In the all-digital high-performance AC speed control system, digital pulse width modulation is usually used instead of the traditional analog pulse width modulation. Space vector pulse width modulation (SVPWM) technology is a hot topic in recent years. SVPWM synthesizes the voltage states of the three-phase converters on the complex plane into a space voltage vector, and forms eight space vectors through different switching states. These eight space vectors are used to approximate the voltage circle to form an SVPWM wave. . It can achieve better harmonic suppression at lower switching frequencies and approximately 15% higher fundamental voltage than SPWM. At the same time, SVPWM modulation technology has an advantage that it is easy to implement digital and real-time control.
Under different approaches, the SVPWM modulation technique will have different effects, ie the harmonics generated will also be different. Based on the mechanism of SVRWM generation, this paper analyzes the harmonic distribution and switching efficiency of SVPWM wave under different approximation methods, as well as the advantages and disadvantages of different approximation methods. At the same time, the influence of harmonics on the electromagnetic torque of AC induction motor is simulated and a new idea for the optimization of SVPWM modulation technology is proposed. The simulation study proves the fund project of the proposed method: Jiangsu Provincial Department of Education Natural Science Foundation (00Kjm70002) 192() space vector PWM harmonic analysis and its correctness and feasibility for motor torque.
2 Basic Principle of Space Vector Pulse Width Modulation From the three-phase inverter main circuit diagram (as shown), the inverter must meet the following two conditions: At any moment, there are always three switches in the on state and the other three in the off state. The upper and lower arms cannot be turned on at the same time.
Three-phase inverter main circuit diagram In the above conditions, the six power triodes have eight switch combinations: eight basic space vectors (of which six are non-zero vectors) are labeled V1 (001), V2 (010), V3 (at the origin, the angle between adjacent non-zero vectors is 60°. As shown in the SVPWM vector, sector and schematic diagram, any voltage space vector can be defined as: the self-conducting state, the upper switch is 1 , The lower switch is 0, U (1 is the DC voltage value.
The basic space vector method is used to estimate the voltage of the motor: the voltage vector of the motor projected at any one time falls within one of the six sectors so that the motor voltage vector at any moment can pass through the adjacent basic space vector The two vector elements are estimated (as shown): The modulation constraint is: Equation (4) determines the maximum voltage at which the inverter supplies power when the DC voltage is Ud. The equation (3) is taken into equation (4) to obtain: 6 action time, T. zero vector action time, T()=Ts-Ti-T2, Ts=l/(Nf), N is to generate PWM carrier Ratio, f is the output voltage frequency, U. is the motor voltage vector at any moment.
It is easy to know that the maximum output voltage of the inverter (that is, the lower limit of the right side of equation (5)) is Ud/N. If it is reflected on a vector diagram, the vector circle is a hexagonal inscribed circle composed of two orthogonal electric zero vectors of two-phase stationary coordinate system given by the six non-perfect control systems. The traditional SPWM input is discussed in the following three approximation methods (taking the first sector as an example). Simulations are performed under the condition of N=36,/=50Hz.
Imitation results are shown in ~5.
Method 1 Method 2 Method 3 The first end of the starting point is inserted with the zero vector and then the vectors V4, V6, and then the zero vector V7> followed by the vector V6, V4, the end point is inserted into the zero vector Vn, as shown in a. Compared with method 1, the zero vector is all served by V. Instead of inserting zero vector in the middle, as shown in a. Compared with method 1, the zero vector is served by V7 instead of V. That is, no zero vector is inserted at both ends, as shown in a.
After the pressure vectors Urdref, Urqef, the SVPWM control algorithm will perform the following functions: determine which sector the U.edrrf and Urqref composite voltage vector Ur falls on.
Calculate the time between two adjacent vectors and zero vectors in the sector.
In equation (3), O 3SVPWM optimisation selection and simulation studies use eight vectors to approximate the circle. There are several ways to approximate the circle. The harmonic distribution of the PWM wave generated by different approximation methods is not the same, and the switch within one sampling period Ts. The number of times is not the same. In other words, the harmonic distribution of the input current has a great relationship with the number of switching times in the sampling period and the arrangement of the space vector approximating the voltage circle.
