Compensation System and Method for Zero-Crossing Voltage of Back Electromotive Force of Brushless DC Motor

Abstract

The present invention discloses a compensation system and method for the back electromotive force zero-crossing voltage of a brushless DC motor. First, a bus current threshold is preset in the voltage compensation module to divide the voltage compensation reference. Secondly, the voltage compensation module compares the bus current value with the bus current threshold to determine whether the bus current value exceeds the bus current threshold. Then, the voltage compensation module outputs a corresponding voltage compensation signal according to the comparison result of the bus current value and the bus current threshold. Finally, the controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, that is, raises the judgment reference of the back electromotive force zero-crossing voltage to make up for the back electromotive force zero-crossing voltage step-down offset caused by excessive bus current, thereby improving the commutation accuracy of the brushless DC motor.

Description

Technical Field

[0001] The present invention relates to the field of brushless motor control, and particularly to a compensation system and method for the back electromotive force zero-crossing voltage of a brushless DC motor.

Background Art

[0002] A brushless DC motor (BLDCM) consists of a motor body and a driver, and is a typical mechatronic product. Due to its own advantages such as high power density, high efficiency, low maintenance cost, simple structure, and easy control, it is widely used in various fields such as automobiles, tools, industrial industrial control, automation, and aerospace, etc.

[0003] In recent years, the sensorless control of brushless DC motors has been a research hotspot at home and abroad. The relatively common rotor position signal detection methods include the back electromotive force method, the stator inductance method, the freewheeling diode method, the magnetic flux estimation method, and the state observer method, etc. Among them, the back electromotive force method is the most effective and practical.

[0004] A brushless DC motor generally adopts a "two-phase conduction three-phase six-state" operation mode. The back electromotive force method obtains the back electromotive force zero-crossing point by comparing the voltages of the two conducting phases with the voltage of the floating phase. For example, for three phases of U phase, V phase, and W phase, when U phase and V phase are conducting, then W phase is the floating phase. When the controller detects that the voltage of W phase (i.e., the back electromotive force) is equal to half of the sum of the voltages of U phase and V phase, the controller determines that the voltage of W phase passes through the zero point at this time (i.e., the back electromotive force zero-crossing point).

[0005] However, it is found in the actual use process of the brushless DC motor that as the load increases, the bus current will increase accordingly, which in turn causes the power supply voltage to drop sharply. This causes a voltage drop offset in the judgment reference of the back electromotive force zero-crossing voltage, that is, the sum of the voltages of the two conducting phases decreases, resulting in two disadvantages: (1) When the back electromotive force is on the rising edge, the controller detects the back electromotive force zero-crossing point in advance, resulting in the brushless DC motor commuting in advance; (2) When the back electromotive force is on the rising edge, the controller detects the back electromotive force zero-crossing point laggingly, resulting in the brushless DC motor commuting laggingly. Both disadvantages lead to inconsistent commutation times of the brushless DC motor, which in turn affects the accurate commutation of the brushless DC motor; in addition, the advance or lag of the brushless motor commutation will cause the bus current frequencies to be inconsistent, and in severe cases, it will cause abnormal operation performance of the brushless DC motor and reduce the working efficiency.

[0006] The existing invention patent No. CN109546903B discloses a compensation method for voltage sampling offset of a sensorless brushless DC motor. First, exclusive OR operations are performed on three virtual Hall signals Ta, Tb, and Tc to obtain Tpos. Secondly, the duration of Tpos being at a high level is calculated, and the duration of Tpos being at a low level is calculated. Then, the motor speed is calculated through the speed formula. Finally, the lag times of the three virtual Hall signals Ta, Tb, and Tc are calculated, so as to obtain new commutation signals Sa, Sb, and Sc to drive the brushless DC motor to operate. The above technical solution solves the problem that due to the insufficient sensitivity of the voltage sensor and the zero-point drift of the operational amplifier in the filtering circuit, the sampled line voltage difference or terminal voltage signal generates an offset, forming an unbalanced zero-crossing point. However, it does not solve the problem that when the load increases, the bus current increases accordingly, causing the power supply voltage to drop sharply, and further causing the judgment reference of the back electromotive force zero-crossing point voltage to have a step-down offset.

[0007] Therefore, it is necessary to design a compensation system and method for the back electromotive force zero-crossing point voltage of a brushless DC motor to solve the above problems.

Summary of the Invention

[0008] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a compensation system and method for the back electromotive force zero-crossing point voltage of a brushless DC motor, which solves the problem that when the load increases, the bus current increases accordingly, causing the power supply voltage to drop sharply, and further causing the judgment reference of the back electromotive force zero-crossing point voltage to have a step-down offset.

[0009] To achieve the above purpose, the present invention adopts the following technical solutions:

[0010] A compensation system for the back electromotive force zero-crossing point voltage of a brushless DC motor includes a DC power supply, a three-phase inverter, a back electromotive force detection module, and a controller. The DC power supply is connected to the three-phase inverter, the three-phase inverter is connected to the brushless DC motor, and the controller is connected to the three-phase inverter through the back electromotive force detection module. The back electromotive force detection module is used to detect the three-phase terminal voltages when the brushless DC motor is operating, and these three phases are two conducting phases and one floating phase. A current sampling module is connected between the DC power supply and the controller, and the current sampling module is used to collect the bus current value when the brushless DC motor is operating.

[0011] The controller includes a voltage compensation module connected to the back electromotive force detection module, and a bus current threshold is preset in the voltage compensation module.

[0012] The voltage compensation module is used to compare the magnitude of the bus current value with the bus current threshold, and output a corresponding voltage compensation signal according to the comparison result of the two.

[0013] The controller compensates the zero-crossing voltage of the back electromotive force of the brushless DC motor according to the voltage compensation signal, and is used to control the commutation of the brushless DC motor.

[0014] Further, the voltage compensation signal is the voltage proportionality coefficient of half of the sum of the voltages at the two conducting phase terminals, and the zero-crossing voltage of the back electromotive force of the brushless DC motor is equal to half of the sum of the voltages at the two conducting phase terminals multiplied by the voltage proportionality coefficient.

