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Power supply and frequency conversion speed regulation of AC motor
Ac motor power supply
Generally, three-phase AC motor is adopted, because compared with single-phase motor, three-phase AC motor has obvious advantages in performance index, raw material utilization rate and price. A three-phase motor with the same power is smaller, lighter and cheaper than a single-phase motor. Three-phase motor has self-starting ability. Single-phase motor has no starting torque, so some special measures need to be taken to solve the starting problem. The torque of single-phase motor is pulsating and the noise is relatively loud, but the power supply required is relatively simple, especially at home. Therefore, small household motors and instrument motors mostly use single-phase motors.
Frequency conversion speed regulation of AC motor
Frequency converter is an electric drive element that uses frequency conversion technology and microelectronics technology to control AC motor by changing the frequency and amplitude of motor power supply. Characteristics of AC motor speed-regulating frequency converter: AC motor speed-regulating frequency converter ■ Low-frequency torque output 180%, good low-frequency operation characteristics ■ Maximum output frequency of 600Hz, which can control high-speed motor ■ Omni-directional detection and protection functions (overvoltage, undervoltage and overload), instantaneous power-off and restart ■ Protection functions such as acceleration, deceleration and stall prevention during rotation ■ Automatic identification function of motor dynamic parameters. Ensure the stability and accuracy of the system ■ Quick response when stopping at high speed ■ Rich and flexible input/output interfaces and control modes, strong versatility ■ Adopt SMT full-patch production and three-proof paint treatment process, high product stability ■ The whole series adopts the latest Siemens IGBT power devices to ensure high quality.
[Edit this paragraph] Direct torque control of AC motor
At present, among several common direct torque control strategies, for small and medium capacity, the control scheme focuses on deadbeat control of torque and flux linkage and improving carrier frequency. For large capacity, the difference is that indirect torque control is adopted at low speed to reduce torque ripple at low speed.
Summary of Direct Torque Control Technology
Compared with DC motor, it has sufficient advantages in simple structure, convenient maintenance, low environmental requirements, energy saving and productivity improvement, which makes AC speed regulation widely used in industrial and agricultural production, transportation, national defense and daily life. With the rapid development of power electronics technology, microelectronics technology and control theory, AC speed regulation technology has also made great progress. At present, there are vector control and direct torque control in the field of high performance AC speed regulation. In 1968, Dr. Hasse of Damme Staade University of Technology initially put forward the field orientation theory, and then in 197 1 year, F. blaschke of Siemens summarized and realized this theory, published it in the form of patent, and gradually improved and formed various vector control methods. As for direct torque control, it is generally believed that it was first proposed by Professor M.Depenbrock of Ruhr University in Germany and I.Takahashi of Japan in 1985 respectively. For the direct torque control of circular flux linkage, the basic idea is to control the instantaneous speed of stator flux linkage by controlling the instantaneous input voltage of the motor, so as to change its instantaneous slip to the rotor and achieve the purpose of directly controlling the motor output. Under the condition of accurately observing the spatial position and size of stator flux linkage and keeping its amplitude basically constant, the load torque can be accurately calculated. Different from vector control in control idea, direct torque control indirectly controls current by directly controlling torque and flux linkage, so it has the advantages of simple structure, fast torque response and good robustness to parameters. In fact, 1977 A B Plunkett once put forward a direct flux linkage and torque adjustment method similar to the current direct torque control structure and idea in IEEE industrial application journals. In this method, the slip frequency is obtained by PI adjustment of the difference between the given torque and the feedback torque, and the slip frequency is added to the mechanical speed of the motor rotor to obtain the voltage stator frequency that the inverter should output. The ratio of voltage to frequency is obtained by integrating the difference between stator flux reference value and feedback value, and the voltage should be output by inverter by multiplying the stator frequency. Finally, SPWM method is used to control the motor. Direct torque control (DTC) has been proposed for nearly 20 years. At present, many control strategies and their digital realization schemes, flux observation and speed identification methods are developed on this basis. This paper will classify, analyze and compare them.
