A protection circuit, method and air conditioning equipment for a motor drive system

Abstract

The present invention discloses a protection circuit, method and air-conditioning equipment for a motor drive system. Among them, the circuit includes: a three-phase rectifier bridge, a capacitor, a detection circuit, and a switching circuit; the three-phase rectifier bridge is connected to the motor stator winding; two ends of the capacitor are respectively connected to a first terminal and a second terminal of the DC output end of the three-phase rectifier bridge, and are used for storing electric energy and supplying power to the switching circuit; an input end of the detection circuit is connected to two ends of the capacitor, an output end is connected to a first end of the switching circuit, a second end of the switching circuit is connected to a first end of the capacitor, and a third end and a fourth end are respectively connected to the first terminal and the second terminal of the DC output end of the three-phase rectifier bridge. Through the present invention, the back electromotive force generated during the deceleration process can be fed back to the motor stator winding, thereby avoiding its reverse charging to the inverter and the DC bus capacitor and improving safety.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic circuits, and in particular, to a protection circuit, method and air-conditioning equipment for a motor drive system. Background Art

[0002] Permanent magnet synchronous motors have been widely used in the air-conditioning industry due to their advantages such as high power density and good dynamic response. Especially in variable-frequency air conditioners, the motor is generally speed-controlled through a drive board to achieve the purpose of frequency conversion. The circuit topology of the drive board is usually a "AC-DC-AC" structure, and the bus voltage in the DC part is constant. However, when the motor suddenly experiences overcurrent or other faults during high-speed operation, the motor will immediately protect and stop. During the process of the motor running from high speed to stationary, a large back electromotive force will be generated. If not processed, this back electromotive force will be reverse-charged to the DC bus. In a drive system without a storage capacitor, since the bus capacitor is a thin-film capacitor with a very small capacitance value and cannot store a large amount of electrical energy, the back electromotive force will cause the bus voltage to rise rapidly, thereby damaging components such as the inverter and the bus capacitor.

[0003] Regarding the problem in the prior art that a large back electromotive force is generated during the process of the motor running from high speed to stationary, which easily causes damage to the components of the drive system, no effective solution has been proposed yet. Summary of the Invention

[0004] Embodiments of the present invention provide a protection circuit, method and air-conditioning equipment for a motor drive system to solve the problem in the prior art that a large back electromotive force is generated during the process of the motor running from high speed to stationary, which easily causes damage to the components of the drive system.

[0005] To solve the above technical problem, the present invention provides a protection circuit for a motor drive system, wherein the circuit includes: a three-phase rectifier bridge, a capacitor, a detection circuit, and a switching circuit;

[0006] The three-phase rectifier bridge is connected to the stator winding of the motor;

[0007] The capacitor, its first end is connected to the first terminal of the DC output end of the three-phase rectifier bridge, and its second end is connected to the second terminal of the DC output end of the three-phase rectifier bridge, and is used for storing electrical energy and supplying power to the switching circuit;

[0008] The detection circuit, its input end is connected to both ends of the capacitor, and its output end is connected to the first end of the switching circuit, and is used for controlling the conduction or cut-off of the switching circuit according to the operating state of the motor;

[0009] The switch circuit has its second end connected to the first end of the capacitor, its third end connected to the first terminal of the DC output end of the three-phase rectifier bridge, and its fourth end connected to the second terminal of the DC output end of the three-phase rectifier bridge, and is used to turn off when the motor is in the normal state and turn on when the motor is in the deceleration state.

[0010] Further, the detection circuit includes:

[0011] A first resistor and a first voltage regulator diode connected in series, the first end of the first branch formed by the series connection is connected to the first end of the capacitor, and the second end of the first branch is connected to the second end of the capacitor;

[0012] A second resistor and a third resistor connected in series, the first end of the second branch formed by the series connection is connected to the first end of the capacitor, and the second end of the second branch is connected to the second end of the capacitor;

[0013] A comparator, its first input terminal is connected between the first resistor and the first voltage regulator diode, its second input terminal is connected between the second resistor and the third resistor, and the output terminal is connected to the switch circuit. The comparator is used to output a high-level signal when the voltage at the first input terminal is greater than the voltage at the second input terminal, and output a low-level signal when the voltage at the first input terminal is less than the voltage at the second input terminal.

