Protection method for automatic switching of power supply of three-phase BLDC motor

Abstract

Disclosed in the present invention is a protection method for automatic switching of a power supply of a three-phase BLDC motor. A surge protection circuit comprises a relay driving circuit, a relay, and a PTC resistor; an electromagnetic switch JK in the relay is connected in parallel to the PTC resistor; a microprocessor MCU outputs a signal to the relay driving circuit to control a coil RY in the relay so as to control the electromagnetic switch JK to be turned on or off; and a signal input end of a three-phase power supply phase loss detection circuit is connected to three-phase input power supplies U, V and W. The protection method comprises: the microprocessor MCU determining an output pulse signal of the three-phase power supply phase loss detection circuit; and if the three-phase power supply phase loss detection circuit outputs a pulse signal having a larger cycle or does not generate a pulse, determining that an abnormal input signal is present in the three-phase power supply phase loss detection circuit, and turning off the electromagnetic switch JK. The present invention achieves the turn-off of the relay in the order of 10 ms, meets market requirements, and has improved universality and extremely high anti-interference capability.

Description

Protection method for automatic switching of power supply for three-phase BLDC motor Technical field: The invention relates to a protection method for automatic switching of power supply of a three-phase BLDC motor. Background technology: At present, BLDC motors are widely used in various electrical equipment due to their advantages such as environmental protection, power saving and easy control, such as fans, air conditioners, electric drills, vacuum cleaners, etc. The electrical wiring diagram is shown in Figure 1. The three-phase AC input power of the mains is connected to the motor controller of the BLDC motor through an air switch. In order to prevent the surge current impact caused by power failure to power on, a surge protection circuit is set behind the three-phase rectifier circuit in the motor controller, as shown in Figure 2. The surge protection circuit includes a relay and a PTC resistor. The electromagnetic switch JK in the relay is connected in parallel with the PTC resistor, and the coil RY in the relay is input by the microprocessor MCU in the motor controller. The output signal (generally through the relay drive circuit) is controlled. Before the motor controller is powered on, the electromagnetic switch JK in the relay is disconnected. When the power is turned on, the current flows through the PTC resistor into the charging capacitor and the inverter circuit. The PTC resistor has a good resistance to surge current and effectively protects the charging capacitor and the IGBT switch in the inverter circuit. When the current stabilizes, the microprocessor MCU outputs a signal to control the coil RY in the relay to close the electromagnetic switch JK, thereby short-circuiting the PTC resistor. The current coming out of the three-phase rectifier circuit passes through the electromagnetic switch JK into the charging capacitor and the inverter circuit, avoiding the loss of electric energy by the PTC resistor during motor operation. UPS (Uninterruptable Power Supply) is widely used in occasions where reliable power supply is required, such as control equipment and its emergency protection system, emergency lighting, system network system, medical system, fire safety alarm system, etc. The main advantage of UPS lies in its uninterruptible power supply capability. When the AC input of the mains fails, the UPS energy storage battery supplies power to the load. Most of the load electrical equipment or systems using UPS power supply mentioned above contain BLDC motors. As the core power component, BLDC motors should be protected. However, during the UPS power switching process (i.e., the AC input failure switches to UPS power supply), the BLDC motor will trigger a huge impact current into the motor controller, which can easily damage the controller hardware. The electrical wiring diagram is shown in Figure 3. The main reason for the damage to the motor controller hardware is that in the event of a power failure due to the AC input failure, the coil RY in the relay in the surge protection circuit is It takes at least a few seconds to release all the electrical energy and disconnect the electromagnetic switch JK in the relay. The switching time from the AC input power failure to the UPS power supply is less than 2 seconds. Therefore, the huge impact current generated during the UPS power switching process directly passes through the electromagnetic switch JK in the relay, causing a great impact on the electronic devices behind it and damaging the motor controller hardware. In addition, the air switch used in the middle connection may also trip, and there is a high probability that the power supply will trip. The patent applicant in this case