expanded.

As the above-mentioned threshold value, for example, 50% of the rated speed is selected. However, in the case of Δ connection, the loss of the inverter circuit 12 increases as compared with the case of Y connection. Therefore, in order to reduce the loss, it is desirable to select the rotation speed as small as possible within the range where stable control can be performed by the Y connection.

[Control procedure of inverter circuit and switching circuit]
FIG. 4 is a flowchart showing a control procedure of the inverter circuit and the switching circuit in the motor drive system of FIG. The above description will be summarized below with reference to FIGS. 1 and 4.

First, it is assumed that the control unit 20 receives an operation start command of the three-phase motor 14 at an externally specified rotation speed (step S100). The control unit 20 determines whether or not the designated rotation speed is equal to or higher than the threshold value Nth (step S105).

When the designated rotation speed is less than the threshold value Nth (NO in step S105), the control unit 20 switches the switching circuit 13 so that the connection method of the three-phase motor 14 is Δ connection (step S110). After switching the wiring system, the control unit 20 controls the inverter circuit 12 so that the three-phase motor 14 is operated at the designated rotation speed (step S115).

After that, unless the rotation speed change command is received (NO in step S125), the operation of the three-phase motor 14 is continued at the rotation speed. When the rotation speed change command is received (YES in step S125), the control unit 20 determines whether the new designated rotation speed is equal to or higher than the threshold value Nth (step S130). As a result, when the designated rotation speed is less than the threshold Nth (NO in step S130), the control unit 20 controls the inverter circuit 12 so that the three-phase motor 14 is operated at the new designated rotation speed (step S130). S115).

On the other hand, when the new designated rotation speed is equal to or higher than the threshold value Nth (YES in step S130), the control unit 20 temporarily stops the operation of the three-phase motor 14 so that the output becomes zero. 12 is controlled (step S135). After stopping the three-phase motor, the control unit 20 switches the switching circuit 13 so that the connection system of the three-phase motor 14 becomes the Y connection (step S150). After switching the wiring system, the control unit 20 controls the inverter circuit 12 so that the three-phase motor 14 is operated at the designated rotation speed (step S155).

After that, unless the rotation speed change command is received (NO in step S165), the operation of the three-phase motor 14 is continued at the rotation speed. When the rotation speed change command is received (YES in step S165), the control unit 20 determines whether the new designated rotation speed is equal to or higher than the threshold value Nth (step S170). As a result, when the designated rotation speed is equal to or higher than the threshold Nth (YES in step S170), the control unit 20 controls the inverter circuit 12 so that the three-phase motor 14 is operated at this new designated rotation speed (step S170). S155).

On the other hand, when the new designated rotation speed is less than the threshold value Nth (NO in step S170), the control unit 20 temporarily stops the operation of the three-phase motor 14, so that the output of the inverter circuit becomes zero. 12 is controlled (step S175). After the stop of the three-phase motor, the control unit 20 switches the switching circuit 13 so that the connection method of the three-phase motor 14 is Δ connection (step S110). After switching the wiring system, the control unit 20 controls the inverter circuit 12 so that the three-phase motor 14 is operated at the designated rotation speed (step S115). Hereinafter, the same control is repeated.

When the motor stop command is received during the operation of the three-phase motor 14 (YES in steps S120 and S160), the control unit 20 controls the inverter circuit 12 so that the output of the inverter circuit 12 becomes zero. As a result, the operation of the three-phase motor 14 is stopped (steps S140, S180).

In the above, when switching the connection method of the three-phase motor 14 from Δ connection to Y connection or vice versa, the three-phase motor 14 was once stopped. On the other hand, if the switching circuit 13 can be switched within a short time such as within a range of about 3 pulses in the PWM control signal having the carrier frequency of 16 kHz, the stop of the three-phase motor 14 is unnecessary. Practically, the switching between the Δ connection and the Y connection is performed when the motor rotation speed decreases, so the above-mentioned time constraint does not pose a problem. When switching the switching circuit 13, it is necessary to change the magnitude of the current and voltage output from the inverter circuit 12 by changing the duty ratio of the PWM signal.

[effect]
As described above, according to the motor drive system of the first embodiment, when the load having the squared-down torque characteristic is driven by the motor, if the motor rotation speed is less than the threshold value, the connection method of the stator winding of the motor is Y. The connection can be switched to the Δ connection. As a result, the magnitude of the motor current increases by √3 times, so that the detection error of the motor current can be reduced. As a result, the motor can be stably inverter-controlled to a range of a lower rotation speed.