[0014] Further, the third resistor is a variable resistor.

[0015] Further, the third input terminal of the comparator is connected to the first end of the capacitor, and the fourth end is connected to the second end of the capacitor, so that the capacitor supplies power to the comparator.

[0016] Further, the switch circuit includes:

[0017] A first switch, its first pole is connected to the first terminal of the DC output end of the three-phase rectifier bridge, its second pole is connected to the second terminal of the DC output end of the three-phase rectifier bridge, and is used to control the back electromotive force generated during motor deceleration to feedback back to the motor stator winding;

[0018] A second switch, its first pole is connected to the output terminal of the comparator, its second pole is connected to the first end of the capacitor, and its third pole is connected to the third pole of the first switch, and is used to control the conduction or cutoff of the first switch according to the low-level signal or high-level signal output by the comparator.

[0019] Further, the first switch is a thyristor switch, and the second switch is a triode switch or a MOS transistor switch.

[0020] Further, the switch circuit further includes:

[0021] The fourth resistor is arranged between the output end of the comparator and the first pole of the second switch, and is used to limit the magnitude of the current input to the second switch.

[0022] Further, the circuit further includes:

[0023] The fifth resistor is arranged between the first end of the first branch and the first end of the capacitor, and is used to limit the charging current of the capacitor.

[0024] Further, the circuit further includes:

[0025] The second zener diode is arranged in parallel at both ends of the capacitor, and is used to limit the maximum voltage across the capacitor.

[0026] The present invention also provides an air conditioning device, which includes a motor and also includes the protection circuit of the above motor drive system.

[0027] Applying the technical solution of the present invention, the state of the motor is detected by the detection circuit, and a control signal is output according to the detection result to control the conduction or cut-off of the switch circuit. When the motor suddenly fails during high-speed operation and causes the motor to decelerate, the switch circuit is controlled to conduct, so that the back electromotive force generated during the deceleration process is fed back to the motor stator winding through the three-phase rectifier bridge and absorbed by the motor stator winding, thereby preventing it from being recharged to the inverter and the DC bus capacitor, protecting the components of the motor drive system from being damaged, and improving safety. Description of the Drawings

[0028] Figure 1 is the topological structure diagram of the existing motor drive system;

[0029] Figure 2 is the structure diagram of the protection circuit of the motor drive system according to the embodiment of the present invention;

[0030] Figure 3 is the structure diagram of the protection circuit of the motor drive system according to another embodiment of the present invention;

[0031] Figure 4 is the connection relationship diagram of the protection circuit and the motor drive system according to the embodiment of the present invention. Detailed Embodiments

[0032] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0033] The terms used in the embodiments of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The singular forms "a", "said", and "the" used in the embodiments of the present invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. "Plural" generally includes at least two.

[0034] It should be understood that the term "and / or" used herein is merely a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " herein generally represents an "or" relationship between the associated objects before and after.

[0035] It should be understood that although the terms first, second, etc. may be used to describe switches in the embodiments of the present invention, these switches should not be limited to these terms. These terms are only used to distinguish switches with different functions. For example, without departing from the scope of the embodiments of the present invention, the first switch may also be referred to as the second switch, and similarly, the second switch may also be referred to as the first switch.

[0036] Depending on the context, the words "if", "when" as used herein may be interpreted as "when...", "while...", "in response to determining", or "in response to detecting". Similarly, depending on the context, the phrase "if determined" or "if detecting (stated condition or event)" may be interpreted as "when determined", "in response to determining", "when detecting (stated condition or event)", or "in response to detecting (stated condition or event)".

[0037] It should also be noted that the term "comprising", "including" or any other variation thereof is intended to cover non-exclusive inclusion, such that a commodity or device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such commodity or device. Without further limitation, an element defined by the statement "comprising one..." does not exclude the existence of additional identical elements in the commodity or device comprising said element.