filed an invention patent application on February 13, 2023 (patent application number: 202310100580.0, patent name: An invention patent for a three-phase power supply phase loss detection circuit and a BLDC motor controller). This patent application proposes a technical solution to the above problems. The working principle of this patent is: See Figures 5, 6, 10 and 11C of the patent disclosure and the relevant description of the embodiment part of the specification: The level output by the three-phase power supply phase loss detection circuit (hardware circuit) to the microprocessor MCU is a high level or a low level, without alternating high and low levels. When the power is disconnected, the level output by the three-phase power supply phase loss detection circuit to the microprocessor MCU is a low level, and when the power is closed, the level output by the three-phase power supply phase loss detection circuit to the microprocessor MCU is a high level. When the microprocessor MCU detects that the signal Vdp voltage is low, it will turn off the relay to protect the subsequent circuit. See the patent disclosure

[0050] Paragraph description: "When the three-phase AC input of the mains is in a power-off state, the signal Vdp is a stable low level, the reference voltage Vref2 input of the comparison circuit 2 and the comparison circuit 3 is provided by the reference voltage power supply circuit, the reference voltage Vref2 is much larger than the zero voltage, the output signal Vdp-2 of the comparison circuit 3 and the output signal Vdp-1 of the comparison circuit 2 are both low levels and before the power is off, the electromagnetic switch JK of the relay is in the disconnected state, and the motor stops. At this time, when the UPS power supply is switched to about 500ms-1 second, the relay is in the disconnected state, and the current flows through the PTC resistor into the bus charging capacitor C5. The function of the PTC resistor is to prevent current mutations. At this time, it can effectively solve the high current caused by switching the power supply and protect the electronic components of the BLDC motor relay from high current shocks." The above circuit and control have the following deficiencies: 1) As shown in Figure 4, due to the setting of X capacitors at the three-phase input power supply U, V, and W (the function of X capacitors is AC filtering), when the external power supply is cut off, the voltages of the three-phase input power supply U, V, and W are still at the value before disconnection. The X capacitor needs to discharge for a period of time before the three-phase power supply phase loss detection circuit can output a low level, which is slow to react, causing the relay to remain in a closed state, and the PTC resistor cannot play a role in preventing current surges. Change effect; 2) When the Vdp signal output by the three-phase power phase loss detection circuit received by the microprocessor MCU receiving end is disturbed, the microprocessor MCU will mistakenly believe that the power is turned off, thereby turning off the relay and causing malfunction. 3) This patent enables the PTC resistor to prevent current mutation when switching to UPS power supply in about 500ms-1 second, which can effectively solve the high current caused by switching power supply; but the switching time of the ATS automatic switching switch commonly used in emergency power supply systems may be within 100ms, resulting in poor adaptability of the solution. Summary of the invention: An object of the present invention is to provide a protection method for automatic switching of power supply of a three-phase BLDC motor, so as to solve the problem that in the prior art, an X capacitor is set at the three-phase input power supply U, V, and W of the three-phase BLDC motor. When the external power supply is powered off, the X capacitor needs to be discharged for a period of time before the three-phase power supply phase loss detection circuit can output a low level, and the reaction is slow and the switching time is long, resulting in the technical problem that the PTC resistor cannot play a role in preventing current mutation and has weak anti-interference ability. The purpose of the present invention is achieved through the following technical solutions. A protection method for automatic switching of power supply of a three-phase BLDC motor, wherein the three-phase BLDC motor comprises a three-phase power supply phase loss detection circuit, a three-phase rectifier circuit, a surge protection circuit, a bus charging capacitor C5, an inverter circuit, an IGBT drive circuit and a microprocessor MCU, wherein the three-phase input power supplies U, V and W are connected to the input end of the three-phase rectifier circuit, and the output end of the three-phase rectifier circuit is connected to the surge protection circuit to charge the bus charging capacitor C5, and the bus charging capacitor C5 provides a DC bus voltage Vbus for the inverter circuit, and the surge protection circuit comprises a