A method for detecting the internal power angle of a brushless DC motor that does not depend on motor parameters

Abstract

The present invention relates to a method for detecting the internal power angle of a brushless DC motor that is independent of motor parameters, and a method for detecting the internal power angle by using the integral relationship between the terminal voltage and the bus voltage. The method first finds the logical relationship between the integral difference between the terminal voltage and 1 / 2 bus voltage and the internal power angle before and after the back electromotive force passes through the zero point, and then mathematically expresses and verifies the values ​​of the two integrals, thereby detecting the size of the internal power angle. The method describes all the characteristics of the back electromotive force during the calculation process, including the fundamental wave and higher harmonics, and is suitable for the non-ideal back electromotive force of the actual motor and for most working conditions. The method does not involve the parameters of the motor, such as the internal resistance that increases with the temperature rise of the motor, and the inductance that decreases with the increase of the motor current, and has the advantage of strong adaptability. The method does not require the addition of additional hardware circuits, does not require a large number of sensors and phase detectors, and has a simple software algorithm. It lays a foundation for the subsequent control of the internal power angle and the improvement of the power density and efficiency of the motor.

Description

Technical Field

[0001] The invention belongs to a method for detecting an internal power angle of a brushless DC motor, and relates to a method for detecting an internal power angle of a brushless DC motor that is independent of motor parameters. Background Art

[0002] In order to improve the power density and efficiency of the brushless DC motor, it is necessary to ensure that the phase difference between the back EMF and the current is the same, that is, the internal power angle is zero. However, due to the error of the high-speed brushless motor without position detection, filter delay, and the motor itself is a first-order lag link, the motor internal power angle cannot be zero, which increases the current and copper loss during the operation of the motor, which is not conducive to improving the power density and efficiency of the motor. Therefore, the detection of the internal power angle is a key step in controlling and eliminating it.

[0003] Currently, there are several main schemes for detecting the internal power angle. The first is to detect the internal power angle through the phase difference between the rotor flux and the integral of the phase current. However, this method requires parameters such as motor inductance, which are sensitive and the motor inductance will change with the operating conditions. The second is to determine the internal power angle by using the Fourier series of the phase voltage waveform to determine the phase lead angle of the square wave waveform of the phase current delay. This method is complex in calculation and software design. Summary of the invention

[0004] Technical issues to be solved

[0005] In order to avoid the shortcomings of the prior art, the present invention proposes a brushless DC motor internal power angle detection method that is independent of motor parameters, has simple software and hardware, and has strong adaptability to working conditions.

[0006] Technical Solution

[0007] A method for detecting the internal power angle of a brushless DC motor that does not rely on motor parameters, characterized by the following steps:

[0008] Step 1: According to the working conditions of the high-speed brushless DC motor, derive the back electromotive force and terminal voltage equations:

[0009] The three-phase windings of the motor are symmetrical, and the Fourier expansion of the three-phase back electromotive force of the motor is:

[0010]

[0011] Where: A1 is the amplitude of the back EMF fundamental voltage, A 2n+1 is the amplitude of the 2n+1th voltage, θ is the electrical angle of the motor rotor;

[0012] When the upper tube of phase A and the lower tube of phase B are turned on and phase C is turned off, the three-phase terminal voltage u aM ,u bM and u cM It is expressed as:

[0013]

[0014] The power device is in saturated conduction state, i a =-i b ,u aM =U,u bM =0

[0015]

[0016] When the upper tube of phase C and the lower tube of phase B are turned on and phase A is turned off, the three-phase terminal voltage u aM ,u bM and u cM It is expressed as:

[0017]

[0018] The power device is in saturated conduction state, i b =-i c ,u bM =U,u cM =0,

[0019]

[0020] When the upper tube of phase C and the lower tube of phase A are turned on and phase B is turned off, the three-phase terminal voltage u aM ,u bM and u cM It is expressed as:

[0021]

[0022] The power device is in saturated conduction state, i a =-i c ,u aM =U,u cM =0

