Since the first real rotating motor was invented by Jacobi in 1834, the motor has been playing a more and more important role in people’s daily production and life. Nowadays, motors are widely used in industrial, agricultural production, transportation, military equipment and in every aspects of life. A motor transforms electrical energy to mechanical energy. In order to achieve the highest efficiency of the mechanical and electrical energy conversion, and to save the maximum cost, we must find an efficient and suitable motor control strategy. Therefore, the control of motor has become an important topic.
Because of the speed of DC motor control system is convenient, it can control the excitation current and input voltage of the motor so that the motor can change the speed smoothly in a very wide range. Based on this advantage, the DC speed regulating system has been widely used in the last 70s century, which needs wide range of response, good dynamic performance and high control accuracy. DC speed regulation has become the prevailing motor control mode at that time. But the DC motor has some shortcomings, such as high cost of production, large maintenance cost, large volume of equipment, due to the existence of commutator and brush, it is easy to produce sparks in the process of operation, causing the motor to burn and even explode.
So, the researchers started to think the ways of replacing DC motors with AC motors. Compared with DC motor, AC motor has the advantages of simple structure, sturdy and durable, low cost, safe operation, convenient maintenance, strong adaptability to environment and so on. The key problem of the motor speed control system is to maintain the air gap magnetic field and control the electromagnetic torque of the motor. However, because of the coupling between the flux and torque of the AC motor, the flux linkage and torque cannot be independently adjusted. But with the rapid development of power electronics and microprocessor technology, it not only promotes the research and development of AC motors, but also greatly optimizes the control strategy of AC motors, which solves the difficult problem of AC motor speed regulation. This makes AC motors widely cited and occupies a leading position.
There are two main types of AC motors: asynchronous motor and synchronous motor. Asynchronous motor, also known as induction motor, is named because its rotor speed is not synchronized with the speed of the rotating magnetic field of the stator. The asynchronous motor is ordinary design, minimal in production cost, secure and efficient in operation, easy to install sensors and feedback devices, and the torque ripple is relatively small. Therefore, it is widely used in production and life. But it also has the disadvantages of poor speed regulation, difficult to achieve smooth speed regulation and low power factor. Synchronous motor is named because the speed of rotor rotation is the same as the speed of stator rotating magnetic field. In synchronous machines, the most frequently used ones are permanent magnet synchronous motors. There are three main reasons:
1. Permanent magnet synchronous motor rotor is permanent magnet, so no external excitation system is needed, which brings convenience for operation. Moreover, the torque damping effect is large, the torque response is good, and the power factor in operation is higher than that of induction motor.
2. China is a large resource country with abundant magnetite and rare earth minerals. And master the advanced technology of permanent magnetic material refining. This has laid the material foundation for mass production of permanent magnet synchronous motor.
3. The control strategy for PMSM is more and more mature. In recent years, a new control strategy — direct torque control (DTC) has emerged. It abandons the traditional vector control decoupling and then controls the amount of control separately. The torque is directly controlled to control the operation of PMSM. This eliminates the cumbersome coordinate transformation and saves a lot of computation time.
Development in this Related Areas
Since 1980s, power electronics technology has been developing rapidly, which solves the problem of difficult speed regulation of AC motor. It includes gate controlled GTO, power field effect transistor MOSFET and power bipolar transistor BJT. Their advantages mainly include the following two aspects: by sending a signal to the gate, it can easily and quickly control the switching of the circuit; the switching frequency is high, so the switching loss is small. By the late 80s, the composite devices represented by insulated gate bipolar transistors (IGBT) have been developing rapidly. Insulated gate bipolar transistor IGBT is composed of BJT and MOSFET. It combines well the advantages of both, such as high voltage resistance, large current carrying capacity and high switching frequency. Therefore, it has become the mainstream power electronic device nowadays.
With the development of power electronic devices, the corresponding PWM control technology has also developed rapidly. Scholars from different countries not only have developed the traditional PWM, but also put forward some new control strategies.
PWM (Pulse Width Modulation), namely pulse width modulation, mainly modulates the width of a series of pulses, so as to get an ideal output waveform. It plays an important role in inverting, rectifying, DC chopping, and AC to AC control, and greatly improves the control accuracy of the circuit. The traditional PWM control technology relies mainly on the comparison of carrier signal and modulation signal, confirming the intersection point, thus playing a regulatory role.