Motor model and direct torque control strategy
Direct torque control is based on static coordinate system, as shown in figure 1. In the traditional direct torque control, stator flux observation and torque calculation are carried out by detecting stator two-phase current, DC bus voltage and motor speed (speed measurement is not needed in speed sensorless DTC), which are subtracted from stator flux reference value and torque reference value respectively, and the difference is compared with their respective hysteresis loops. Output torque and flux increase and decrease signals, input these two signals into the optimization vector switch table, and add the sector where the stator flux is located to obtain a voltage vector that satisfies the requirement that the flux is circular and the torque output follows the given torque. The hysteresis of flux linkage and torque can be set in multiple stages, and its width is variable. The smaller the hysteresis width, the higher the switching frequency and the more accurate the control. Direct torque control has the advantages of simple structure, fast torque response and good robustness to parameters, but it is a control method based on Bang-Bang control, with single vector, torque and flux hysteresis, which inevitably leads to low switching frequency, unstable switching frequency and large torque ripple, which limits the application of direct torque control in low speed region. In view of this, many scholars at home and abroad have put forward various methods to improve switching frequency, fix switching frequency and reduce torque ripple. This section will list and compare them one by one.
Deadbeat space vector modulation method
T.G.Habetler's space vector modulation method applies deadbeat method to direct torque control, which was first proposed by T.G.Habetler in the United States. The main idea of this method is to get the difference between the given torque value and the feedback value in this sampling period. The amplitude and phase of the space voltage vector are arbitrary and can be obtained by synthesizing two adjacent basic voltage vectors. The calculated space voltage vector can be used to achieve the goal of deadbeat torque and flux linkage. Using the deadbeat method of Habetler, theoretically, the flux linkage and torque errors can be completely zero, thus eliminating torque ripple, making up for the deficiency of traditional DTC Bang-Bang control, and making the motor run at extremely low speed. In addition, the space voltage vector obtained by deadbeat control can greatly improve the switching frequency and make it fixed, which is very helpful to reduce voltage harmonics and motor noise. However, the action time of space voltage vector may be longer than the sampling period, which shows that the deadbeat control of flux linkage and torque cannot be satisfied at the same time. Therefore, the author puts forward three steps: first, whether the torque meets the dead zone, if not, whether the flux linkage meets the dead zone, and if not, selecting a single voltage vector for the next cycle according to the original direct torque control vector table. Therefore, according to Habetler's deadbeat method, the maximum calculation amount has four steps, which will consume a lot of calculation resources and is not easy to realize. In addition, the whole calculation process depends heavily on motor parameters, which will reduce the robustness of control. Predictive control method of torque or flux linkage is difficult to realize in deadbeat direct torque control method of T G Habetler, so a series of simplified deadbeat direct-to-AC motor-South Korea SPG AC motor full-range torque control appears, and the typical method is torque tracking prediction. This method analyzes the situation of low-speed torque ripple and draws the conclusion that the sawtooth of torque ripple is asymmetric. Non-zero voltage vector and zero voltage vector have different effects on torque variation. The former can increase or decrease the torque, while the latter always decreases the torque. In addition, the change rate of torque action is also changing in different speed ranges. In the torque predictive control method, the position of voltage vector in space is fixed, and it is synthesized between two single voltage vectors, but the voltage vector does not affect the whole sampling period, but has a certain duty cycle, which can be divided into non-zero voltage vector and zero voltage vector in a sampling period. If the non-zero voltage vector and the zero voltage vector * * * in the next sampling period work together, the torque change is equal to the torque error calculated in this period. The torque error will be eliminated, thus achieving the purpose of torque deadbeat control. Even if the calculated voltage vector action time exceeds the sampling period, it can be replaced by a full voltage vector, so it is very easy to realize. From the experimental results, the sawtooth of torque ripple is basically symmetrical, which shows that torque ripple has been greatly reduced. Previous methods thought that flux control was accurate or slow, and deadbeat control of flux was not considered, and flux was also predicted in the literature. In this method, it can be approximately obtained by the relationship between the space vector of flux linkage and the voltage vector: where Δ ψ s is the amplitude change of flux linkage under the action of the voltage vector, and θ v ψ is their space angle. Assuming that the flux linkage error in the kth sampling period is δ ψ so, the vector action time that makes the flux linkage error in the kth+1period zero can be obtained according to formula (5); based on the principle of giving priority to torque control, the