[0038] The optional embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0039] Embodiment 1

[0040] In a variable-frequency air conditioner, the motor is generally speed-regulated through a drive system to achieve the purpose of frequency conversion. Figure 1 For the topological structure diagram of an existing motor drive system, such as Figure 1As shown, the drive circuit topology is usually an "AC-DC-AC" structure, that is, the alternating current output by the power grid is converted into direct current by the rectifier 1 and output to the inverter 2, and then the direct current output by the rectifier is converted into alternating current by the inverter 2 and output to the motor 3. The voltage of the two DC buses between the rectifier 1 and the inverter 2 is constant. A DC bus capacitor C is connected in parallel on these two DC buses. An inductor L is also connected in series between one of the DC buses and one end of the capacitor C. When the motor suddenly experiences overcurrent or other faults during high-speed operation, the motor will immediately protect and stop. During the process of the motor running from high speed to stationary, a large back electromotive force will be generated. If not processed, this back electromotive force will be reverse-charged to the DC bus. In a drive system without an energy storage capacitor, since the DC bus capacitor C is a thin-film capacitor with a very small capacitance value and cannot store a large amount of electrical energy, the back electromotive force will cause the bus voltage to rise rapidly, thereby damaging components such as the inverter and the bus capacitor.

[0041] To solve the above problems, this embodiment provides a protection circuit for a motor drive system. Figure 2 As shown in the structure diagram of the protection circuit for the motor drive system according to the embodiment of the present invention, Figure 2As shown, the circuit includes: a three-phase rectifier bridge 11, which includes three parallel branches, each branch includes two diodes connected in series end to end, and the wires between the diodes in each branch are respectively connected to the U, V, and W phase lines of the motor stator winding 14, and the motor stator winding 14 is connected to the drive system 15; a capacitor C1, the first end of the capacitor C1 is connected to the first terminal of the DC output end of the three-phase rectifier bridge 11, and the second end of the capacitor C1 is connected to the second terminal of the DC output end of the three-phase rectifier bridge 11, which is used to store the electric energy output by the three-phase rectifier bridge and supply power to the switch circuit 13; a detection circuit 12, its input end includes four connection terminals, the first connection terminal and the second connection terminal are connected between the first end of the capacitor C1 and the first terminal of the DC output end of the three-phase rectifier bridge 11, the third connection terminal and the fourth connection terminal are connected between the second end of the capacitor C1 and the second terminal of the DC output end of the three-phase rectifier bridge 11, and the output end of the detection circuit 12 is connected to the first end of the switch circuit 13. Since a back electromotive force will be generated when the motor decelerates, therefore, when the motor decelerates, the voltage at the detection point between the second connection terminal and the fourth connection terminal at the input end of the detection circuit 12 will increase. The detection circuit 12 can judge whether the motor is in a normal state or a deceleration state by detecting the increase in voltage, and control the conduction of the switch circuit 13 when the motor is in a deceleration state and turn it off when the motor is in a normal state; the second end of the switch circuit 13 is connected to the first end of the capacitor C1, the third end is connected to the first terminal of the DC output end of the three-phase rectifier bridge 11, and the fourth end is connected to the second terminal of the output end of the three-phase rectifier bridge 11. It is turned off when the motor is in a normal state and is not connected to the circuit. It is turned on when the motor stator winding 14 is in a deceleration state, so that the inverter and rectifier connected to the motor stator winding 14 are short-circuited, and the three-phase rectifier bridge 11 is connected to the motor stator winding 14, so as to control the back electromotive force generated when the motor decelerates to feedback back to the motor stator winding 14, rather than being reverse-charged to the inverter, DC bus capacitor or rectifier in the drive system.

[0042] In this embodiment, the state of the motor is detected by the detection circuit, and a control signal is output according to the detection result to control the conduction or turn-off of the switch circuit. When a sudden failure occurs when the motor is running at high speed, resulting in the motor decelerating, the switch circuit is controlled to conduct, so that the back electromotive force generated during the deceleration process is fed back to the motor stator winding through the three-phase rectifier bridge and absorbed by the motor stator winding, thereby preventing it from being reverse-charged to the inverter, DC bus capacitor or rectifier, and protecting the components of the motor drive system from being damaged and improving safety.