[0015] Further, when the bus current value is less than the bus current threshold, the voltage proportionality coefficient is 1, and the controller determines that the zero-crossing voltage of the back electromotive force of the brushless DC motor is half of the sum of the voltages at the two conducting phase terminals; when the bus current value is greater than or equal to the bus current threshold, the voltage proportionality coefficient is greater than 1, and the controller compensates the zero-crossing voltage of the back electromotive force of the brushless DC motor according to the voltage proportionality coefficient.

[0016] Further, the voltage compensation module includes an operation module and a storage unit. The operation module is connected to the current sampling module and the back electromotive force detection module, and the storage unit is connected to the operation module.

[0017] Further, the operation module is used to compare the magnitude of the bus current value with the bus current threshold and output a corresponding voltage compensation signal according to the comparison result of the bus current value and the bus current threshold.

[0018] Further, the bus current threshold is preset in the storage unit.

[0019] Further, when the floating phase terminal voltage is equal to half of the sum of the voltages at the two conducting phase terminals, the controller determines the zero-crossing point of the back electromotive force of the brushless DC motor and executes the zero-crossing commutation program, where the floating phase terminal voltage is the back electromotive force of the brushless DC motor.

[0020] Further, the zero-crossing commutation program is a commutation operation for the three phases of the brushless DC motor, that is, after the operation module detects the zero-crossing point of the back electromotive force of the brushless DC motor, according to the commutation point lagging behind the zero-crossing point by 30° electrical angle, the controller sets a corresponding delay time, and when the delay time arrives, the brushless DC motor commutes to the next working state.

[0021] The present invention provides a method for compensating the zero-crossing voltage of the back electromotive force of a brushless DC motor. The compensation method is applied to a compensation system for the zero-crossing voltage of the back electromotive force of the brushless DC motor, and includes the following steps:

[0022] S1: The brushless DC motor operates, and the current sampling module collects the bus current value and transmits it to the controller;

[0023] S2: The controller collects the three-phase terminal voltages of the brushless DC motor in real time through the back electromotive force detection module, and calculates half of the sum of the terminal voltages of the two conducting phases.

[0024] S3: The controller detects in real time whether the floating-phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, and at the same time, the controller judges in real time whether the bus current value reaches the bus current threshold.

[0025] S4: When the bus current value is less than the bus current threshold, the controller judges that the back electromotive force zero-crossing voltage of the brushless DC motor is half of the sum of the terminal voltages of the two conducting phases, and enters S6.

[0026] S5: When the bus current value is greater than or equal to the bus current threshold, the controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage proportionality coefficient, and enters S7.

[0027] S6: When the controller judges that the floating-phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, the controller judges that the back electromotive force of the brushless DC motor passes through the zero-crossing point, executes the zero-crossing commutation program, and enters S8.

[0028] S7: The voltage compensation module multiplies half of the sum of the terminal voltages of the two conducting phases by the voltage proportionality coefficient to compensate the back electromotive force zero-crossing voltage of the brushless DC motor. When the controller judges that the floating-phase terminal voltage is equal to the compensated back electromotive force zero-crossing voltage of the brushless DC motor, the controller judges that the back electromotive force of the brushless DC motor passes through the zero-crossing point, executes the zero-crossing commutation program, and enters S8.

[0029] S8: After the controller detects that the back electromotive force of the brushless DC motor passes through the zero-crossing point, according to the commutation point lagging behind the zero-crossing point by 30° electrical angle, the controller sets the corresponding delay time. When the delay time arrives, the brushless DC motor commutates and enters the next working state.

[0030] The present invention also relates to a compensation system for the back electromotive force zero-crossing voltage of a brushless DC motor, including a DC power supply, a three-phase inverter, a back electromotive force detection module and a controller. The DC power supply is connected to the three-phase inverter, the three-phase inverter is connected to the brushless DC motor, and the controller is connected to the three-phase inverter through the back electromotive force detection module; the back electromotive force detection module is used to detect the three-phase terminal voltages when the brushless DC motor is running, and the three phases are two conducting phases and one floating phase; a current sampling module is connected between the DC power supply and the controller, and the current sampling module is used to collect the bus current value when the brushless DC motor is running.

[0031] The controller includes a voltage compensation module connected to the back electromotive force detection module, and multiple groups of bus current threshold intervals are preset in the voltage compensation module.

[0032] The voltage compensation module is used to judge a set of bus current threshold intervals where the bus current value is located, and output a corresponding voltage compensation signal according to the set of bus current threshold intervals;

[0033] The controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, and is used to control the brushless DC motor to perform commutation.

[0034] Further, each set of bus current threshold intervals respectively corresponds to a set of voltage compensation signals. The voltage compensation signal is the voltage proportionality coefficient of half of the sum of the terminal voltages of the two conducting phases. The back electromotive force zero-crossing voltage of the brushless DC motor is equal to half of the sum of the terminal voltages of the two conducting phases multiplied by the voltage proportionality coefficient.

[0035] Further, the bus current threshold intervals include a low threshold interval and a high threshold interval. When the bus current value is in the low threshold interval, the voltage proportionality coefficient is 1; when the bus current value is in the high threshold interval, the voltage proportionality coefficient is greater than 1.

[0036] Further, the high threshold interval includes a first threshold interval, a second threshold interval and a third threshold interval. The voltage proportionality coefficients include a first proportionality coefficient, a second proportionality coefficient and a third proportionality coefficient. The first threshold interval corresponds to the first proportionality coefficient, the second threshold interval corresponds to the second proportionality coefficient, and the third threshold interval corresponds to the third proportionality coefficient.

[0037] Further, the first threshold interval, the second threshold interval and the third threshold interval show an increasing trend, and the first proportionality coefficient, the second proportionality coefficient and the third proportionality coefficient show an increasing trend.

[0038] Further, when the bus current value is in the low threshold interval, the controller judges that the back electromotive force zero-crossing voltage of the brushless DC motor is half of the sum of the terminal voltages of the two conducting phases;

[0039] When the bus current value is in the first threshold interval, the controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the first proportionality coefficient;

[0040] When the bus current value is in the second threshold interval, the controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the second proportionality coefficient;

[0041] When the bus current value is in the third threshold interval, the controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the third proportionality coefficient;

[0042] When the busbar current value exceeds the high threshold range, the controller controls the brushless DC motor to stop for overcurrent protection.