relay drive circuit, a relay and a PTC resistor, and the electromagnetic switch JK in the relay is connected in parallel with the PTC resistor, and the microprocessor MCU outputs a signal to the relay drive circuit to control the coil RY in the relay, so as to control the closing or disconnection of the electromagnetic switch JK; the signal input end of the three-phase power supply phase loss detection circuit is respectively connected to the three-phase input power supplies U, V and W; The protection method of automatic switching of power supply is: the microprocessor MCU judges the output pulse signal of the three-phase power supply phase loss detection circuit, if the output pulse signal of the three-phase power supply phase loss detection circuit is If the cycle becomes larger or no pulse is generated, it is determined that there is an abnormal input signal in the three-phase power supply phase loss detection circuit, and the microprocessor MCU outputs a signal to the relay drive circuit to control the electromagnetic switch JK in the relay to disconnect. The above-mentioned microprocessor MCU determines whether the output signal of the three-phase power supply phase loss detection circuit has a level rising edge signal or a level falling edge signal at every time T2. If it is detected, it is in a normal power supply state, and if it is not detected, it is in a power-off state. When the microprocessor MCU determines that the output signal of the three-phase power supply phase loss detection circuit has a level rising edge signal or a level falling edge signal, it is also necessary to determine the time period between adjacent level rising edge signals and level falling edge signals. If the detection time period becomes longer, it is determined that it is in a phase loss state, and the microprocessor MCU outputs a corresponding signal for protection. The above time T2 satisfies: T0 / 2 <t2<t0，t0是三相输入电源u、v、w输入正常时三相电源缺相检测电路所输出的方波信号的周期。The above-mentioned three-phase power supply phase loss detection circuit includes a U-phase detection circuit, a V-phase detection circuit, a W-phase detection circuit and a comparison circuit 2. The circuit structures of the U-phase detection circuit, the V-phase detection circuit and the W-phase detection circuit are the same, and all include a rectifier circuit, a step-down circuit and a comparison circuit 1. The input end of the U-phase detection circuit is connected to the three-phase input power supply U-phase, the input end of the V-phase detection circuit is connected to the three-phase input power supply V-phase, and the input end of the W-phase detection circuit is connected to the three-phase input power supply W-phase. The output ends of the U-phase detection circuit, the V-phase detection circuit and the W-phase detection circuit are connected as the signal input end of the comparison circuit 2, and the signal output end Vdp of the comparison circuit 2 is connected to the signal input end of the microprocessor MCU. The output ends of the above-mentioned U-phase detection circuit, V-phase detection circuit, and W-phase detection circuit are connected and stabilized by a voltage stabilizing circuit. Compared with the prior art, the present invention has the following effects: 1. The three-phase power supply phase loss detection circuit uses the high and low level pulse signals integrated by the phase difference of the three-phase input power supply to judge the abnormality of the input power supply. It can avoid the problem that the relay shutdown delay is caused by the slow discharge of the X capacitor set at the three-phase input power supply U, V, and W during power switching, and the PTC resistor cannot play the role of preventing current mutation, so as to achieve the shutdown of the relay at the 10ms level, meet market demand, and improve its versatility. 2. The anti-interference ability is extremely strong. When there is an interference signal in the square wave pulse signal output by the three-phase power supply phase loss detection circuit, it will not affect the judgment of the microprocessor MCU. 3. Other advantages of the present invention are described in detail in the embodiment section. Description of the drawings: FIG1 is a schematic diagram of electrical wiring of various electrical equipment with BLDC motors in the prior art; FIG. 2 is a circuit block diagram of a controller of a BLDC motor in the prior art; FIG3 is a schematic diagram of electrical wiring of various electrical equipment with BLDC motors in the prior art after using a UPS power supply; 4 is a circuit diagram of suppressing AC surge current and X-capacitor filtering provided between a three-phase input power supply U, V, W with a BLDC motor and a three-phase rectifier circuit in the prior art; 5 is a circuit block diagram of a three-phase power supply phase loss detection circuit provided in Embodiment 1 of the present invention; FIG6 is a circuit diagram of a U-phase detection circuit corresponding to FIG5 ; FIG7 is a perspective view of a BLDC motor according to