[0043] Embodiment 2

[0044] This embodiment provides another protection circuit for a motor drive system. Figure 3The figure is a structural diagram of a protection circuit for a motor drive system according to another embodiment of the present invention. To further achieve detecting the increase in voltage during motor deceleration and then controlling the conduction of the switching circuit, as Figure 3 shown, the detection circuit 13 in the protection circuit of the motor drive system includes: a first resistor R1 and a first voltage regulator diode D1. The first resistor R1 and the first voltage regulator diode D1 are connected in series. The first end of the first branch formed by the series connection is connected to the first end of the capacitor C1, and the second end of the first branch is connected to the second end of the capacitor C1. It also includes: a second resistor R2 and a third resistor R3. The second resistor R2 and the third resistor R3 are connected in series. The first end of the second branch formed by the series connection is connected to the first end of the capacitor C1, and the second end of the second branch is connected to the second end of the capacitor C1. The detection circuit 13 further includes a comparator U1. The first input terminal of the comparator U1 is connected between the first resistor R1 and the first voltage regulator diode D1. Since the voltage across the first voltage regulator diode D1 remains constant, the voltage output at the connection point between the first resistor R1 and the first voltage regulator diode D1 remains constant. This voltage serves as the reference voltage Vr of the comparator U1. The second input terminal of the comparator U1 is connected between the second resistor R2 and the third resistor R3. When the motor decelerates, the voltage V1 output at the connection point between the second resistor R2 and the third resistor R3 will increase. Therefore, the magnitude relationship between it and the reference voltage Vr also changes. By configuring the resistance values of the first resistor R1, the second resistor R2, and the third resistor R3 and the voltage across the first voltage regulator diode D1, it is possible to make the voltage V1 less than the reference voltage Vr when the motor is operating normally, causing the comparator U1 to output a high-level signal. When the motor decelerates, the voltage V1 rises to be greater than the reference voltage Vr, causing the comparator U1 to output a low-level signal.

[0045] In the embodiment, the resistance value of the third resistor R3 is variable. By adjusting the resistance value of the third resistor R3, the proportional relationship between the voltage V1 and the actual back electromotive force can be changed, thereby ensuring that when the back electromotive force increases to a certain value, the first switch T1 in the protection circuit is immediately turned on to activate the protection function.

[0046] To ensure the normal operation of the comparator U1, a stable power supply voltage needs to be provided to it. Therefore, the comparator U1 also includes a third input terminal and a fourth input terminal. The third input terminal is connected to the first end of the capacitor C1, and the fourth terminal is connected to the second end of the capacitor C1. The capacitor C1 supplies power to the comparator U1.

[0047] The output terminal of the above comparator U1 is connected to the switching circuit 13. To achieve feeding back the reverse electromotive force generated during motor deceleration to the motor stator winding 14, it is necessary to make the switching circuit 13 turn off when the above comparator U1 outputs a high-level signal and the switching circuit 13 turn on when the above comparator U1 outputs a low-level signal. To achieve this purpose, asFigure 3 As shown, the above-mentioned switching circuit 13 includes: a first switch T1, whose first pole is connected to the first terminal of the DC output terminal of the three-phase rectifier bridge 11, and whose second pole is connected to the second terminal of the output terminal of the three-phase rectifier bridge 11 to control the back electromotive force generated when the motor decelerates to feedback back to the motor stator winding 14; a second switch T2, whose first pole is connected to the output terminal of the comparator U1, the second pole is connected to the first end of the capacitor C1, and the third pole is connected to the third pole of the first switch T1. When the comparator U1 outputs a low-level signal, the second switch T2 is turned on, and then the first end of the capacitor C1 is controlled to provide a high voltage to the first switch T1 to control the first switch T1 to turn on. When the comparator U1 outputs a high-level signal, the second switch T2 is turned off, and then the first switch T1 is controlled to turn off. In specific implementation, the first switch T1 can adopt a thyristor switch, and the second switch T2 can adopt a triode switch or a MOS tube switch. If a triode switch is adopted, it is preferably a PNP type triode switch, which is turned on when a low-level signal is input to the base (i.e., the first end of the second switch T2), and is cut off when a high level is input to the base.

[0048] Since the second switch T2 is a triode switch or a MOS tube switch, if the input current is too large, it will cause the device to fail to work properly or even be damaged. Therefore, in order to control the working current of the second switch T2, as Figure 3 shown, the switching circuit 13 further includes: a fourth resistor R4, which is arranged between the output terminal of the comparator U1 and the first pole of the second switch T2, and is used to limit the magnitude of the current input to the second switch T2 so that it does not exceed the working current of the second switch T2, and then the second switch T2 can work normally.