[0023]

[0024] Where: u NM is the neutral point voltage;

[0025] Step 2: Integrate the terminal voltage of the shut-off phase of one of the three phases according to the back electromotive force and terminal voltage equation:

[0026] Integrate the voltage at the off phase terminal before the back EMF crosses zero minus half the bus voltage, and the result is S1;

[0027] Integrate half of the bus voltage after the back EMF passes through zero minus the voltage at the turned-off phase terminal, and the result is S2:

[0028] Step 3: Determine the internal power angle based on the integral characteristics of the terminal voltage before and after the back electromotive force passes through zero:

[0029] When S1=S2, the current and the back EMF are in the same direction, and the internal work angle is 0;

[0030] When S1<S2, the phase current lags the back EMF, and the internal work angle is greater than zero;

[0031] When S1>S2, the phase current leads the back EMF and the internal work angle is less than zero.

[0032] Beneficial Effects

[0033] The present invention proposes a method for detecting the internal power angle of a brushless DC motor that is independent of motor parameters. First, according to the working conditions of the high-speed brushless DC motor in a working state, the back electromotive force and the three-phase terminal voltage equations are derived, and the terminal voltage characteristics are summarized; then, the difference between the terminal voltage and 1 / 2 bus voltage before and after the back electromotive force passes through a zero point is integrated to obtain two triangular areas S1 and S2 enclosed by the terminal voltage and 1 / 2 bus voltage; finally, the difference S1-S2 between the two integral results is calculated, and the relationship between the difference S1-S2 and the internal power angle is analyzed and summarized, so as to detect the internal power angle according to the integral result.

[0034] The beneficial effects of the present invention are as follows: the method describes all the characteristics of the back electromotive force during the calculation process, including the fundamental wave and higher harmonics, and is applicable to the non-ideal back electromotive force of the actual motor and to most working conditions. The method does not involve the parameters of the motor, such as the internal resistance that increases with the temperature rise of the motor and the inductance that decreases with the increase of the motor current, and has the advantage of strong adaptability. The method does not require the addition of additional hardware circuits, does not require a large number of sensors and phase detectors, and the software algorithm is simple. It lays the foundation for the subsequent control of the internal power angle and the improvement of the power density and efficiency of the motor. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a simplified circuit when the lower tube of phase C is turned off and the current is continued;

[0036] Figure 2 is the relationship between the back EMF and phase current when the motor is ideally commutated: where S1 is the area of ​​the triangle, U represents the bus voltage, and u cM represents the voltage at the phase C terminal, e c represents the opposite potential of C, i c Indicates the C phase current;

[0037] Figure 3 is the relationship between the back EMF and phase current when the internal power angle is zero: where S1 is the area of ​​the triangle, U represents the bus voltage, and u cM represents the voltage at the phase C terminal, e c represents the opposite potential of C, i c Indicates the C phase current.

[0038] Figure 4 When S1<S2, the internal power angle waveform is c Indicates the C phase current, u cM Indicates the voltage at the phase C terminal, u cZP Indicates the zero-crossing waveform of the C-phase voltage. is the inner power angle;

[0039] Figure 5 When S1=S2, the internal power angle waveform is: c Indicates the C phase current, u cM Indicates the voltage at the phase C terminal, u cZP Indicates the zero-crossing waveform of the C-phase voltage. Internal power angle DETAILED DESCRIPTION

[0040] The present invention will now be further described with reference to the embodiments and the accompanying drawings:

[0041] The present invention solves the technical problem through the following technical solutions:

[0042] This method first derives the back electromotive force, phase current, and terminal voltage equations of the high-speed brushless DC motor in working state.

[0043] Based on the terminal voltage equation, find out the pattern between the zero-crossing point of the back electromotive force and the zero-crossing point of the phase current.

[0044] Based on the terminal voltage equation, integral calculation is performed to derive the two triangular areas enclosed by the terminal voltage and 1 / 2 bus voltage before and after the back electromotive force passes through the zero point.