SPWM (Sinusoidal PWM), which is the pulse width regulation of sine wave, is the most widely used and the most mature method of pulse width adjustment. By comparing the sine wave and the carrier signal, the sine wave is replaced by a series of pulses which are changed by a series of width according to the sine law. By adjusting the width of these pulses, the characteristics of the sine wave are adjusted indirectly, thus the function of the control circuit is played. There are mainly two ways to realize SPWM basic control:
The natural sampling method compares the sine wave with the carrier signal (often isosceles triangle wave), and uses their natural intersection point as the time to break the circuit. The advantage is simple operation, and the waveform obtained is very close to the original sine wave. However, because of the arbitrariness of the intersection, the center of the pulse is different in each cycle, so that the calculation involves the transcendental equation, which increases the difficulty of the mathematical operation and prolongs the time of the operation.
The standard sampling method takes a range of trigonometric waves to sample the sine wave, and gets the ladder wave similar to the sine wave shape. Then the step wave is compared with the triangle wave to determine the intersection point, thus the pulse is obtained, and the opening and closing of the circuit is controlled. The standard sampling method is also divided into the symmetric regular sampling method and the asymmetric regular sampling method. If the sampling at the peak or the lowest point of the sine wave, the center of the pulse obtained in each sampling period is the same distance, which is the symmetric regular sampling method. If the sampling time is not at the vertex or the lowest point of the sine wave, the center distance of the pulse is not equal in each sampling period, and this is the asymmetric regular sampling method.
With the continuous progress of technology, researchers have improved the traditional PWM control method and proposed SVPWM (Space Vector Pulse Width Modulation) pulse width adjustment of space vector. SVPWM is a three phase symmetric ideal magnetic field circle produced by the stator of a three-phase symmetrical sinusoidal voltage supply, which is the reference standard of the three-phase symmetrical ideal magnetic field circle. The switching modes of the three-phase inverter are properly converted to get the waveform of the PWM, thus forming the actual magnetic chain vector to track the accurate magnetic field circle.
DSP is a kind of high-speed special-purpose microprocessor appearing in recent years. Its main characteristic is to adopt Harvard structure to separate the program storage space from the data storage space, and each of them have its own data bus and address bus. The hardware multiplier and the accumulator are pipelined so that the multiplier and the accumulator can be carried out continuously at high speed. In-chip also integrates more and more peripheral interfaces, thus greatly improving its function, and it has a complete development and debugging tools. The development cycle is short, making DSP in the control field of application more attention. In the late 1990s, foreign companies introduced DSP controllers for motor control, such as Texas Instrument’s TMS320C/F24x series and Analog Devices’ADMC4xx series. On the basis of high-speed DSP core, three-phase PWM generator with dead-time function, input interface of photoelectric encoder, rich I/O were added. The source provides a powerful controller for the fully digital AC motor control system.
Control method and strategy is the key to decide the performance of frequency converter. At present, the output voltage of universal frequency converter is 380-650V, the output power is 0.75-400kW, and the working frequency is 0-400Hz and main circuit adopts AC-DC-AC circuit. Inverter control mode mainly through the following ways.
1. Voltage-frequency (V/Hz) ratio control mode: it is based on the equivalent circuit of asynchronous motor to determine the linear frequency conversion speed. Its characteristics are: the control circuit is simple in structure and low in cost. Voltage refers to the effective value of fundamental wave. Changing U/f can only adjust the steady-state flux and torque of the motor, but cannot be dynamically controlled. The control curve will change with the load change, the torque response is slow, and the motor torque utilization rate is not high.
2. Vector control mode (VC control): The theory and practice of AC drive control finally made a breakthrough in the 1970s, that is, the emergence of vector control technology. In essence, the AC motor is equivalent to a DC motor, which controls the speed and magnetic field separately. The torque and magnetic field components are obtained by controlling the rotor flux, orientating the rotor flux and decomposing the stator current. Orthogonal or decoupling control is realized by coordinate transformation. In this way, the motor model reconstructed by coordinate transformation can be equivalent to a DC motor. The control method of vector control realizes the decoupling control of flux and torque of asynchronous motor, improves the dynamic characteristics of AC drive system significantly, and opens up a new era of AC drive. However, in the actual system, because the rotor flux is difficult to observe accurately, and the complexity of vector rotation transformation, the actual control effect is not as good as the theoretical analysis. This is the deficiency of vector control technology in practice. Experts and scholars in the field of AC drive have also done a lot of research on the defects of vector control, such as parameter identification (or compensation) and the use of state observers and other modern control theory, but the introduction of these schemes makes the system more complex, real-time control and reliability is reduced.