[0049] In the above-mentioned embodiment, the capacitor C1 is charged through the three-phase rectifier bridge 11 so that the capacitor C1 charges the switching circuit and the detection circuit. In order to avoid excessive charging current during the charging process and cause overcharging of the capacitor C1, as Figure 3 shown, the protection circuit of this motor drive system further includes: a fifth resistor R5, which is arranged between the first end of the first branch formed by the series connection of the first resistor R1 and the first voltage regulator diode D1 and the first end of the capacitor C1, and is used to limit the charging current of the capacitor C1.

[0050] Since the capacitance value of the capacitor C1 is fixed, in order to limit the voltage across the capacitor C1 and prevent the capacitor C1 from being broken down, as Figure 3 shown, the protection circuit of this motor drive system further includes: a second voltage regulator diode D2, which is connected in parallel across the capacitor C1 and is used to limit the maximum voltage across the capacitor C1.

[0051] Embodiment 3

[0052] This embodiment provides another protection circuit for a motor drive system, which is used to protect the drive system of the motor. In a small bus capacitor drive system, due to the small bus capacitor and limited stored energy, the back electromotive force generated during the motor deceleration process is easily reversed to the DC bus between the rectifier and the inverter, resulting in an increase in the DC bus voltage and causing the bus capacitor to be broken down. Therefore, an overvoltage protection circuit for a small bus capacitor motor drive system is proposed. When the motor suddenly experiences overcurrent or other faults during high-speed operation, the back electromotive force generated during the deceleration process is absorbed by the motor stator winding, thereby preventing it from being reversed to the DC bus and protecting the components from being damaged.

[0053] Figure 4 FIG. is a connection diagram of the protection circuit and the motor drive system according to an embodiment of the present invention. As Figure 4 shown, during normal operation, the three-phase voltage input from the power grid passes through the rectifier 41 in the motor drive system 400 and is filtered by the inductor L4 and the bus capacitor C4 to obtain a stable DC bus voltage, and then the motor operation is controlled through the inverter 42. After the terminal voltage of the motor passes through the three-phase rectifier bridge 44 of the protection circuit, it charges the capacitor C41 through the resistor R41, and the maximum voltage across the capacitor C41 is limited by the zener diode D42. The thyristor switch T41 is normally in the off state. The protection circuit further includes a comparator U1, a first resistor R41, a second resistor R42, a third resistor R43, and a zener diode D41. The voltage division structure formed by the first resistor R41 and the zener diode D41 is used to provide a stable reference voltage Vr for the comparator U41; the voltage division structure formed by the second resistor R42 and the third resistor R43 outputs a comparison voltage V1. When the motor is operating normally, the reference voltage Vr > the comparison voltage V1, and the comparator U41 outputs a high level. The protection circuit further includes a triode T42, which conducts when the comparator U41 outputs a low level. In this embodiment, the third resistor R43 is a variable resistor. By adjusting the resistance value of the resistor R43, the proportional relationship between the comparison voltage V1 and the actual back electromotive force can be changed. Thus, it is ensured that when the back electromotive force increases to a certain extent, the thyristor switch T41 of the protection circuit immediately conducts and the protection function is enabled.

[0054] When the motor suddenly experiences overcurrent or other faults during high-speed operation, the motor will immediately decelerate. A large back electromotive force will be generated during the deceleration process. At this time, the back electromotive force is divided by the second resistor R42 and the third resistor R43 in the protection circuit, resulting in a rapid increase in the comparison voltage V1, making V1 > Vr. Then the comparator U41 outputs a low level, causing the triode T42 to conduct. Then the capacitor C1 gives a voltage signal to the thyristor T41 through the triode T42, making the thyristor switch T41 conduct;

[0055] After the thyristor switch T41 is turned on, the motor drive system 400, the first resistor R41, the second resistor R42, the third resistor R43, and the zener diode D41 are all short-circuited. The loop formed by the motor stator winding 43, the three-phase rectifier bridge 44 of the overvoltage protection circuit, and the thyristor switch T41 is turned on, and the back electromotive force is consumed on the motor stator winding 43, preventing it from being reverse-charged to the DC bus and causing component damage.