The motor drive system of the first embodiment is widely applied to equipment including a load having a gradual reduction torque characteristic, such as a blower fan, a pump, a fan, an air purifier, and an indoor fan device and an outdoor fan device of an air conditioner. be able to.

<Second Embodiment>
In the motor drive system according to the second embodiment, a case where the fan device has two blades and a case where the three-phase motor is a permanent magnet synchronous motor using a rare earth magnet will be described. In this specification, a fan having two blades is called a two-blade fan, and a fan having three blades is called a three-blade fan.

FIG. 5 is a schematic diagram showing the relationship between the motor rotation speed and the U-phase current amplitude, showing a comparison between a two-blade fan and a three-blade fan and a comparison between a case where a ferrite magnet is used and a case where a neodymium magnet is used. ing. In FIG. 5, it is assumed that the stator winding connection method is Y connection.

[Comparison between 2-blade fan and 3-blade fan]
Since the load torque of the fan motor increases as the number of blades increases, the motor current at the same rotation speed also increases. Therefore, when the three-blade fan is used, there is a problem that the motor current reaches the limit current of the semiconductor switching element used in the inverter circuit, and the motor rotation speed is limited. Therefore, a 2-blade fan tends to be used especially in a fan device having a high output.

However, as described above, in the fan motor, the magnitude of the motor current is significantly reduced when the motor rotation speed is low. The degree of reduction of the motor current is more remarkable in the 2-blade fan than in the 3-blade fan, as schematically shown in FIG. Therefore, particularly in the case of a fan device using a two-blade fan, as described in the first embodiment, by operating by switching the connection method of the stator windings to the Δ connection, the operation in the low rotation speed region can be improved. The controllability of the fan motor can be increased.

[Comparison of ferrite magnet and neodymium magnet]
In the permanent magnet synchronous motor, as the magnetic force of the permanent magnet increases, that is, the field magnetic flux increases, the same torque can be generated even when the magnitude of the motor current is reduced. A rare earth magnet is known as a permanent magnet having a particularly large magnetic force. Rare earth magnets include neodymium magnets and samarium cobalt magnets.

FIG. 5 schematically shows a case where a ferrite magnet that does not use rare earth elements is used and a case where a neodymium magnet is used as a typical example of rare earth magnets. As shown in FIG. 5, when the neodymium magnet is used, the magnitude of the motor current at the same rotation speed is smaller than when the neodymium magnet is not used. In particular, when a rare earth magnet such as a neodymium magnet is used in a permanent magnet synchronous motor that drives a two-blade fan, the motor current is significantly reduced. Therefore, as described in the first embodiment, the controllability of the fan motor in the low rotation speed region can be increased by switching the stator winding connection method to the Δ connection for operation.

<Third Embodiment>
In the third embodiment, an example in which the motor drive system 10 of the first embodiment is applied to an outdoor fan device and an indoor fan device of an air conditioner will be described.

[Overall structure of air conditioner]
FIG. 6 is a block diagram showing an example of the overall configuration of the air conditioner according to the third embodiment. With reference to FIG. 6, the air conditioner 30 includes a refrigeration cycle (also referred to as a heat pump cycle) 65 including a compressor 34, a four-way valve 35, an outdoor heat exchanger 36, an expansion valve 37, and an indoor heat exchanger 38. .. Further, the air conditioner 30 includes an outdoor fan device 40, an indoor fan device 50, a converter circuit 32, inverter circuits 33, 44, 54, and a control unit 60.

In the refrigeration cycle 65, the compressor 34 compresses the refrigerant. The four-way valve 35 switches the circulating direction of the refrigerant in the cooling operation and the heating operation. The outdoor heat exchanger 36 exchanges heat between the outdoor air and the refrigerant. The opening degree of the expansion valve 37 is controlled in order to adjust the flow rate of the refrigerant. The indoor heat exchanger 38 exchanges heat between indoor air and refrigerant.

The circulation direction of the refrigerant when the four-way valve 35 is switched will be described. In the cooling operation mode, as shown by the solid arrow CL in FIG. 6, the compressor 34, the four-way valve 35, the outdoor heat exchanger 36, the expansion valve 37, the indoor heat exchanger 38, the four-way valve 35, and the compressor 34 are operated. The refrigerant circulates in sequence. In this case, the outdoor heat exchanger 36 functions as a condenser for condensing and liquefying the compressed high-temperature refrigerant, and the indoor heat exchanger 38 evaporates the liquefied refrigerant to cool the refrigerant to a low temperature. It functions as an evaporator for changing to gas. On the other hand, in the heating operation mode, as shown by the dashed arrow HT in FIG. 2, the compressor 34, the four-way valve 35, the indoor heat exchanger 38, the expansion valve 37, the outdoor heat exchanger 36, the four-way valve 35, and the compressor. The refrigerant circulates in the order of 34. In this case, the outdoor heat exchanger 36 functions as an evaporator, and the indoor heat exchanger 38 functions as a condenser.