The grouping of voltage vectors in each sector is: Sector number 123456 Vector group Compare the spectrum of the SVPWM waveforms generated by the above three approximation methods. It can be found that the difference of the lower harmonics is not very large, and the higher harmonics are mainly generated at the switching frequency. (1. 8kHz) near the integer multiple. The higher harmonics produced by methods 1 and 2 are smaller, and the lower harmonics generated by method 3 are smaller. Comparing the line voltage spectra of the three methods, it can be seen that their spectral distribution is consistent with the corresponding SVPWM spectral distribution. From the point of view of switching loss, in a PWM cycle, method 1 is to be switched 6 times, method 2 is to be switched 4 times, and method 3 is switched 5 times.
These three approximation methods are low-power approximations. According to the limitation of working frequency and switching times in practical applications, optimization selection can be made with reference to simulation results in different frequency bands.
4 Harmonic Influence on Motor Torque Ripple Because the inverter output voltage contains fundamental and various harmonics, each of them generates a corresponding electromagnetic torque in the motor. In order to obtain high-performance electromagnetic torque, it is necessary to reduce the influence of harmonic torque on the motor as much as possible. Because the harmonic torque will cause the motor to generate torque ripple, the 5th and 7th harmonics are more significant. Take the 5th and 7th harmonics as examples for detailed analysis.
Assuming that the phase sequence of the fundamental voltage is a positive phase sequence, then the harmonic analysis shows that the 5th harmonic is the reverse phase sequence and the 7th phase is the positive phase sequence. The induced currents generated by the 5th and 7th harmonic voltages in the rotor are all 6th harmonics. If the saturation of the magnetic circuit and hysteresis loss are not considered, the superposition principle can be considered to consider the electromagnetic torque. The maximum electromagnetic torque produced by 5 and 7 cycles are as follows: 5, 7, Ux represents the voltage rms value of the xth harmonic, /x represents the xth harmonic frequency, LS1 represents the stator leakage inductance per phase, and LS2 represents the conversion to Stator-side rotor leakage inductance per phase.
The simulation motor model is a fifth-order nonlinear equation based on a two-phase stationary coordinate system. The motor parameters are: rated power 2.2 kW, rated voltage 380 V, rated current 8. Hz, pole pair number 2, rated speed 183. 26 rad/s, rated electromagnetic torque 12 Nm, stator resistance 0.6878, stator inductance 83.97 mH, rotor resistance 0. 8428, Rotor Inductance 85. The fundamental and 5th harmonics and 7th harmonics and various combinations of electromagnetic torque effects such as ~ 9 (all motor simulation results under no-load conditions) are shown.
From the above simulation analysis, it can be seen that the 5th and 7th harmonics cancel out the pulsation of the electromagnetic torque, and the harmonic suppression of the electromagnetic 5 harmonic optimization program is a very important indicator of the optimization of the SVPWM modulation technology. From the current perspective, SVPWM modulation technology is almost in a state of stagnation, and the technology has not been fundamentally optimized. Because of harmonic suppression, optimization of power consumption of the switching device is also considered. At present, many harmonic optimization methods focus on the distribution of zero vectors. The main methods are: a reasonable distribution of zero vectors, so that the suppression of harmonics and the reduction of the number of switches to achieve a comprehensive optimal.
With the same sampling period, a random pulse sequence is generated and the conduction time is changed randomly, so that harmonics are distributed higher.
Through a certain control strategy, the carrier ratio is changed in real time to improve the low-frequency characteristics. 15. The purpose of these methods is to eliminate harmonics, but some of them eliminate the harmonics and impose heavy burdens on real-time performance and switching devices. From the above simulations, we can see that the full use of the mutual suppression between harmonics, such as in the elimination of the 5th and 7th harmonics, as long as the compensation of some 5th harmonics, you can find a new reduction or elimination of harmonic effects The idea is to simplify the algorithm, reduce the burden on switching devices, and eliminate the effects of these harmonics without eliminating certain harmonics.
It is worth noting that none of these algorithms found the parameters that limit the harmonics in real-time control, that is, the suitable nonlinear description function of the inverter could not be found, because the ideal continuous function of the control pulse sequence could not be found. Began to conduct research in this area.
6 Summary In this paper, the principle of the space vector pulse width modulation technique and the harmonic distribution of several main methods are analyzed through simulation research, and some summary of its characteristics are made. At the same time, based on the analysis of the effect of harmonics on the pulsation of the electromagnetic torque of AC induction motors, a new idea of ​​harmonic influence suppression is proposed. The research in this paper has practical significance for the optimization of SVPWM in the application of AC induction motor variable frequency drive and the determination of harmonic suppression scheme.
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