[0045] According to the two triangle area formulas, the relationship between them and the internal power angle is obtained, so that the internal power angle is detected according to the integration results.

[0046] The embodiment of the present invention detects the internal power angle based on the integral relationship between the terminal voltage and 1 / 2 bus voltage before and after the back electromotive force of the brushless DC motor passes through the zero point. The embodiment of the present invention is implemented in the following steps:

[0047] Step 1: Perform mathematical analysis on the characteristics of back EMF and phase current when the high-speed brushless DC motor is in operation.

[0048] The actual back EMF of the brushless DC motor can be expressed as the synthesis of the back EMF fundamental wave and multiple harmonics. Assuming that the three-phase winding of the motor is symmetrical, the Fourier expansion of the three-phase back EMF of the motor is:

[0049]

[0050] A1 is the amplitude of the back EMF fundamental voltage, A 2n+1 is the amplitude of the 2n+1th voltage, and θ is the electrical angle of the motor rotor.

[0051] The Fourier expansion of the current can be expressed as

[0052]

[0053] B1 is the amplitude of the fundamental wave of the phase current, B 6n±1 is the amplitude of the 6n±1th phase current harmonic, θ i is the angle of the phase current. Since the sum of the three-phase current is zero, the harmonics of multiples of 3 in the three-phase current are eliminated. Figure 1 It is a high-speed brushless DC motor source topology.

[0054] In the non-commutation stage, for example, when the upper tube of phase A and the lower tube of phase B are turned on, and phase C is turned off, the three-phase voltage equation can be expressed as

[0055]

[0056] u aN ,u bN and u cN are the three-phase voltages, i a ,i b and i c They are the three-phase current, e a , e b and e c They are the three-phase reverse electromotive force, R is the motor phase resistance, and L is the motor phase inductance. The three-phase terminal voltage u aM ,u bM and u cM It can be expressed as

[0057]

[0058] u NM is the neutral point voltage. The power device is in saturated conduction state, and its tube voltage drop can be ignored. At this time, i a =-i b ,u aM =U,u bM =0, we can get,

[0059]

[0060] From formulas (4) and (5), we can get

[0061]

[0062] Similarly, the characteristics of the three-phase terminal voltage of the output motor in other working states can be obtained.

[0063] Step 2: Analyze the integral relationship between the terminal voltage and 1 / 2 bus voltage before and after the back EMF passes through zero.

[0064] When the inner angle When the current phase angle is equal to the back EMF phase angle, that is, θ i =θ. According to the relationship between formula (1) and formula (2), the fundamental wave and higher harmonics of the back electromotive force are equal in angle to the fundamental wave and higher harmonics of the phase current. That is to say, the zero crossing point of the back electromotive force coincides with the zero crossing point of the phase current. Due to the three-phase six-state working mode of the brushless DC motor, its phase current is similar to a square wave. There are two shutdown areas in each conduction cycle. The zero crossing point of the phase current can be described as the midpoint of the shutdown area of ​​the phase current. Therefore, the present invention proposes that when the internal power angle is zero, the zero crossing point of the back electromotive force coincides with the midpoint of the shutdown area of ​​the phase current.

[0065] The relationship between back EMF and phase current during ideal motor commutation is shown in Figure 2 As shown. The black circle is the zero-crossing point of the back EMF. Since Δθ1<Δθ2, this point is ahead of the midpoint of the phase current cut-off region. On the contrary, when the internal power angle is equal to zero, Δθ1=Δθ2, and the relationship between the back EMF and the phase current is shown in Figure 3 shown.

[0066] Although the internal power angle detection and control can be achieved by detecting the deviation between the midpoint of the phase current shutdown region and the back electromotive force zero crossing point, the detection of the midpoint of the phase current shutdown region requires the addition of three additional phase current sensors, which makes the circuit complex and increases the cost. The present invention utilizes a regular feature of the terminal voltage and proposes a method for achieving the above-mentioned similar control effect without relying on the current sensor.