comprehensive vector action time can be obtained according to the vector action time calculated by torque predictive control and the action time calculated by flux predictive control. The deadbeat control considering flux linkage is better than the pure torque deadbeat control, which not only eliminates torque ripple, but also does not produce flux linkage distortion and the calculation amount is not too large. In addition to the above-mentioned torque deadbeat control method, similar methods are also adopted in the literature, and the final voltage vector calculation action time is basically the same, so I will not repeat them here. Like Habetler's deadbeat method, the prediction method also needs more motor parameters. If the stator resistance and rotor time constant can be identified on-line in real time, the control accuracy will be greatly improved. The discrete-time direct torque control based on detecting back electromotive force is introduced in detail in the literature. In the literature, this method is first applied to direct torque control. The basic method is as follows: the voltage equation and flux linkage equation obtained from the basic circuit model of the motor are discretized as follows: the definitions of A and B also discretize the torque equation. Substitute Equation (7) into it, and at the same time, substitute Equation (7) into the square expression of amplitude of flux linkage. Using the discrete torque equation and the discrete flux linkage amplitude leveling method, the increments VSx and VSy of the space voltage vector in the next cycle can be solved, and the voltage vector controlled by the torque and flux linkage deadbeat can be obtained by substituting into the following formula, with limited amplitude. The torque and flux linkage that k+ 1 cycle should reach are derived, so that the deadbeat control of torque and flux linkage can be realized at the same time, which is very suitable for digital control in terms of implementation mode. In addition, this method is mainly based on stator side control, and the required motor parameters are only stator resistance and inductance, which is more robust to the change of motor parameters. From the experimental results, the dynamic response performance of the system is good. However, in this method, it is necessary to detect the phase voltage of the motor, which increases the complexity of the system hardware, in addition, the amount of calculation is also relatively large. Dead-beat control based on geometry In the literature, stator flux equation, rotor flux equation and torque equation represented by stator flux are discretized, and then the first two equations are brought into the torque equation. Through the analysis of discrete torque equation, it can be known that the torque error can be zero by applying voltage vector, and the torque becomes a straight line on the plane, parallel to the direction of rotor flux vector. Similarly, it can be analyzed that applying voltage vector can make the magnetic flux error zero, and the magnetic flux becomes a circle on the plane, which is concentric with the magnetic flux circle. Therefore, the voltage vector that makes torque and flux linkage control deadbeat can be obtained by using the intersection of straight line and circle. Of course, this voltage vector is limited by the voltage that the inverter can output. It is a good idea to introduce geometry into deadbeat control, which can get the optimal voltage vector of deadbeat control and also contribute to theoretical analysis. However, there are still some difficulties in how to realize the combination of graphic mode and digital control.
Discrete space vector modulation (DSVM) method
Dead-beat direct torque control can theoretically eliminate the errors of torque and flux linkage and overcome the weakness of inaccurate Bang-Bang control, but it needs a lot of calculations, and these calculations are related to motor parameters, which are easy to cause calculation errors. Therefore, a discrete space vector modulation method is proposed in the literature, which can improve the accuracy of torque and flux control without too much calculation. In the discrete space vector modulation method, the adjacent voltage vector and the zero voltage vector among the six basic voltage vectors output by the two-level inverter are synthesized regularly, as shown in Figure 3, which is a space voltage vector synthesized by using the adjacent single vector 2, single vector 3 and zero voltage vector. As can be seen from Figure 3, the synthesis method is to divide the whole sampling period into three segments on average, and each segment consists of a non-zero voltage vector or a zero voltage vector. For example, the space voltage vector 23Z is composed of vector 2, vector 3 and zero voltage vector, and each vector acts on the sampling period of 1/3, so it can be synthesized in five or seven segments (not illustrated in this paper), and 10 can be synthesized by this conventional synthesis method. Accurate voltage vector can control torque and flux more accurately. In the literature, the traditional two-stage hysteresis Bang-Bang control is adopted for flux linkage, but considering that the series torque of AC motor-South Korea SPG small motor needs fast dynamic response, it is divided into five-stage hysteresis Bang-Bang control, as shown in Figure 4, and different voltage vectors are used for different error bands. In addition, the influence formula of voltage vector on torque change is as follows: From the formula (10), it can be seen that the same voltage vector has different influence on torque change at low speed and high speed. Therefore, different speed ranges use different voltage vectors, as shown in Figure 3. On