a first embodiment of the present invention; FIG8 is a perspective view of a BLDC motor controller according to a first embodiment of the present invention; FIG9 is a cross-sectional view of a BLDC motor according to a first embodiment of the present invention; 10 is a circuit block diagram of a BLDC motor controller according to a first embodiment of the present invention; FIG11A is a circuit diagram corresponding to a portion of a three-phase rectifier circuit in FIG10 ; FIG11B is a circuit diagram corresponding to the inverter circuit portion in FIG10 ; FIG11C is a circuit diagram corresponding to the relay drive circuit portion in FIG10 ; FIG12 is a voltage waveform diagram of the U-phase input power supply to the ground GND among the three-phase input power supplies U, V, and W; 13 is a pulse square wave diagram of the three-phase input power U, V, W respectively outputted with a phase difference of 120 degrees after being processed by the rectifier circuit, the step-down circuit and the comparison circuit 1; 14 is a waveform diagram of the integrated square wave signals output by the U-phase detection circuit, the V-phase detection circuit, and the W-phase detection circuit; 15 is a diagram of a pulse square wave signal outputted from the output ends of the U-phase detection circuit, the V-phase detection circuit, and the W-phase detection circuit after being processed by the second comparison circuit; FIG16 is a graph showing the voltage waveform of the U-phase input power supply to the ground GND and the pulse square wave output by the three-phase power supply phase failure detection circuit; 17 is a test waveform diagram of the X capacitor discharge voltage after being processed by the three-phase power supply phase loss detection circuit to output a high level; 18 is a test waveform diagram of the X capacitor discharge voltage after being processed by the three-phase power supply phase loss detection circuit to output a low level; 19 is a waveform diagram of the output of the three-phase power supply phase loss detection circuit when the three-phase input power U, V, W phase is lost; FIG. 20 is a software flow chart of the present invention. Specific implementation method: The present invention is further described in detail below through specific embodiments in conjunction with the accompanying drawings. Embodiment one: As shown in Figures 5, 6 and 10, the present embodiment provides a three-phase BLDC motor including a three-phase power supply phase loss detection circuit, a three-phase rectifier circuit, a surge protection circuit, a bus charging capacitor C5, an inverter circuit, an IGBT drive circuit and a microprocessor MCU. The three-phase input power supplies U, V and W are connected to the input end of the three-phase rectifier circuit, and the output end of the three-phase rectifier circuit is connected to the surge protection circuit to charge the bus charging capacitor C5. The bus charging capacitor C5 provides a DC bus voltage Vbus for the inverter circuit. The surge protection circuit includes a relay drive circuit, a relay and a PTC resistor. The electromagnetic switch JK in the relay is connected in parallel with the PTC resistor. The microprocessor MCU outputs a signal to the relay drive circuit to control the coil RY in the relay, thereby controlling the closing or disconnection of the electromagnetic switch JK; the signal input end of the three-phase power supply phase loss detection circuit is respectively connected to the three-phase input power supplies U, V, and W; it is characterized in that the protection method for automatic switching of the power supply is: the microprocessor MCU determines the output pulse signal of the three-phase power supply phase loss detection circuit, if the period of the pulse signal output by the three-phase power supply phase loss detection circuit becomes larger or no pulse is generated, it is determined that the three-phase power supply phase loss detection circuit has an abnormal input signal, and the microprocessor MCU outputs a signal to the relay drive circuit to control the electromagnetic switch JK in the relay to disconnect. The above-mentioned three-phase power supply phase loss detection circuit includes a U-phase detection circuit, a V-phase detection circuit, a W-phase detection circuit and a comparison circuit 2. The circuit structures of the U-phase detection circuit, the V-phase detection circuit and the W-phase detection circuit are as follows: The two circuits are similar, and both include a rectifier circuit, a step-down circuit, and a comparison circuit 1. The input end of the U-phase detection circuit is connected to the three-phase input power supply U phase, the input end of the V-phase detection circuit is connected to the three-phase input power supply V phase, and the input end of the W-phase detection circuit is connected to the three-phase input power supply W phase. The output ends of the U-phase detection circuit, the V-phase detection circuit, and the W-phase detection circuit are connected as