[0043] Further, when the floating-phase terminal voltage is equal to half of the sum of the two conducting-phase terminal voltages, the controller determines the zero-crossing point of the back electromotive force of the brushless DC motor and executes the zero-crossing commutation program, where the floating-phase terminal voltage is the back electromotive force of the brushless DC motor.

[0044] Further, the zero-crossing commutation program is to perform a commutation operation on the three phases of the brushless DC motor, that is, after the operation module detects the zero-crossing point of the back electromotive force of the brushless DC motor, according to the commutation point lagging the zero-crossing point by 30° electrical angle, the controller sets the corresponding delay time, and when the delay time arrives, the brushless DC motor commutates to enter the next working state.

[0045] The present invention also provides a method for compensating the zero-crossing point voltage of the back electromotive force of a brushless DC motor, and the compensation method is applied to the compensation system for the zero-crossing point voltage of the back electromotive force of the brushless DC motor, including the following steps:

[0046] S1: The brushless DC motor operates, and the current sampling module collects the busbar current value and transmits it to the controller;

[0047] S2: The controller collects the three-phase terminal voltages of the brushless DC motor in real time through the back electromotive force detection module and calculates half of the sum of the two conducting-phase terminal voltages;

[0048] S3: The controller detects in real time whether the floating-phase terminal voltage is equal to half of the sum of the two conducting-phase terminal voltages, and at the same time the controller judges in real time the busbar current threshold range where the busbar current value is located;

[0049] S4: When the busbar current value is in the low threshold range, the controller determines that the zero-crossing point voltage of the back electromotive force of the brushless DC motor is half of the sum of the two conducting-phase terminal voltages and enters S7;

[0050] S5: When the busbar current value is in the high threshold range, the controller compensates the zero-crossing point voltage of the back electromotive force of the brushless DC motor according to the voltage compensation signal and enters S8;

[0051] S6: When the busbar current value exceeds the high threshold range, the controller controls the brushless DC motor to stop for overcurrent protection;

[0052] S7: When the controller determines that the floating-phase terminal voltage is equal to half of the sum of the two conducting-phase terminal voltages, the controller determines the zero-crossing point of the back electromotive force of the brushless DC motor, executes the zero-crossing commutation program, and enters S9;

[0053] S8: The voltage compensation module multiplies half of the sum of the voltages at the two conducting phase terminals by the voltage ratio coefficient to compensate for the back electromotive force zero-crossing voltage of the brushless DC motor. When the controller determines that the voltage at the floating phase terminal is equal to the compensated back electromotive force zero-crossing voltage of the brushless DC motor, the controller determines that the back electromotive force of the brushless DC motor passes through the zero-crossing point, executes the zero-crossing commutation program, and enters S9;

[0054] S9: After the controller detects the zero-crossing point of the back electromotive force of the brushless DC motor, according to the commutation point lagging behind the zero-crossing point by 30° electrical angle, the controller sets the corresponding delay time. When the delay time arrives, the brushless DC motor commutates and enters the next working state.

[0055] Compared with the prior art, the present invention has the following beneficial effects:

[0056] (1) Firstly, the present invention presets a bus current threshold value in the voltage compensation module to divide the voltage compensation reference; secondly, the voltage compensation module compares the bus current value with the bus current threshold value to determine whether the bus current value exceeds the bus current threshold value; then, the voltage compensation module outputs a corresponding voltage compensation signal according to the comparison result of the bus current value and the bus current threshold value; finally, the controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, that is, improves the judgment reference of the back electromotive force zero-crossing voltage to make up for the voltage drop offset of the back electromotive force zero-crossing voltage caused by the excessive bus current, thereby improving the commutation accuracy of the brushless DC motor.

[0057] (2) Firstly, the present invention presets multiple groups of bus current threshold value intervals in the voltage compensation module to further divide the voltage compensation reference; secondly, the voltage compensation module determines the group of bus current threshold value intervals where the bus current value is located and outputs a corresponding voltage compensation signal according to the group of bus current threshold value intervals to further improve the voltage compensation accuracy; finally, the controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, that is, further improves the judgment reference of the back electromotive force zero-crossing voltage to make up for the voltage drop offset of the back electromotive force zero-crossing voltage caused by the excessive bus current, thereby further improving the commutation accuracy of the brushless DC motor.

Description of the Drawings

[0058] The following further describes in detail the specific embodiments of the present invention with reference to the drawings:

[0059] Figure 1 is a comparison diagram of the back electromotive force waveforms of three phases U, V, and W and the bus current during the operation of a brushless DC motor in the prior art;

[0060] Figure 2 is the circuit structure diagram of the compensation system for the back electromotive force zero-crossing voltage of the brushless DC motor of the present invention.

[0061] Figure 3It is the first flowchart of the compensation method for the back electromotive force zero-crossing voltage of the brushless DC motor of the present invention;

[0062] Figure 4 It is the second flowchart of the compensation method for the back electromotive force zero-crossing voltage of the brushless DC motor of the present invention;

[0063] Figure 5 It is the comparison diagram of the back electromotive force waveforms of three phases (U phase, V phase, and W phase) and the bus current when the brushless DC motor of the present invention is running.

[0064] Meanings of the reference numerals in the figure:

[0065] 1. DC power supply; 2. Three-phase inverter; 3. Back electromotive force detection module; 4. Controller; 5. Brushless DC motor; 6. Current sampling module; 7. Voltage compensation module; 8. Operation module; 9. Storage

[0066] unit

Detailed implementation manners

[0067] The present invention will be further described in detail below in conjunction with the drawings and implementation manners.