[0056] The protection circuit further includes: a fourth resistor R44, disposed between the output terminal of the comparator U41 and the first pole of the triode T42, for limiting the magnitude of the current input to the triode T42 so that it does not exceed the operating current of the triode T42, thereby enabling the triode T42 to operate normally.

[0057] The protection circuit further includes: a fifth resistor R45, disposed between the first end of the branch formed by the series connection of the first resistor R41 and the first zener diode D41 and the positive electrode of the capacitor C41, for limiting the charging current of the capacitor C41.

[0058] The overvoltage protection of the motor drive system protection circuit in this embodiment is completely achieved through hardware, with a very fast response speed, having great advantages compared to the existing software overvoltage protection method.

[0059] Embodiment 4

[0060] This embodiment provides an air conditioning device, including a motor, and the device further includes the protection circuit of the above-mentioned motor drive system.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A protection circuit for a motor drive system, characterized in that, the circuit includes: a three-phase rectifier bridge, a capacitor, a detection circuit, and a switching circuit; the three-phase rectifier bridge is connected to the motor stator winding; the capacitor, its first end is connected to the first terminal of the DC output end of the three-phase rectifier bridge, and its second end is connected to the second terminal of the DC output end of the three-phase rectifier bridge, for storing electrical energy and supplying power to the switching circuit; the detection circuit, its input end is connected to both ends of the capacitor, and its output end is connected to the first end of the switching circuit, for controlling the conduction or cut-off of the switching circuit according to the operating state of the motor; the switching circuit, its second end is connected to the first end of the capacitor, its third end is connected to the first terminal of the DC output end of the three-phase rectifier bridge, and its fourth end is connected to the second terminal of the DC output end of the three-phase rectifier bridge, for turning off when the motor is in a normal state and turning on when the motor is in a deceleration state; the switching circuit includes: a first switch, its first pole is connected to the first terminal of the DC output end of the three-phase rectifier bridge, and its second pole is connected to the second terminal of the DC output end of the three-phase rectifier bridge, for controlling the back electromotive force generated during motor deceleration to feedback back to the motor stator winding; a second switch, its first pole is connected to the output end of the detection circuit, its second pole is connected to the first end of the capacitor, and its third pole is connected to the third pole of the first switch, for controlling the conduction or cut-off of the first switch according to the low-level signal or high-level signal output by the detection circuit; the first switch is a thyristor switch, and the second switch is a triode switch or a MOS transistor switch.

2. The circuit according to claim 1, characterized in that, the detection circuit includes: a first resistor and a first zener diode connected in series, the first end of the first branch formed by the series connection is connected to the first end of the capacitor, and the second end of the first branch is connected to the second end of the capacitor; a second resistor and a third resistor connected in series, the first end of the second branch formed by the series connection is connected to the first end of the capacitor, and the second end of the second branch is connected to the second end of the capacitor; a comparator, its first input end is connected between the first resistor and the first zener diode, its second input end is connected between the second resistor and the third resistor, and the output end is connected to the switching circuit, the comparator is used to output a high-level signal when the voltage at the first input end is greater than the voltage at the second input end, and output a low-level signal when the voltage at the first input end is less than the voltage at the second input end.

3. The circuit according to claim 2, characterized in that, the third resistor is a variable resistor.

4. The circuit according to claim 2, characterized in that, the third input end of the comparator is connected to the first end of the capacitor, and the fourth end is connected to the second end of the capacitor, so that the capacitor supplies power to the comparator.

5. The circuit according to claim 2, characterized in that, the switching circuit further includes: a fourth resistor, arranged between the output end of the comparator and the first pole of the second switch, for limiting the magnitude of the current input to the second switch.

6. The circuit according to claim 2, characterized in that, the circuit further comprises: a fifth resistor, disposed between the first end of the first branch and the first end of the capacitor, for limiting the charging current of the capacitor.

7. The circuit according to claim 1, characterized in that, the circuit further comprises: a second zener diode, connected in parallel across the capacitor, for limiting the maximum voltage across the capacitor.

8. An air-conditioning device, comprising a motor, characterized in that, the air-conditioning device further comprises the protection circuit of the motor drive system according to any one of claims 1 to 7.

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