The outdoor fan device 40 promotes heat exchange in the outdoor heat exchanger by blowing outdoor air (that is, outside air) to the outdoor heat exchanger 36. The outdoor fan device 40 includes a fan 41 (for example, a propeller fan), a motor 42 that rotationally drives the fan 41, and a switching circuit 43 that switches the stator winding connection method of the motor 42 between Y connection and Δ connection. Including. Although the motor 42 and the switching circuit 43 are shown as separate configurations in FIG. 6, in practice, the switching circuit 43 is incorporated in the motor 42 so that the two are integrated. May be.

The indoor fan device 50 blows the air that has been heat-exchanged by the indoor heat exchanger 38 into the room. The indoor fan device 50 switches the fan 51 (for example, a cross flow fan or a propeller fan), a motor 52 that rotationally drives the fan 51, and a connection method of a stator winding of the motor 52 between Y connection and Δ connection. And a circuit 53. The motors 42 and 52 are, for example, permanent magnet synchronous motors. Although the motor 52 and the switching circuit 53 are shown as separate components in FIG. 6, in practice, the switching circuit 53 is incorporated in the motor 52, so that the two are integrated. May be.

The converter circuit 32 converts the AC voltage supplied from the external AC power supply 5 into a DC voltage. The inverter circuits 33, 44, 54 convert the DC voltage output from the converter circuit 32 into a three-phase AC voltage. Specifically, the inverter circuit 33 drives a motor (not shown) built in the compressor 34, the inverter circuit 44 drives the motor 42 of the outdoor fan device 40, and the inverter circuit 54 drives the motor 52 of the indoor fan device 50. To drive.

The control unit 60 is configured based on a microcomputer including a CPU and a memory. The control unit 60 controls the entire air conditioner 30. Here, a portion of the control unit 60 that controls the refrigeration cycle 65 is referred to as a refrigeration cycle control unit 61, and a portion that controls the inverter circuit 54 and the switching circuit 53 for the indoor fan device 50 is referred to as an indoor fan control unit 55. A portion that controls the inverter circuit 44 and the switching circuit 43 for the outdoor fan device 40 is referred to as an outdoor fan control unit 45.

The air conditioner 30 further includes a temperature sensor 71 for measuring the temperature of the outdoor heat exchanger 36, a temperature sensor 70 for measuring a discharge temperature which is a refrigerant temperature at the outlet of the compressor 34, and an indoor heat. A temperature sensor 73 for measuring the temperature of the exchanger 38. These temperature sensors 71, 70, 73 are, for example, thermistors. The temperature sensor 71 for the outdoor heat exchanger 36 and the temperature sensor 73 for the indoor heat exchanger 38 are both arranged between the inlet and the outlet of the heat exchanger. Therefore, in the normal case, the temperature detected when these heat exchangers function as a condenser is the condensation temperature of the refrigerant, and the temperature detected when they function as an evaporator is the evaporation temperature of the refrigerant. is there. The air conditioner 30 is further provided with a temperature sensor 72 for measuring the outside air temperature and a temperature sensor 74 for measuring the indoor temperature. These temperature sensors 72 and 74 can also be constituted by, for example, thermistors.

In the above configuration, the motor 42 and the switching circuit 43 of the outdoor fan device 40, the inverter circuit 44, and the outdoor fan control unit 45 correspond to the motor drive system 10 of the first embodiment. Similarly, the motor 52 and the switching circuit 53 of the indoor fan device 50, the inverter circuit 54, and the indoor fan control unit 55 correspond to the motor drive system 10 of the first embodiment.

As described in the first embodiment, when the outdoor fan 41 is driven at a rotational speed lower than the threshold value, the switching circuit 43 switches the connection of the stator winding of the motor 42 to the Δ connection, thereby the outdoor fan 41 is driven. The device 40 can be controlled more stably. Similarly, when the indoor fan 51 is driven at a rotational speed lower than the threshold value, the switching circuit 53 switches the connection of the stator winding of the motor 52 to the Δ connection, thereby more stably controlling the indoor fan device 50. can do.