[0068] Refer to Figure 1 As shown, taking the floating phase W phase as an example, the intersection points n and h of the straight line where the horizontal coordinate ωt is located and the back electromotive force waveform of the W phase represent the zero-crossing points of the back electromotive force of the brushless DC motor judged by the controller in the prior art, that is, the points when the voltage at the W phase terminal is equal to half of the sum of the voltages at the U phase and V phase terminals; the dotted line a represents the voltage at the W phase terminal when the brushless DC motor is running with a relatively small bus current; the dotted line b represents the voltage at the W phase terminal when the brushless DC motor is running with a relatively large bus current. Among them, the method for judging the back electromotive force zero-crossing in the prior art is: when the voltage at the floating phase terminal is equal to half of the voltages at the two conducting phase terminals (that is, when the voltage at the W phase terminal is equal to half of the sum of the voltages at the U phase and V phase terminals), the controller judges that the back electromotive force is zero-crossing, and then performs subsequent commutation operations.

[0069] From Figure 1 it can be seen that when the bus current is relatively large, the three-phase terminal voltages are pulled down, that is, the sum of the voltages at the U phase and V phase terminals decreases, resulting in a lower judgment reference for the back electromotive force zero-crossing voltage. Just as Figure 1 the intersection points m and f of the dotted line b and the back electromotive force waveform of the W phase in the figure, the voltage at point m is detected significantly earlier, resulting in the brushless DC motor commuting in advance, while the voltage at point f is detected significantly later, resulting in the brushless DC motor commuting late. Eventually, it leads to inconsistent bus current frequencies. In severe cases, it will cause abnormal operating performance of the brushless DC motor, such as abnormal motor shutdown, uneven rotation speed, etc., greatly reducing the working efficiency.

[0070] Example 1: Refer to Figure 2A compensation system for the back electromotive force zero-crossing voltage of a brushless DC motor, as shown, includes a DC power supply 1, a three-phase inverter 2, a back electromotive force detection module 3 and a controller 4. The controller 4 generally uses an MCU controller. Among them, the DC power supply 1 is connected to the three-phase inverter 2, the three-phase inverter 2 is connected to the brushless DC motor 5, and the controller 4 is connected to the three-phase inverter 2 through the back electromotive force detection module 3; the back electromotive force detection module 3 is used to detect the three-phase terminal voltage when the brushless DC motor 5 is running, and the three phases are two conducting phases and one floating phase; a current sampling module 6 is connected between the DC power supply 1 and the controller 4, and the current sampling module 6 is used to collect the bus current value when the brushless DC motor 5 is running; the controller 4 includes a voltage compensation module 7 connected to the back electromotive force detection module 3. A bus current threshold is preset in the voltage compensation module 7, and this bus current threshold is the benchmark for determining whether the back electromotive force zero-crossing voltage of the brushless DC motor needs to be compensated.

[0071] Among them, the three-phase inverter 2 includes upper bridge switching tubes Q1~Q3 and lower bridge switching tubes Q4~Q6. The drains of the upper bridge switching tubes Q1~Q3 are all electrically connected to the positive pole of the DC power supply 1, and the sources of the lower bridge switching tubes Q4~Q6 are all electrically connected to the negative pole of the DC power supply 1. The source of the upper bridge switching tube Q1 is electrically connected to the U phase, the source of the upper bridge switching tube Q2 is electrically connected to the V phase, the source of the upper bridge switching tube Q3 is electrically connected to the W phase, and the gates of the upper bridge switching tubes Q1~Q3 and the lower bridge switching tubes Q4~Q6 are all electrically connected to the controller 4; the back electromotive force detection module 3 includes voltage dividing resistors R1~R6. The voltage dividing resistor R1 is connected in series with the voltage dividing resistor R2, and the upper end of the voltage dividing resistor R1 is electrically connected between the source of the switching tube Q1 and the U phase, and the lower end of the voltage dividing resistor R2 is grounded; the voltage dividing resistor R3 is connected in series with the voltage dividing resistor R4, and the upper end of the voltage dividing resistor R3 is electrically connected between the source of the switching tube Q2 and the V phase, and the lower end of the voltage dividing resistor R4 is grounded; the voltage dividing resistor R5 is connected in series with the voltage dividing resistor R6, and the upper end of the voltage dividing resistor R5 is electrically connected between the source of the switching tube Q3 and the W phase, and the lower end of the voltage dividing resistor R6 is grounded; between the voltage dividing resistors R1 and R2, between R3 and R4, and between R5 and R6 are all electrically connected to the controller 4; the controller 4 calculates the terminal voltages Ua, Ub, and Uc of the U phase, V phase, and W phase after voltage division by the voltage dividing resistors R1~R6, and calculates the terminal voltages Uu, Uv, and Uw of the U phase, V phase, and W phase and stores them in the storage unit 9. Half of the terminal voltage of the two conducting phases is: (Uu + Uv) / 2 or (Uu + Uw) / 2 or (Uv + Uw) / 2, where Uu, Uv, and Uw represent the voltages of the three-phase lines of the U phase, V phase, and W phase of the brushless DC motor to the ground.

[0072] In this embodiment, the voltage compensation module 7 is used to compare the bus current value with the bus current threshold, and output a corresponding voltage compensation signal according to the comparison result of the two; the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, and is used to control the brushless DC motor 5 to perform commutation.

[0073] In this embodiment, the voltage compensation signal is the voltage proportionality coefficient of half of the sum of the voltages at the two conducting phase terminals. The back electromotive force zero-crossing voltage of the brushless DC motor is equal to half of the sum of the voltages at the two conducting phase terminals multiplied by the voltage proportionality coefficient. When the bus current value is less than the bus current threshold, the voltage proportionality coefficient is 1, and the controller 4 determines that the back electromotive force zero-crossing voltage of the brushless DC motor is half of the sum of the voltages at the two conducting phase terminals. When the bus current value is greater than or equal to the bus current threshold, the voltage proportionality coefficient is greater than 1, and the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage proportionality coefficient.

[0074] Continue to refer to Figure 1 As shown, the voltage compensation module 7 includes an operation module 8 and a storage unit 9. The operation module 8 is connected to the current sampling module 6 and the back electromotive force detection module 3, and the storage unit 9 is connected to the operation module 8. Among them, the operation module 8 is used to compare the bus current value with the bus current threshold and output a corresponding voltage compensation signal according to the comparison result of the bus current value and the bus current threshold. The storage unit 9 pre-sets a bus current threshold, and this bus current threshold is the benchmark for dividing whether the back electromotive force zero-crossing voltage of the brushless DC motor needs to be compensated.