the other hand, using a small voltage vector at low speed and a large voltage vector at high speed also conforms to the law of V/F = C. The traditional direct torque control continuously uses more zero voltage vectors at low speed, which makes the switching frequency very low and the torque ripple very large. In the discrete space vector modulation mode, because the voltage vector with small amplitude is used at low speed, the zero voltage vector used continuously is less, the switching frequency is high and the torque ripple is small. In addition, because there are many voltage vectors at high speed, it can be divided into 12 sectors, and two voltage vector tables are used, so that more accurate control can be carried out. From the above analysis, it can be seen that the discrete space vector modulation method is simple to realize, does not need as much calculation as deadbeat control, and maintains the advantages of traditional Bang-Bang control, so it has good robustness, but compared with traditional direct torque control, it can improve the accuracy of torque and flux linkage control and reduce low-speed torque ripple. However, the higher the control accuracy, the finer the vector division and the larger the voltage vector control table, which will increase the control complexity. Therefore, if we can combine discrete space vector modulation with deadbeat control, it will help to overcome this shortcoming.
Method of output space voltage vector of PI regulator
In direct torque control, if the space voltage vector of any phase can be obtained, it will help to reduce the torque ripple at low speed and realize the steady-state performance of vector control at low speed. The deadbeat control in the third section can get the space voltage vector of any phase, but the calculation is complicated and it is difficult to realize. Another method to obtain voltage vector of arbitrary phase space is to use PI regulator. A. B. Plunkett's direct torque and flux regulation method is a PI regulation method, but there was no concept of space voltage vector at that time, and only SPWM method was used to output the motor control voltage. In the literature, the proposed direct torque control adopts PI regulation method, while SVM adopts the method of outputting space voltage vector. The torque error obtained by torque reference and torque feedback is input into PI regulator, and the Q-axis voltage vector is obtained by PI regulation. The stator flux error obtained by stator flux reference and stator flux feedback is input into PI regulator, and the D-axis voltage vector is obtained by PI regulation. Then, the D-axis and Q-axis voltage vectors are rotated to the α-axis and β-axis of the static coordinate system, and the space voltage vector is output. Obviously, the phase of this space voltage vector in the space position is arbitrary. In structure, the direct torque control based on PI regulation is similar to the vector control of stator flux orientation, but there are differences between them. The vector control of stator flux orientation is based on synchronous rotating coordinate system, which is oriented to the D axis of stator flux, and the Q axis flux is zero. In addition, flux linkage and current in Q-axis direction should be decoupled, which is unnecessary for direct torque control based on PI regulation. Among them, only the torque output and stator flux feedback need to track the given value through PI adjustment method, which is relatively simple to implement and has good robustness. Compared with the traditional direct torque control, it can increase the switching frequency and reduce the torque ripple at low speed, but this method needs to choose appropriate PI parameters, otherwise it will affect the dynamic and static performance of the control system. In addition to the direct torque control of PI regulation mentioned above, a further study is made on the basis of the direct torque and flux adjustment method of A B Plunkeet in the literature, and the output is in the form of space voltage vector, which will not be described here.
Inject high frequency jitter to improve switching frequency
In the previous direct torque control strategy, it is said that increasing the switching frequency at low speed can reduce torque ripple and noise. In the literature, a method of injecting high-frequency jitter on the basis of traditional direct torque control is proposed to improve the switching frequency, in which the author graphically shows that the switching frequency increases with the decrease of torque and hysteresis width of flux linkage, but this improvement is limited. One of the main reasons is the delay of flux linkage and torque control. The greater the delay, the lower the switching frequency. For example, from the simulation, 10μs delay has a switching frequency of 14kHz, but when there is a delay of 20μs, it has only a switching frequency of 8kHz. The method to improve the switching frequency proposed in the literature is to superimpose a high-frequency triangular wave in the hysteresis loop of torque and flux linkage, and its amplitude is equivalent to the width of the hysteresis loop. When the feedback value is greater than the triangular wave, the voltage vector decreases, and when the feedback value is less than the triangular wave, the voltage vector increases. Therefore, even if there is delay in control, the switching frequency will increase with the increase of triangular wave frequency. For example, when the frequency of triangular wave is 30kHz, the switching frequency can reach 10kHz. The single voltage vector method is used in the literature. If the method of spatial arbitrary voltage vector is adopted, the switching frequency can be further improved.