3. Direct Torque Control (DTC): Its essence is not only control the current, flux, etc., but also the torque. The advantages are: direct torque control and good dynamic performance. Direct Torque Control (DTC) is a new high performance AC variable frequency speed regulation technology developed after vector control technology (1974). In 1985, Professor M. Depenbrock, a German scholar, first put forward the theory of direct torque control (DTC), and then Professor I. Takahashi, a Japanese scholar, put forward a similar control scheme and achieved exciting results. Although the methods of derivation and implementation are different, their basic ideas are consistent. That is to say, the stator current is decoupled into the excitation current component and the torque current shunt without considering how to decouple it. The mathematical model of AC motor is directly analyzed in the stator coordinate system. By measuring the stator voltage and current, the stator magnetic field orientation is used to directly control the flux and torque of the motor, focusing on the rapid response of the torque, in order to obtain high-efficiency control performance. Direct Torque Control (DTC) has attracted widespread attention for its novel control idea, concise system structure and excellent dynamic and static performance. Compared with the vector control technology, it is insensitive to the parameters of the motor, simple and feasible, and to a great extent overcomes the shortcomings of the vector control technology.
Development and Related Knowledge in Direct Torque Control of PMSM
With the improvement of technology, the research study of PMSM, such as direct torque control, has been improved. DTC has many advantages, like faster torque control, high torque and high-speed sensitivity at low speed. The essence of DTC is to apply motor flux and torque as the basic control variables, the same as the DC drive. So as to imitate the magnetic operating conditions of DC motor, the rotor state information is required to perform the flux vector generated electromagnetic field orientation technique. The information should be acquired by using the pulse coder to feed rotor speed and angular position feedback. Encoders required a lot, and they increase the complexity of the entire system.
A number of techniques have been established to acquire rotor position and angular velocity from electrical measurements and calculations just to get rid of the need for sensors. The concept responsible for these approaches is to adjust the motor equations to ensure that the motor position and speed are conveyed as functions of terminal amounts. In 2004, Adreescu ; Rabinovici, the self-adjusting velocity operator and Luenberger observer are suggested, but the encoder is yet required for rotor position recognition 18. Most techniques, nevertheless, work just if the rotor is anisotropic and the inductance relies on the rotor position is recognized precisely; additionally, speed is not determined, so the actuator is not completely without mechanical sensors19.
Alternatively, it can be stated that the techniques present by Bolognani can be utilized to establish the rotor position and speed of PMSM on-line feasible solutions20. The expanded electromotive force version is utilized to determine the rotor position21. Nevertheless, this technique is according to experienced knowledge. A few specialists have suggested a sequence of current and flux estimates or techniques including Kalman filter, fuzzy logic and neural network observers to acquire rotor position angles 22, 23. Lately, specialists have attempted to minimize torque pulses and harmonics in PMSM. Furthermore, an inverter output filter method for PWM recommended decreasing the harmonics of surface mounted permanent magnet synchronous motor (SPMSM)24. It can easily minimize the switching harmonics partially, but the distributing current between inverters is huge and the filter elements have to be improved. It might disregard the motor terminal voltage of inverter current limit.
Ever since the speed of direct driving PMSM is less than that of induction motors with gearboxes, the possibility of torque harmonics happening at mechanical vibrations is enhanced within the regular functioning speed range. Permanent magnet motors have been utilized for decades in low-power requests for instance, servo drives and residential appliances. Currently, the PMSM driver has been additionally designed and utilized in industrial requests involving low speed and high torque. PMSM drives are substituting standard induction motors with gearboxes, including paper and textile industries, for particular marine administrations. Nevertheless, the PMSM cannot be precisely delivered from the primary power supply, and it has to be operated by an AC motor driver. Just like induction motors, vector control techniques are approached PMSM to obtain elevated bandwidth torque control efficiency.
The flux angle of rotor has to be understood by AC motor driver when it comes to vector control. For that reason, sensors on the motor shaft including incremental encoders are applied to sense the rotor flux angle, and AC motor drivers work with this angle details for vector correspondent modification. Through vector coordinate transformation, the AC motor in the supervision coordinate is exchanged DC motor, and torque control is a simple issue of current control understood by current regulator. For that reason, the vector control method has high-bandwidth torque control for AC machines, which carries high-bandwidth speed and position control25.
Regardless of the quality and effectiveness that present day control technology has offered motion control, the advancement of AC motor and drive techniques keeps on to be studied. The enthusiasm is to develop technology to accomplish high effectiveness and quality. This chapter presents the literature research study on direct torque control of permanent magnet synchronous motor.The direct torque control theory is initially established for asynchronous motors26. Takahashi and Noguchi designed the direct torque control simply applying the torque routines for asynchronous motors10.
Pillay and Krishnan created permanent magnet synchronous motor by state space variables, and complete the torque analysis. This analysis is a very essential step in the research study of permanent magnet synchronous motor27.