[0075] In this embodiment, when the floating phase terminal voltage is equal to half of the sum of the voltages at the two conducting phase terminals, the controller 4 determines the back electromotive force zero-crossing of the brushless DC motor and executes the zero-crossing commutation program. Among them, the floating phase terminal voltage is the back electromotive force of the brushless DC motor 5. The zero-crossing commutation program is to perform a commutation operation on the three phases of the brushless DC motor 5. That is, after the operation module 8 detects the back electromotive force zero-crossing of the brushless DC motor, according to the commutation point lagging the zero-crossing by 30° electrical angle, the controller 4 sets a corresponding delay time. When the delay time arrives, the brushless DC motor 5 commutates and enters the next working state.

[0076] This reference Figure 3 As shown in a compensation method for the back electromotive force zero-crossing voltage of a brushless DC motor, this compensation method is applied to a compensation system for the back electromotive force zero-crossing voltage of a brushless DC motor, and includes the following steps:

[0077] S1: The brushless DC motor 5 operates, and the current sampling module 6 collects the bus current value and transmits it to the controller 4;

[0078] S2: The controller 4 collects the three-phase terminal voltages of the brushless DC motor 5 in real time through the back electromotive force detection module 3 and calculates half of the sum of the voltages at the two conducting phase terminals;

[0079] S3: The controller 4 detects in real time whether the floating phase terminal voltage is equal to half of the sum of the voltages at the two conducting phase terminals, and at the same time the controller 4 determines in real time whether the bus current value reaches the bus current threshold;

[0080] S4: When the bus current value is less than the bus current threshold, the controller 4 determines that the back electromotive force (EMF) zero-crossing voltage of the brushless DC motor is half of the sum of the voltages of the two conducting phase terminals, and enters S6;

[0081] S5: When the bus current value is greater than or equal to the bus current threshold, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage proportionality coefficient, and enters S7;

[0082] S6: When the controller 4 determines that the floating phase terminal voltage is equal to half of the sum of the voltages of the two conducting phase terminals, the controller 4 determines that the back electromotive force of the brushless DC motor passes through the zero-crossing point, executes the zero-crossing commutation program, and enters S8;

[0083] S7: The voltage compensation module 7 multiplies the sum of the voltages of the two conducting phase terminals by the voltage proportionality coefficient to compensate the back electromotive force zero-crossing voltage of the brushless DC motor. When the controller 4 determines that the floating phase terminal voltage is equal to the compensated back electromotive force zero-crossing voltage of the brushless DC motor, the controller 4 determines that the back electromotive force of the brushless DC motor passes through the zero-crossing point, executes the zero-crossing commutation program, and enters S8;

[0084] S8: After the controller 4 detects that the back electromotive force of the brushless DC motor passes through the zero-crossing point, according to the commutation point lagging behind the zero-crossing point by 30° electrical angle, the controller 4 sets the corresponding delay time. When the delay time arrives, the brushless DC motor 5 commutates to enter the next working state.

[0085] In this embodiment, the compensation principle of the back electromotive force zero-crossing voltage of the brushless DC motor is as follows:

[0086] Assume that the bus current threshold is 40 A; the three phases are denoted as the U phase, V phase, and W phase, and the three-phase terminal voltages are denoted as Uu, Uv, and Uw; the voltage proportionality coefficient greater than 1 is 110%; the power supply voltage is 16 V, and the voltage drop of the MOS tube is ignored. Then, half of the sum of the voltages of the two conducting phase terminals is 8 V; when the bus current value is greater than 40 A, the power supply voltage is 14.4 V, and half of the sum of the voltages of the two conducting phase terminals is 7.2 V. The specific compensation method is as follows:

[0087] S1: When the brushless DC motor 5 is running, the current sampling module 6 real-time collects the bus current value and transmits it to the controller 4;

[0088] S2: The controller 4 real-time collects the U-phase, V-phase, and W-phase terminal voltages Uu, Uv, and Uw of the brushless DC motor through the back electromotive force detection module 3, and calculates half of the sum of the voltages of the two conducting phase terminals. Assume that at this time, the U phase and V phase are the two conducting phases, and the W phase is the floating phase. Then, half of the sum of the voltages of the two conducting phase terminals is (Uu + Uv) / 2 = 8 V;

[0089] S3: The controller 4 continuously detects whether the W-phase terminal voltage Uw is equal to 8V, and at the same time, the controller 4 continuously determines whether the bus current value reaches 40A;

[0090] S4: When the bus current value is less than 40A, the controller 4 determines that the back electromotive force zero-crossing voltage of the brushless DC motor is 8V; when the controller 4 detects that Uw is equal to 8V, the controller 4 determines that the back electromotive force of the brushless DC motor passes through zero, and executes the zero-crossing commutation program;

[0091] S5: When the bus current value is greater than or equal to 40A, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage proportionality coefficient of 110%, that is, the back electromotive force zero-crossing voltage of the brushless DC motor is 7.2V * 110% = 7.92V at this time. When the controller 4 determines that Uw is equal to 7.92V, the controller 4 determines that the back electromotive force of the brushless DC motor passes through zero, and executes the zero-crossing commutation program.

[0092] Among them, the voltage proportionality coefficient can be adjusted according to the amplitude of the voltage drop between the two conducting phase terminal voltages during the actual use of the brushless DC motor 5 to ensure that the back electromotive force zero-crossing voltage of the brushless DC motor approaches 8V.

[0093] In this embodiment, first, a bus current threshold is preset in the voltage compensation module 7 to divide the voltage compensation reference; second, the voltage compensation module 7 compares the bus current value with the bus current threshold to determine whether the bus current value exceeds the bus current threshold; then the voltage compensation module 7 outputs a corresponding voltage compensation signal according to the comparison result of the bus current value and the bus current threshold; finally, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, that is, increases the judgment reference of the back electromotive force zero-crossing voltage to make up for the back electromotive force zero-crossing voltage drop offset caused by the excessive bus current, thereby improving the commutation accuracy of the brushless DC motor 5.