the signal input end of the comparison circuit 2, and the signal output end Vdp of the comparison circuit 2 is connected to the signal input end of the microprocessor MCU. FIG6 only shows the circuit diagram of the U-phase detection circuit. As for the circuit structures of the V-phase detection circuit and the W-phase detection circuit, they are basically the same as the circuit structure of the U-phase detection circuit. The reference voltage Vref1 of the comparison circuit 1 in FIG5 is determined by the voltage division of the resistor R69 and the resistor R93. As shown in Figures 7, 8 and 9, the BLDC motor provided in this embodiment is composed of a motor 1 and a BLDC motor controller 2. The motor 1 includes a stator assembly 12, a rotor assembly 13 and a housing assembly 11. The stator assembly 13 is mounted on the housing assembly 11, and the rotor assembly 13 is sleeved on the inner side of the stator assembly 12. The motor controller 2 includes a control box 22 and a control circuit board 21 installed in the control box 22. As shown in Figures 10, 11A, 11B and 11C, the control circuit board 21 in the BLDC motor controller 2 integrates a three-phase power supply phase loss detection circuit, a three-phase rectifier circuit, a surge protection circuit, a bus charging capacitor C5, an inverter circuit, an IGBT drive circuit and a microprocessor MCU. The three-phase input power supplies U, V and W are connected to the input end of the three-phase rectifier circuit. The output end of the three-phase rectifier circuit is connected to the surge protection circuit to charge the bus charging capacitor C5. The bus charging capacitor C5 provides a DC bus voltage Vbus for the inverter circuit. The surge protection circuit includes a relay, a relay drive circuit and a PTC resistor. The electromagnetic switch JK in the relay is connected in parallel with the PTC resistor. The microprocessor MCU outputs a signal to the relay drive circuit to control the relay. The coil RY in the device is used to control the closing or disconnection of the electromagnetic switch JK; the signal input ends of the three-phase power supply phase loss detection circuit are respectively connected to the three-phase input power supplies U, V, and W, and the signal Vdp output by the three-phase power supply phase loss detection circuit is input to the microprocessor MCU. The output signal of the microprocessor MCU controls the inverter circuit to work through the IGBT drive circuit. The output signal K1 of the microprocessor MCU is connected to an input signal end of the relay drive circuit, so as to avoid the relay shutdown delay caused by the slow discharge of the X capacitor set at the three-phase input power supplies U, V, and W when the UPS power is switched, resulting in the problem that the PTC resistor cannot play a role in preventing current mutation, so as to achieve the relay shutdown at the 10ms level, meet market demand, and improve its versatility. The DC bus voltage Vbus is also connected to the input end of the DC-DC buck circuit. The output end of the circuit provides different supply voltages to power the inverter circuit and the microprocessor MCU. As shown in FIG11C , the relay drive circuit includes a resistor R192, a resistor R123, a resistor R6, a transistor Q8, and a transistor Q9 to form a push-pull drive circuit to drive the coil RY in the relay. The +15V power supply reaches the base of the transistor Q8 through the voltage regulator tube ZD7, the resistor R116, the diode D223, and the resistor R192. One end of the capacitor C99 is connected between the diode D223 and the resistor R192, and the other end of the capacitor C99 is grounded GND. The output signal K1 of the microprocessor MCU is connected to an input signal end of the relay drive circuit to control the transistor Q8 and the transistor Q9 to be turned off or on, thereby controlling the electromagnetic switch JK of the relay to be pulled in or out. The present invention is in the working process: 1. The three-phase input power supplies U, V, and W are powered on normally, and the voltages of the three-phase input power supplies U, V, and W to GND are waveforms with a phase difference of 120 degrees: as shown in FIG12 , FIG12 is a voltage waveform of the U-phase input power supply to the ground GND; 2. The three-phase input power supplies U, V, and W are processed by the rectifier circuit, the step-down circuit, and the comparison circuit one respectively, and output three pulse square waves with a phase difference of 120 degrees, as shown in Figure 13. The output ends of the U-phase detection circuit, the V-phase detection circuit, and the W-phase detection circuit are then processed by the comparison circuit two. When the level state of the output signals of the U-phase detection circuit, the V-phase detection circuit, and the W-phase detection circuit is a square wave with two highs and one low or three highs (due to their unique phase difference), the integrated signal of the output square wave signals of the three comparison circuits one is shown in Figure 14. 