[Example of air conditioner control procedure]
FIG. 7: is a figure which shows an example of the control procedure of the air conditioner by the refrigerating-cycle control part of FIG. In the control procedure of FIG. 7, the part related to the rotation speed command to the outdoor fan device 40 and the indoor fan device 50 is simplified and shown.

Referring to FIGS. 6 and 7, when the operation start command of air conditioner 30 is received from the user (YES in step S200), refrigeration cycle control unit 61 detects indoor temperature by temperature sensor 74 (step S210). .. The refrigeration cycle control unit 61 operates the compressor 34 at a rotation speed corresponding to the temperature difference between the detected value of the indoor temperature and the set temperature set by the user.

Further, the refrigeration cycle control unit 61 and the outdoor fan device 40 and the indoor unit 40 perform the necessary and sufficient heat exchange in the outdoor heat exchanger 36 and the indoor heat exchanger 38 in accordance with the set value of the rotation speed of the compressor 34. The command value of the rotation speed of the fan motors 42 and 52 in the fan device 50 is determined (steps S230 and S240).

The outdoor fan control unit 45 controls the inverter circuit 44 and the switching circuit 43 based on the determined command value of the rotation speed of the fan motor 42. Similarly, the indoor fan control unit 55 controls the inverter circuit 54 and the switching circuit 53 based on the determined command value of the rotation speed of the fan motor 52. The specific control method of inverter circuits 44, 54 and switching circuits 43, 53 is similar to that of the first embodiment, and therefore detailed description will not be repeated.

The above-described control of steps S210 to S240 is repeated until a command to stop the operation of the air conditioner 30 is received from the user (until YES in step S250). When the operation stop command of the air conditioner 30 is received from the user (YES in step S250), the refrigeration cycle control unit 61 stops the compressor 34 by setting the output of the inverter circuit 33 to zero (step S260). A stop command for the outdoor fan device 40 and the indoor fan device 50 is output to the outdoor fan control unit 45 and the indoor fan control unit 55 (steps S270 and S280).

<Appendix>
The part of the first to third embodiments is summarized as follows.

(1) The motor drive system 10 includes a three-phase motor 14, an inverter circuit 12, and a switching circuit 13. The three-phase motor 14 drives a load 15 having a squared torque characteristic. The inverter circuit 12 drives the three-phase motor 14 so as to rotate at a predetermined rotation speed. The switching circuit 13 switches the connection method of the stator windings SU, SV, SW of the three-phase motor 14 between Δ connection and Y connection. When rotating the three-phase motor 14 at a rotation speed of less than the threshold Nth, the switching circuit 13 makes the connection of the stator windings SU, SV, SW Δ connection and makes the three-phase motor 14 at the rotation speed of the threshold Nth or more. Is rotated, the stator windings SU, SV, SW are connected in a Y connection.

According to the above configuration, if the motor rotation speed is less than the threshold value, the connection method of the stator windings SU, SV, SW of the three-phase motor 14 is switched from Y connection to Δ connection. As a result, the magnitude of the motor current increases by √3 times, so that the detection error of the motor current can be reduced. As a result, the three-phase motor 14 can be stably inverter-controlled to a lower rotation speed range.

(2) In the above (1), the three-phase motor 14 is a permanent magnet synchronous motor using a rare earth magnet. In the permanent magnet synchronous motor using the rare earth magnet, the field magnetic force becomes large, so that the magnitude of the motor current in the low rotation speed region becomes significantly small. Therefore, switching the connection method of the stator windings SU, SV, SW to the Δ connection is extremely effective for stabilizing the inverter control.

(3) In the above (1) or (2), the load 15 is a two-blade fan. Since the load torque of the fan motor of the 2-blade fan is smaller than that of the 3-blade fan, the magnitude of the motor current in the low rotation speed region is significantly reduced. Therefore, switching the connection method of the stator windings SU, SV, SW to Δ connection is extremely effective for stabilizing the inverter control.

(4) The air conditioner 30 includes a compressor 34 that compresses a refrigerant, a first heat exchanger (36; 38) that condenses the compressed refrigerant, and a first heat exchanger (36; 38). An expansion valve 37 that adjusts the flow rate of the refrigerant that has passed through and a second heat exchanger (38; 36) that evaporates the refrigerant that has passed through the expansion valve 37 and returns the evaporated refrigerant to the compressor 34. The air conditioner 30 further includes fans 41 and 51 for blowing air to the first heat exchanger (36; 38) or the second heat exchanger (38; 36), and a three-phase motor 42 that drives the fan. , 52, the inverter circuits 44, 54 for driving the three-phase motors 42, 52 so as to rotate at a predetermined rotation speed, and the connection method of the stator windings of the three-phase motors 42, 52 are Δ connection and Y connection. And switching circuits 43 and 53 for switching with. When rotating the three-phase motors 42 and 52 at a rotation speed lower than the threshold value, the switching circuits 43 and 53 change the connection of the stator windings to a Δ connection, and the three-phase motors 42 and 52 are rotated at the rotation speed equal to or higher than the threshold value. When rotating, the connection of the stator windings is configured to be a Y connection.