[0094] Among them, after the compensation method of the above-mentioned Embodiment 1, the three-phase back electromotive force waveforms of the U-phase, V-phase, and W-phase during the operation of the brushless DC motor 5 are significantly regular and stable, the durations of the rising edge stage S1 and the falling edge stage S2 of the back electromotive force also tend to be the same, and at the same time, the bus current is also significantly stable and orderly, as specifically shown in Figure 5 shown.

[0095] Embodiment 2: Refer to Figure 2A compensation system for the back electromotive force zero-crossing voltage of a brushless DC motor, as shown, includes a DC power supply 1, a three-phase inverter 2, a back electromotive force detection module 3, and a controller 4. The DC power supply 1 is connected to the three-phase inverter 2, the three-phase inverter 2 is connected to a brushless DC motor 5, and the controller 4 is connected to the three-phase inverter 2 through the back electromotive force detection module 3. The back electromotive force detection module 3 is used to detect the three-phase terminal voltage when the brushless DC motor 5 is running, and the three phases are two conducting phases and one floating phase. A current sampling module 6 is connected between the DC power supply 1 and the controller 4, and the current sampling module 6 is used to collect the bus current value when the brushless DC motor 5 is running. The controller 4 includes a voltage compensation module 7 connected to the back electromotive force detection module 3, and multiple groups of bus current threshold intervals are preset in the voltage compensation module 7. The voltage compensation module 7 is used to determine a group of bus current threshold intervals where the bus current value is located, and output a corresponding voltage compensation signal according to this group of bus current threshold intervals. The controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, and is used to control the brushless DC motor 5 to perform commutation.

[0096] Among them, each group of bus current threshold intervals corresponds to a group of voltage compensation signals respectively. The voltage compensation signal is the voltage proportionality coefficient of half of the sum of the terminal voltages of the two conducting phases. The back electromotive force zero-crossing voltage of the brushless DC motor is equal to half of the sum of the terminal voltages of the two conducting phases multiplied by the voltage proportionality coefficient, so as to further divide the voltage compensation reference and make the compensation of the back electromotive force zero-crossing voltage of the brushless DC motor more accurate.

[0097] In this embodiment, the bus current threshold intervals include a low threshold interval and a high threshold interval. When the bus current value is in the low threshold interval, the voltage proportionality coefficient is 1; when the bus current value is in the high threshold interval, the voltage proportionality coefficient is greater than 1.

[0098] Among them, the high threshold interval includes a first threshold interval, a second threshold interval, and a third threshold interval. The voltage proportionality coefficients include a first proportionality coefficient, a second proportionality coefficient, and a third proportionality coefficient. The first threshold interval corresponds to the first proportionality coefficient, the second threshold interval corresponds to the second proportionality coefficient, and the third threshold interval corresponds to the third proportionality coefficient; and the first threshold interval, the second threshold interval, and the third threshold interval show an increasing trend, and the first proportionality coefficient, the second proportionality coefficient, and the third voltage coefficient show an increasing trend.

[0099] In this embodiment, when the bus current value is in the low threshold range, the controller 4 determines that the back electromotive force zero-crossing voltage of the brushless DC motor is half of the sum of the terminal voltages of the two conducting phases; when the bus current value is in the high threshold range, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal; and when the bus current value exceeds the high threshold range, the controller 4 controls the brushless DC motor 5 to stop for overcurrent protection. Among them, when the bus current value is in the first threshold range, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the first proportionality coefficient; when the bus current value is in the second threshold range, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the second proportionality coefficient; when the bus current value is in the third threshold range, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the third proportionality coefficient.

[0100] In this embodiment, when the floating-phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, the controller 4 determines the zero-crossing point of the back electromotive force of the brushless DC motor and executes the zero-crossing commutation program. Among them, the floating-phase terminal voltage is the back electromotive force of the brushless DC motor 5. The zero-crossing commutation program is to perform commutation operations on the three phases of the brushless DC motor 5, that is, after the operation module 8 detects the zero-crossing point of the back electromotive force of the brushless DC motor, according to the commutation point lagging behind the zero-crossing point by 30° electrical angle, the controller 4 sets the corresponding delay time. When the delay time arrives, the brushless DC motor 5 commutates and enters the next working state.

[0101] Refer to Figure 4 A compensation method for the back electromotive force zero-crossing voltage of a brushless DC motor shown below. This compensation method is applied to a compensation system for the back electromotive force zero-crossing voltage of a brushless DC motor and includes the following steps:

[0102] S1: The brushless DC motor 5 operates, and the current sampling module 6 collects the bus current value and transmits it to the controller 4;

[0103] S2: The controller 4 collects the three-phase terminal voltages of the brushless DC motor 5 in real time through the back electromotive force detection module 3 and calculates half of the sum of the terminal voltages of the two conducting phases;

[0104] S3: The controller 4 detects in real time whether the floating-phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, and at the same time the controller 4 determines in real time the bus current threshold range where the bus current value is located;

[0105] S4: When the bus current value is in the low threshold range, the controller 4 determines that the back electromotive force zero-crossing voltage of the brushless DC motor is half of the sum of the terminal voltages of the two conducting phases and enters S7;

[0106] S5: When the bus current value is in the high threshold range, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal and enters S8;

[0107] S6: When the bus current value exceeds the high threshold range, the controller 4 controls the brushless DC motor 5 to stop for overcurrent protection;

[0108] S7: When the controller 4 determines that the floating-phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, the controller 4 determines that the back electromotive force of the brushless DC motor passes through the zero point, executes the commutation program at the zero crossing, and enters S9;

[0109] S8: The voltage compensation module 7 multiplies the sum of the terminal voltages of the two conducting phases by the voltage proportionality coefficient to compensate the voltage at the zero crossing of the back electromotive force of the brushless DC motor. When the controller 4 determines that the floating-phase terminal voltage is equal to the compensated voltage at the zero crossing of the back electromotive force of the brushless DC motor, the controller 4 determines that the back electromotive force of the brushless DC motor passes through the zero point, executes the commutation program at the zero crossing, and enters S9;

[0110] S9: After the controller 4 detects that the back electromotive force of the brushless DC motor passes through the zero point, according to the commutation point lagging behind the zero crossing by 30° electrical angle, the controller 4 sets the corresponding delay time. When the delay time arrives, the brushless DC motor 5 commutates to enter the next working state.