3. The integrated signal of the square wave signal output by the three comparison circuits 1 is processed by the back-end comparison circuit 2, and after being compared with the reference voltage Vref2 set by the hardware, the high and low level pulse square wave signals are output to enter the input end of the microprocessor MCU. The test waveform output by the comparison circuit 2 is shown in Figure 15. 4. As shown in Figure 16, in the normal mode of the three-phase input power supply U, V, W, the waveform period T0 detected by the microprocessor MCU is 7ms. Assuming that the frequency of the three-phase input power supply U, V, W is 50Hz, the period of the voltage waveform of the U-phase input power supply to the ground GND is 1 / 50 seconds; the period of the test waveform output by the comparison circuit 2 is 1 / 50 seconds divided by 3, which is about 7ms. [The waveform period T0 detected by the microprocessor MCU is related to the power supply frequency. If the frequency of the three-phase input power supply U, V, W is 60Hz, then T0 = 1 / (60×3)] 5. When the three-phase input power supply U, V, and W are disconnected, the voltage waveform after suppressing the AC surge current and filtering the X capacitor in Figure 4 is a continuous fluctuating voltage. At the moment of disconnection, the X capacitors between the three-phase input power supplies U, V, and W will discharge from any voltage value in the fluctuation range. Therefore, there are two states, as shown in Figure 5. The first state: the X capacitor discharge voltage is greater than the reference voltage Vref1 of the first of the three comparison circuits at the front end after passing through the rectification circuit and the step-down circuit, so the input voltage entering the comparison circuit 2 is higher than the reference voltage Vref2, and the comparison circuit 2 outputs a high level. The test is shown in Figure 17; the second state: the X capacitor voltage is less than the reference voltage Vref1 of the comparison circuit 1 after passing through the rectification circuit and the step-down circuit, so the input voltage entering the comparison circuit 2 is lower than the reference voltage Vref2, and the comparison circuit 2 outputs a low level. However, no matter whether the comparison circuit 2 outputs a high level or a low level, the microprocessor MCU cannot recognize the double edges of the square wave signal within the T0 time. The waveform test is shown in Figure 18. 6. When the three-phase input power supply U, V, W is missing, due to the input voltage phase difference of 120 degrees, the output period T1 of the square wave signal output by the comparison circuit 2 will increase by 1 / 2, and T1 = about 10.5ms at this time. The test waveform is shown in Figure 19. It can be concluded that when the three-phase input power U, V, W input is normal, the period T0 of the square wave output by the comparison circuit 2 is about 7ms. When the three-phase input power U, V, W input is missing, the period T1 of the square wave output by the comparison circuit 2 is about 10.5ms. When the three-phase input power U, V, W input is disconnected, there is no square wave output by the comparison circuit 2. We only need the microprocessor MCU to check every T2 (T0 / 2 <t2<t0)时间内判断是否有双边沿输入，如果双边沿输入异常，便可判断三相输入电源u、v、w输入异常，微处理器mcu输出信号进而关断继电器，避免因三相输入电源u、v、w处设置x电容放电慢而导致的继电器关断延时，导致ptc电阻不能发挥防止电流突变作用的问题，从而做到在10ms级别关断继电器，满足市场需求，提高其通用性。Explanation of the technical solution of the present invention: According to the conclusion drawn from the above solution introduction, it can be seen that the hardware circuit of the three-phase power supply phase loss detection circuit outputs a pulse square wave of a specific period under normal power supply, and generates a pulse larger than the specific period or no pulse under abnormal power supply. At this time, the microprocessor MCU only needs to detect whether there is a rising edge or a falling edge of the level within a specific period. If it is detected, it belongs to the normal power supply state. If the detection period becomes longer, it is in the phase loss state. If it is not detected, it is in the power-off state. After the power-off state is detected, the microprocessor MCU The processor MCU closes the relay through the hardware circuit. As shown in Figure 20, when the BLDC motor controller 2 of the present invention is powered on, the microprocessor MCU scans the output signal of the three-phase power supply phase loss detection circuit once every T2 seconds to determine whether there is a double-edge signal. If so, the microprocessor MCU continues to scan the output signal of the three-phase power supply phase loss detection circuit once every T2 seconds and determine. If not, the microprocessor MCU outputs a signal to close the relay through the hardware circuit. Definitions of Abbreviations and Key Terms: Double edge: rising edge and falling edge of the microcontroller TTL level. Rising edge: the process from low level to high level. Falling edge: the process from high level to low level. The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited thereto. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention are equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. Protection method for automatic switching of the power supply of a three-phase BLDC motor. The three-phase BLDC motor includes a three-phase power supply phase loss detection circuit, a three-phase rectification circuit, a surge protection circuit, a bus charging capacitor C5, an inverter circuit, an IGBT drive circuit, and a microprocessor MCU. The three-phase input power supplies U, V, and W are connected to the input end of the three-phase rectification circuit. The output end of the three-phase rectification circuit is connected to the surge protection circuit and then charges the bus charging capacitor C5. The bus charging capacitor C5 provides the DC bus voltage Vbus for the inverter circuit. The surge protection circuit includes a relay drive circuit, a relay, and a PTC resistor. The electromagnetic switch JK in the relay is connected in parallel with the PTC resistor. The microprocessor MCU outputs a signal to the relay drive circuit to control the coil RY in the relay, thereby controlling the closing or opening of the electromagnetic switch JK. The signal input ends of the three-phase power supply phase loss detection circuit are respectively connected to the three-phase input power supplies U, V, and W; It is characterized in that The protection method for automatic switching of the power supply is as follows: The microprocessor MCU judges the output pulse signal of the three-phase power supply phase loss detection circuit. If the period of the output pulse signal of the three-phase power supply phase loss detection circuit becomes larger or no pulse is generated, it is judged that there is an abnormal input signal in the three-phase power supply phase loss detection circuit, and the microprocessor MCU outputs a signal to the relay drive circuit to control the electromagnetic switch JK in the relay to open.

2. The protection method for automatic switching of power supply of a three-phase BLDC motor according to claim 1 is characterized in that: The microprocessor MCU judges whether there is a rising edge signal or a falling edge signal in the output signal of the three-phase power supply phase loss detection circuit every time interval T2. If detected, it belongs to the normal power supply state; if not detected, it is in the power-off state.

3. The protection method for automatic switching of power supply of a three-phase BLDC motor according to claim 2 is characterized in that: When the microprocessor MCU judges that there is a rising edge signal or a falling edge signal in the output signal of the three-phase power supply phase loss detection circuit, it is also necessary to judge the time period between adjacent rising edge signals and falling edge signals. If the detected time period becomes longer, it is judged to be in the phase loss state, and the microprocessor MCU outputs a corresponding signal for protection.

4. The protection method for automatic switching of power supply of a three-phase BLDC motor according to claim 2 or 3, characterized in that: The time interval T2 satisfies: T0 / 2 < T2 < T0, where T0 is the period of the square wave signal output by the three-phase power supply phase loss detection circuit when the three-phase input power supplies U, V, and W are input normally.

5. The protection method for automatic switching of power supply of a three-phase BLDC motor according to claim 4 is characterized in that: The three-phase power supply phase loss detection circuit includes a U-phase detection circuit, a V-phase detection circuit, a W-phase detection circuit, and a comparison circuit two. The circuit structures of the U-phase detection circuit, the V-phase detection circuit, and the W-phase detection circuit are the same, and each includes a rectification circuit, a step-down circuit, and a comparison circuit one. The input end of the U-phase detection circuit is connected to the U phase of the three-phase input power supply. The input end of the V-phase detection circuit is connected to the V phase of the three-phase input power supply. The input end of the W-phase detection circuit is connected to the W phase of the three-phase input power supply. The output ends of the U-phase detection circuit, the V-phase detection circuit, and the W-phase detection circuit are connected together as the signal input end of the comparison circuit two. The signal output end Vdp of the comparison circuit two is connected to the signal input end of the microprocessor MCU.

6. The protection method for automatic switching of power supply of a three-phase BLDC motor according to claim 5 is characterized in that: The output ends of the U-phase detection circuit, the V-phase detection circuit, and the W-phase detection circuit are connected together and stabilized by a voltage stabilization circuit.

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