Low speed control strategy of large capacity direct torque control
Direct torque control was first put forward in Germany to solve the control problem of large-capacity locomotive, and the most important point is to reduce the switching frequency. At present, when GTO is used as the power device of inverter, its switching frequency is generally less than 200Hz, and when IGBT is used, it is generally less than 500Hz. Therefore, the direct torque control strategy described in the above sections will not be suitable for large-capacity direct torque control, otherwise it will cause relatively high switching frequency. At low speed, if direct torque control is used, the sampling period is very short at first, otherwise the torque ripple is large and it is easy to overcurrent. Secondly, annular flux linkage is required, otherwise the torque ripple is large; Thirdly, a single voltage vector with a duty ratio of 100% should be adopted to reduce the switching frequency by at least half; Finally, there should be a large hysteresis between torque and flux linkage, otherwise the switching frequency will be high, but if the hysteresis between torque and flux linkage is too large, it will cause a large torque ripple. Therefore, it is not easy to use traditional direct torque control in large-capacity speed regulation. The most mature method at present is indirect torque control. This control method is actually an improvement of A B Plunkett's direct torque and flux adjustment method. The torque regulator outputs a dynamic slip with a period of integral dynamic increment Δ xd, and the steady slip is calculated by flux and torque. The synchronous angular velocity can be obtained by the sum of the dynamic slip and the mechanical angular velocity of the motor, which can be integrated in a sampling period to obtain the phase steady increment δ x0 of the flux linkage in a period, and the total phase increment δ x of the flux linkage in a sampling period can be obtained by adding it to the dynamic increment. The flux regulator outputs amplitude increment kψ, and the space voltage vector of the control motor can be obtained by using phase increment, amplitude increment and voltage equation. As can be seen from the above analysis, the physical concept of indirect torque control is very clear. By calculating the amplitude increment and phase increment of flux linkage to determine the space voltage vector, not only can the flux linkage trajectory be circular, but also the torque can be stably and dynamically adjusted. In addition, the switching frequency can be reduced by increasing the sampling period like vector control without generating additional torque ripple, mainly because the amplitude increment and phase increment of flux linkage can be accurately calculated within one sampling period. Therefore, indirect torque control has good steady-state and dynamic performance, which can greatly reduce the low-speed torque ripple and increase the speed regulation range in large-capacity speed regulation.
The Future of Direct Torque Control Technology
Compared with the traditional direct torque control, the main improvement methods of small and medium-sized motor control at present are deadbeat control of torque and flux linkage, and raising and fixing switching frequency. It is difficult to realize deadbeat control of torque and flux linkage at the same time, so there are separate predictive tracking control and discrete space voltage vector control of torque and flux linkage between deadbeat control and Bang-Bang control, which not only simplifies the control algorithm, but also improves the control accuracy. Using PI regulator to control torque and flux linkage is a relatively direct method, which saves the complicated calculation of deadbeat control and is easy to realize. No matter deadbeat control or PI regulation, any or many space voltage vectors can be output, which naturally increases and fixes the switching frequency, which is very helpful to reduce torque ripple and noise. However, it should be clearly seen that the low-speed performance of small-capacity direct torque control is not as good as that of vector control, and the torque ripple and noise are also greater than the latter. How to reduce the torque ripple and noise needs further study. In addition, it is also a good idea to introduce indirect torque control into small capacity and low speed control. For the large-capacity direct torque control strategy, the main difference from the small and medium-capacity direct torque control strategy is to limit the switching frequency to a certain range. Because of the indirect torque control at low speed, the torque ripple is relatively small, which can almost achieve the low-speed performance of vector control. With the development of power electronic devices to high power and high frequency, it will contribute to the further development of large capacity direct torque control.
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