[0111] In this embodiment, the compensation principle of the voltage at the zero crossing of the back electromotive force of the brushless DC motor is as follows:

[0112] Assume that the low threshold range is: (0A, 40A], and the high threshold range is: (40A, 80A]; the first threshold range is (40A, 50A], corresponding to the first voltage proportionality coefficient of 110%; the second threshold range is: (50A, 60A], corresponding to the second voltage proportionality coefficient of 115%; the third threshold range is: (60A, 70A], corresponding to the third voltage proportionality coefficient of 120%; the power supply voltage is 16V, and the voltage drop of the MOS tube is ignored, so half of the sum of the terminal voltages of the two conducting phases is 8V; the specific compensation method is as follows:

[0113] (1) When the bus current value is in the low threshold range (0A, 40A], the voltage at the zero crossing of the back electromotive force of the brushless DC motor does not need to be compensated, and the voltage proportionality coefficient at this time is 1. Then, when the controller 4 detects that the floating phase is equal to 8V, the controller 4 determines that the back electromotive force of the brushless DC motor passes through the zero point and executes the commutation program at the zero crossing;

[0114] (2) When the bus current value is in the first threshold range (40A, 50A], the power supply voltage is 14.4V, so half of the sum of the terminal voltages of the two conducting phases is 7.2V. After compensation by the first voltage proportionality coefficient, the voltage at the zero crossing of the back electromotive force of the brushless DC motor is 7.2V * 110% = 7.92V, approaching 8V;

[0115] (3) When the busbar current value is in the second threshold range (50A, 60A], the power supply voltage is 13.6V. Then, half of the sum of the terminal voltages of the two conducting phases is 6.8V. After compensation by the second voltage ratio coefficient, the back electromotive force zero-crossing voltage of the brushless DC motor is 6.8V * 115% = 7.82V, approaching 8V;

[0116] (4) When the busbar current value is in the third threshold range (60A, 70A], the power supply voltage is 12.8V. Then, half of the sum of the terminal voltages of the two conducting phases is 6.4V. After compensation by the third voltage ratio coefficient, the back electromotive force zero-crossing voltage of the brushless DC motor is 6.4V * 120% = 7.68V, approaching 8V;

[0117] (5) When the busbar voltage value exceeds 70A, the controller 4 controls the brushless DC motor 5 to perform overcurrent shutdown protection.

[0118] Among them, the voltage ratio coefficient can be adjusted according to the amplitude of the voltage drop of the terminal voltages of the two conducting phases during the actual use of the brushless DC motor 5 to ensure that the back electromotive force zero-crossing voltage of the brushless DC motor approaches 8V.

[0119] The compensation method in this embodiment further optimizes the compensation method of Embodiment 1. First, multiple groups of busbar current threshold ranges are preset in the voltage compensation module 7 to further divide the voltage compensation reference. Secondly, the voltage compensation module 7 determines the group of busbar current threshold ranges where the busbar current value is located and outputs a corresponding voltage compensation signal according to this group of busbar current threshold ranges to further improve the voltage compensation accuracy. Finally, the controller 4 compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, that is, further improves the judgment reference of the back electromotive force zero-crossing voltage to make up for the voltage drop offset of the back electromotive force zero-crossing voltage caused by excessive busbar current, thereby further improving the commutation accuracy of the brushless DC motor 5.

[0120] Among them, after the compensation method of the above Embodiment 2, the three-phase back electromotive force waveforms during the operation of the brushless DC motor 5 are more regular and more stable, the rising edge and falling edge durations of the back electromotive force are more consistent, and at the same time, the busbar current is also more stable and orderly.

[0121] The present invention is not limited to the above specific embodiments. Those of ordinary skill in the art can easily understand that, without departing from the principles and scope of the present invention, there are many other alternative solutions for the compensation system and method of the back electromotive force zero-crossing voltage of the brushless DC motor of the present invention. The protection scope of the present invention is subject to the content of the claims.

Claims

1. A compensation system for the back electromotive force zero-crossing voltage of a brushless DC motor, comprising a DC power supply, a three-phase inverter, a back electromotive force detection module and a controller. The DC power supply is connected to the three-phase inverter, the three-phase inverter is connected to the brushless DC motor, and the controller is connected to the three-phase inverter through the back electromotive force detection module. The back electromotive force detection module is used to detect the three-phase terminal voltages when the brushless DC motor is running, and the three phases are two conducting phases and one floating phase. It is characterized in that: A current sampling module is connected between the DC power supply and the controller, and the current sampling module is used to collect the bus current value when the brushless DC motor is running. The controller includes a voltage compensation module connected to the back electromotive force detection module, and a bus current threshold is preset in the voltage compensation module. The voltage compensation module is used to compare the magnitude of the bus current value with the bus current threshold, and output a corresponding voltage compensation signal according to the comparison result of the two. The controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage compensation signal, and is used to control the commutation of the brushless DC motor. Wherein, the voltage compensation signal is the voltage proportionality coefficient of half of the sum of the terminal voltages of the two conducting phases. When the bus current value is less than the bus current threshold, the voltage proportionality coefficient is 1, and the controller judges that the back electromotive force zero-crossing voltage of the brushless DC motor is half of the sum of the terminal voltages of the two conducting phases. When the bus current value is greater than or equal to the bus current threshold, the voltage proportionality coefficient is greater than 1, and the controller compensates the back electromotive force zero-crossing voltage of the brushless DC motor according to the voltage proportionality coefficient.

2. The compensation system for the back electromotive force zero-crossing voltage of a brushless DC motor according to claim 1, It is characterized in that: When the floating phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, the controller judges that the back electromotive force of the brushless DC motor passes through the zero point and executes the zero-crossing commutation program, wherein the floating phase terminal voltage is the back electromotive force of the brushless DC motor.