According to the air conditioner having the above-described configuration, the connection method of the three-phase motors 42 and 52 that drives the fans 41 and 51 that blow air to the heat exchangers 36 and 38 is changed from the Y connection when the motor rotation speed is less than the threshold value. Can be switched to wiring. As a result, the magnitude of the motor current is increased by √3 times, so that the detection error of the motor current can be reduced, and as a result, the fan motors 42 and 52 can be stably controlled to a lower rotation speed range. You can

(5) In (4) above, the rotation speeds of the three-phase motors 42 and 52 are set according to the rotation speed of the compressor 34. Therefore, the heat exchangers 36 and 38 can be controlled so that necessary and sufficient heat exchange is performed according to the rotation speed of the compressor 34.

The embodiments disclosed this time are to be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description but by the scope of the claims, and is intended to include meanings equivalent to the scope of the claims and all modifications within the scope.

10 motor drive system, 11, 32 converter circuit, 12, 33, 44, 54 inverter circuit, 13, 43, 53 switching circuit, 14 three-phase motor, 15 load, 20, 60 control unit, 21 microcomputer, 30 air conditioning Machine, 34 compressor, 35 four-way valve, 36 outdoor heat exchanger, 37 expansion valve, 38 indoor heat exchanger, 40 outdoor fan device, 41,51 fan, 42,52 fan motor (three-phase motor), 45 outdoor fan Control unit, 50 indoor fan device, 55 indoor fan control unit, 61 refrigeration cycle control unit, 70 to 74 temperature sensor, N1, N2, N3 output node, NC common node, Nn negative side power supply node, Np positive side power supply node, Q1-Q6 switching element, SU, SV, SW stator winding, SWΔ, SWY switch group.

Claims (5)

A three-phase motor for driving a load having a squared torque characteristic,
An inverter circuit that drives the three-phase motor so as to rotate at a predetermined rotation speed,
A switching circuit for switching the connection method of the stator windings of the three-phase motor between Δ connection and Y connection ;
With a control unit ,
The control unit is
When rotating the three-phase motor at a rotational speed less than a threshold value, the switching circuit is controlled so that the connection of the stator winding is a Δ connection,
When rotating the three-phase motor at a rotation speed equal to or higher than the threshold value, the switching circuit is controlled so that the connection of the stator winding is a Y connection ,
When changing the rotation speed of the three-phase motor from the first rotation speed larger than the threshold value to the second rotation speed smaller than the threshold value, the connection of the stator winding is changed from the Y connection to the Δ connection. A motor drive system for controlling the switching circuit as described above . The motor drive system according to claim 1, wherein the three-phase motor is a permanent magnet synchronous motor using a rare earth magnet. The motor drive system according to claim 1, wherein the load is a two-blade fan. A compressor for compressing the refrigerant,
A first heat exchanger for condensing the compressed refrigerant,
An expansion valve that adjusts the flow rate of the refrigerant that has passed through the first heat exchanger;
A second heat exchanger that evaporates the refrigerant that has passed through the expansion valve and returns the evaporated refrigerant to the compressor;
A fan for blowing air to the first heat exchanger or the second heat exchanger,
A three-phase motor for driving the fan,
An inverter circuit that drives the three-phase motor so as to rotate at a predetermined rotation speed,
A switching circuit for switching the connection method of the stator windings of the three-phase motor between Δ connection and Y connection ;
With a control unit ,
The control unit is
When rotating the three-phase motor at a rotational speed less than a threshold value, the switching circuit is controlled so that the connection of the stator winding is a Δ connection,
When rotating the three-phase motor at a rotation speed equal to or higher than the threshold value, the switching circuit is controlled so that the connection of the stator winding is a Y connection ,
When changing the rotation speed of the three-phase motor from the first rotation speed larger than the threshold value to the second rotation speed smaller than the threshold value, the connection of the stator winding is changed from the Y connection to the Δ connection. An air conditioner that controls the switching circuit as described above . The air conditioner according to claim 4, wherein the rotation speed of the three-phase motor is set according to the rotation speed of the compressor.

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