Adnanes established the torque analysis of permanent magnet synchronous motor in unit mode, and understood the mathematical relationship between flux and torque thoroughly28. Pelczewski accomplished the finest design monitoring control of PMSM. So as to do the estimations, the motor design and its linearization are required29. Matsui and Ohashi, submitted a PMSM flexible controller according to DSP. Thus, they have demonstrated that, DSP can certainly also be understood in motor control30.
Chern and Wu presented PMSM position control by utilizing an adjustable driven controller. The controller computes promptly depending on unidentified load and motor criteria. The method versions important and the estimation take a long period of time31.
The position estimation for PM motors with zero and low speed conditions was accomplished built upon the saliency32. Zhong accomplished some of the very first academic researchers in the area of direct torque control of permanent magnet synchronous motor. Direct torque control of permanent magnet synchronous motor using two stage torque hysteresis controller is proposed by them16. Direct Torque Control (DTC) is obtained by utilizing a method according to obtaining d and q-axis voltages by applying specific coefficients33. Luukko establishes a switching table for direct torque control with including zero vectors to the vector selection algorithm34. A discovery has made on continuous torque control on a vector controller using a TMS320C31 DSP. In that particular study, a DSP technique is insufficient to obtain the dynamic attributes of the motor, so the torque does not perform well in the response of the required torque and feedback time35.
A multi-level inverter is put forward to minimize torque ripple and stabilize switching frequency in AC drive systems36, 37. These techniques can generate much better waveforms, minimize distortion, and operate at much lower switching frequencies. However, they need to have more switching devices. Furthermore, the control techniques of these methods are very complex. Space vector modulation (SVM) and direct torque control (DTC) are applied utilizing DSP by Dariusz 38. A technical guideline regarding torque control and vector control of permanent magnet synchronous motors has applied in many similar industries and academic research39.
A new technique using space vector modulation has established to minimize torque ripple and acquire good results. The established control algorithm demands two PI controllers, estimation of reference voltage and switching sequence of selected vectors40. Using DSP, the regulation of super high speed (200000rpm) permanent magnet synchronous motor is understood and accomplished by41.
Luukko recommended other rotor and load angle estimation techniques for direct torque control (DTC). They determine the load angle straight from the PMSM formula34. All these estimations, they employ tangent functions. Whenever the effects of DSP regulated inverter and motor test setups are examined, it can possibly be seen that rotor angle estimation comes with oscillation. During substantial oscillation, the inaccuracy between the real and expected rotor angle into larger form. . The results present that the PID coefficient should be maintained high to minimize this flaw. An output filter for direct torque control inverter is developed 42. The filter is comprised of RLC filter and isolation transformer. This analysis is interesting because it features transformer design and soft switching technology in power electronics. Furthermore, and RLC-based filters maximize the system’s cost compared to the progression of switching signals that makes inverters in control.
A fuzzy logic controller for DTC has been constructed in 2007 43. In that technique, torque error and stator current were utilized as fuzzy logic membership functions. Furthermore for the simulation researches, they utilize an AC motor drive setting named Platform III to execute fuzzy logic functions via the software for the setup. The simulation and experimental results demonstrate that the stator current is not waveform and has some unstable and arbitrary shapes. Compared to the DTC technique regulated by PI, the stator current is even more distorted.
The reference flux vector estimation is established in the space vector modulation of DTC by Wang and Gao 44. They draw out the voltages a trigonometric function of the time frame and use a frame transform, which computes the lifetime of the zero vector depended on the angular frequency of the current. Nonetheless, this structure has been executed in simulation, but it has not been accomplished experimentally. There is a long delay time among the real value and the computed value in the torque layout.
The DTC method is applicable for high-power PMSM employing inverters operated by space vector modulation 45 In this particular research study, the DSPACE1103 was used to control unit to instantly employ the MATLAB Simulink simulation model. For this study, it is not important to develop speed controller. This is primarily because of that vehicle operators can manipulate speed in order to unique driving conditions.
Direct torque control (DTC) is carried out without having speed sensors, but only current and voltage sensors are employed to determine the stator voltage vector46. In the outcome of their application of closed loop controllers, they suggest that the speed of calculation is excessive for data oscillations. Siahbalaee analyzed the flux-optimized copper loss of DTC PMSP to minimize torque and flux oscillation47.The minimal use of predictive control in direct torque control is researched in the literary works 48. Experimental results were acquired by utilizing DSP. Nonetheless, in the experimental results, they recognized complicated trigonometric functions. In the experimental results, additional flux drops can be monitored compared to the simulation results. They can employ the suggested usages by decreasing flux references.
Direct torque control (DTC) of PMSM is understood by implementing model predictive control (MPC) algorithm, which minimizes switching frequency and thus switching loss25. The algorithm could minimize the switching loss by 50% and minimize the THD by 25%.