3. A compensation method for the back electromotive force zero-crossing voltage of a brushless DC motor, and the compensation method is applied to the compensation system for the back electromotive force zero-crossing voltage of a brushless DC motor according to any one of claims 1-2, It is characterized in that: It includes the following steps: S1: The brushless DC motor runs, and the current sampling module collects the bus current value. S2: The controller collects the three-phase terminal voltages of the brushless DC motor in real time through the back electromotive force detection module, and calculates half of the sum of the terminal voltages of the two conducting phases. S3: The controller detects in real time whether the floating phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, and at the same time the controller judges in real time whether the bus current value reaches the bus current threshold. S4: When the bus current value is less than the bus current threshold, the controller judges that the back electromotive force zero-crossing voltage of the brushless DC motor is half of the sum of the terminal voltages of the two conducting phases. When the controller judges that the floating phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, the controller judges that the back electromotive force of the brushless DC motor passes through the zero point and executes the zero-crossing commutation program. S5: When the bus current value is greater than or equal to the bus current threshold, the controller compensates the zero-crossing voltage of the back electromotive force of the brushless DC motor according to the voltage proportionality coefficient. When the controller determines that the floating-phase terminal voltage is equal to the compensated zero-crossing voltage of the back electromotive force of the brushless DC motor, the controller determines the zero-crossing of the back electromotive force of the brushless DC motor and executes the zero-crossing commutation program.

4. A compensation system for the zero-crossing voltage of the back electromotive force of a brushless DC motor, comprising a DC power supply, a three-phase inverter, a back electromotive force detection module and a controller. The DC power supply is connected to the three-phase inverter, the three-phase inverter is connected to the brushless DC motor, and the controller is connected to the three-phase inverter through the back electromotive force detection module. The back electromotive force detection module is used to detect the three-phase terminal voltages during the operation of the brushless DC motor, and the three phases are two conducting phases and one floating phase. Characterized in that: A current sampling module is connected between the DC power supply and the controller, and the current sampling module is used to collect the bus current value during the operation of the brushless DC motor. The controller includes a voltage compensation module connected to the back electromotive force detection module, and multiple groups of bus current threshold intervals are preset in the voltage compensation module. The voltage compensation module is used to determine a group of the bus current threshold intervals where the bus current value is located, and output a corresponding voltage compensation signal according to this group of bus current threshold intervals. The controller compensates the zero-crossing voltage of the back electromotive force of the brushless DC motor according to the voltage compensation signal, and is used to control the commutation of the brushless DC motor. Wherein, each group of the bus current threshold intervals corresponds to a group of voltage compensation signals, and the voltage compensation signal is the voltage proportionality coefficient of half of the sum of the two conducting-phase terminal voltages. The bus current threshold intervals include a low threshold interval and a high threshold interval. When the bus current value is in the low threshold interval, the voltage proportionality coefficient is 1. When the bus current value is in the high threshold interval, the voltage proportionality coefficient is greater than 1.

5. The compensation system for the zero-crossing voltage of the back electromotive force of a brushless DC motor according to claim 4, Characterized in that: The high threshold interval includes a first threshold interval, a second threshold interval and a third threshold interval. The voltage proportionality coefficients include a first proportionality coefficient, a second proportionality coefficient and a third proportionality coefficient. The first threshold interval corresponds to the first proportionality coefficient, the second threshold interval corresponds to the second proportionality coefficient, and the third threshold interval corresponds to the third proportionality coefficient.

6. The compensation system for the zero-crossing voltage of the back electromotive force of a brushless DC motor according to claim 5, Characterized in that: The first threshold interval, the second threshold interval and the third threshold interval show an increasing trend, and the first proportionality coefficient, the second proportionality coefficient and the third proportionality coefficient show an increasing trend.

7. The compensation system for the zero-crossing voltage of the back electromotive force of a brushless DC motor according to claim 6, Characterized in that: When the bus current value is in the low threshold interval, the controller determines that the zero-crossing voltage of the back electromotive force of the brushless DC motor is half of the sum of the two conducting-phase terminal voltages. When the busbar current value is within the first threshold range, the controller compensates the zero-crossing voltage of the back electromotive force of the brushless DC motor according to the first proportionality coefficient; When the busbar current value is within the second threshold range, the controller compensates the zero-crossing voltage of the back electromotive force of the brushless DC motor according to the second proportionality coefficient; When the busbar current value is within the third threshold range, the controller compensates the zero-crossing voltage of the back electromotive force of the brushless DC motor according to the third proportionality coefficient; When the busbar current value exceeds the high threshold range, the controller controls the brushless DC motor to stop for overcurrent protection.

8. The compensation system for the zero-crossing voltage of the back electromotive force of a brushless DC motor according to claim 4, characterized in that: When the floating-phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, the controller determines the zero-crossing point of the back electromotive force of the brushless DC motor and executes the zero-crossing commutation program, where the floating-phase terminal voltage is the back electromotive force of the brushless DC motor.

9. A method for compensating the zero-crossing voltage of the back electromotive force of a brushless DC motor, the compensation method being applied to the compensation system for the zero-crossing voltage of the back electromotive force of a brushless DC motor according to any one of claims 4-8, characterized in that: comprises the following steps: S1: The brushless DC motor runs, and the current sampling module samples the busbar current value; S2: The controller collects the three-phase terminal voltages of the brushless DC motor in real time through the back electromotive force detection module and calculates half of the sum of the terminal voltages of the two conducting phases; S3: The controller detects in real time whether the floating-phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, and at the same time the controller determines in real time the busbar current threshold range in which the busbar current value is located; S4: When the busbar current value is within the low threshold range, the controller determines that the zero-crossing voltage of the back electromotive force of the brushless DC motor is half of the sum of the terminal voltages of the two conducting phases. When the controller determines that the floating-phase terminal voltage is equal to half of the sum of the terminal voltages of the two conducting phases, the controller determines the zero-crossing point of the back electromotive force of the brushless DC motor and executes the zero-crossing commutation program; S5: When the busbar current value is within the high threshold range, the controller compensates the zero-crossing voltage of the back electromotive force of the brushless DC motor according to the voltage compensation signal. When the controller determines that the floating-phase terminal voltage is equal to the compensated zero-crossing voltage of the back electromotive force of the brushless DC motor, the controller determines the zero-crossing point of the back electromotive force of the brushless DC motor and executes the zero-crossing commutation program; S6: When the busbar current value exceeds the high threshold range, the controller controls the brushless DC motor to stop for overcurrent protection.

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