[0067] Taking phase C as an example, when phase C is disconnected and the freewheeling ends, define S1 as the integral of the voltage at the phase C terminal before the back EMF crosses the zero point minus half the bus voltage, and S2 as the integral of the voltage at the phase C terminal minus half the bus voltage after the back EMF crosses the zero point. Figure 2 and Figure 3 When the phase current lags behind the back electromotive force, S1<S2; when the internal work angle is eliminated, S1=S2; similarly, when the phase current leads the back electromotive force, S1>S2.

[0068] S1 can be expressed as:

[0069]

[0070] S2 can be expressed as:

[0071]

[0072] Step 3: Determine the internal power angle based on the integral characteristics of the terminal voltage before and after the back electromotive force passes through zero.

[0073] When Δθ1=Δθ2=Δθ, formulas (7) and (8) can be expressed as:

[0074]

[0075] That is, S1=S2, the current and the back electromotive force are in the same direction, and the internal work angle is 0.

[0076] When Δθ1<Δθ2, according to formulas (7) and (8), S1<S2, the phase current lags the back EMF, and the internal work angle is greater than zero;

[0077] When Δθ1>Δθ2, according to formulas (7) and (8), S1>S2, the phase current leads the back electromotive force, and the internal work angle is less than zero.

[0078] Therefore, the internal power angle can be detected based on the integrated difference between the terminal voltage and 1 / 2 bus voltage before and after the back electromotive force passes through the zero point.

[0079] Simulation Results Figure 4 , Figure 5 It shows that when S1<S2, the average value of the internal power angle is negative, and the phase current lags behind the back electromotive force; when S1=S2, the average value of the internal power angle is 0, and the phase current is in phase with the back electromotive force.

Claims

1. A method for detecting the internal power angle of a brushless DC motor that does not rely on motor parameters, characterized in that Here are the steps: Step 1: According to the working condition of the high-speed brushless DC motor, derive the back electromotive force and terminal voltage equations: the three-phase winding of the motor is symmetrical, and the Fourier expansion of the three-phase back electromotive force of the motor is: Where: A1 is the amplitude of the back EMF fundamental voltage, A 2n+1 is the amplitude of the 2n+1th voltage, θ is the electrical angle of the motor rotor; When the upper tube of phase A and the lower tube of phase B are turned on and phase C is turned off, the three-phase terminal voltage u aM ,u bM and u cM It is expressed as: The power device is in saturated conduction state, i a =-i b ,u aM =U,u bM =0 Among them, U is the bus voltage, R is the motor phase resistance, and L is the motor phase inductance; When the upper tube of phase C and the lower tube of phase B are turned on and phase A is turned off, the three-phase terminal voltage u aM ,u bM and u cM It is expressed as: The power device is in saturated conduction state, i a =-i c ,u aM =U,u cM =0, Among them, i a is the phase A current, i b is the B phase current, i c is the C phase current; When the upper tube of phase C and the lower tube of phase A are turned on and phase B is turned off, the three-phase terminal voltage u aM ,u bM and u cM It is expressed as: The power device is in saturated conduction state, i a =-i c ,u aM =U,u cM =0 Where: u NM is the neutral point voltage; Step 2: Integrate the terminal voltage of the shut-off phase of one of the three phases according to the back electromotive force and terminal voltage equation: The voltage at the off phase terminal before the back EMF crosses zero minus half of the bus voltage is integrated, and the result is S1; the voltage at the off phase terminal minus half of the bus voltage after the back EMF crosses zero is integrated, and the result is S2; Step 3: Determine the internal power angle based on the integral characteristics of the terminal voltage before and after the back electromotive force passes through zero: When S1=S2, the current and the back EMF are in the same direction, and the internal work angle is 0; When S1<S2, the phase current lags the back EMF, and the internal work angle is greater than zero; When S1>S2, the phase current leads the back EMF and the internal work angle is less than zero.

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