AC motor has the advantages of simple structure, reliable operation, long service life and low price. The disadvantage of AC motor is its complex control. On the basis of consulting a large number of documents, AIP has understood the research status of AC motor control at home and abroad as well as the existing achievements, and understood the basic principle of vector control. Matlab / Simulink is used to simulate the vector control system of AC motor, and the corresponding speed, torque and current curves are obtained, which verify the reliability and effectiveness of the vector control system.

1 Application of motor testing machine (motor testing system) 

The motor testing machine is widely used in the motor manufacturers to test the product performance and quality of the motor, while the high-performance testing machine needs to control the motor drive accurately, using the high-performance control method to control the AC motor, which can meet the requirements of the system. The most important part of the motor testing machine is the AC part. The quality of the AC part is directly related to the performance of the motor test bench. At present, there are many control methods of AC motor, such as direct torque control, neural network control and fuzzy matrix control. Here we mainly discusse the control of AC motor from the point of view of vector control.

2 Development and Prospect of AC motor control

After the induction motor vector transformation control method was put forward in the 1970s, it has been developed rapidly. Compared with AC motor, DC motor has better dynamic speed regulation characteristics. The vector transformation control is to use the coordinate transformation method to simulate the AC motor into the DC motor, control the excitation current component and torque current component respectively, so as to obtain the same good dynamic speed regulation characteristics as the DC motor. Because this method uses coordinate transformation, it requires high performance of hardware facilities.

Good hardware equipment is the key to ensure that the vector control method can be applied, especially to improve the reliability and real-time performance of the control system. This is the basic requirement of the control system and the key element. The development of motor control system has experienced the stages of analog circuit, integrated circuit, large-scale integrated circuit interpretation and special integrated circuit. So far, most of the motor control circuits are analog digital hybrid circuits. Because it can not only improve the reliability and real-time of the system, but also has smaller volume and less R & D cost, it has developed rapidly. As a special integrated circuit (ASIC) for motor control, there are many kinds and specifications of ASIC, and the product data and application data are very rich. But at the same time, there are some technical barriers due to the lack of uniform standards among manufacturers. Because the current motor control is always towards diversification and more and more complex, direct use of manufacturers' products sometimes can not meet the requirements. So many times, engineers need to develop their own products. DSP is a very good development tool. DSP (digital Signal Processor) is a unique microprocessor, which processes a large amount of information with digital signals. Its working principle is to receive the analog signal, convert it to 0 or 1 digital signal, modify, delete and strengthen the digital signal, and interpret the digital data back to the analog data or actual environment format in other system chips. It not only has programmability, but also can run tens of millions of complex instruction programs per second in real time, which is far more than the general microprocessor. It is an increasingly important computer chip in the digital electronic world. Its powerful data processing ability and high running speed are the two most praiseworthy characteristics.

3 Basic idea for vector control

The most important thing for motor control is to control the speed, which is achieved by the torque control. By controlling the magnitude of FS mode of stator magnetic potential or fr mode of rotor magnetic potential and their position in space, the torque of motor can be controlled. Controlling the magnitude of the mode value of FS or fr can be realized by controlling the magnitude of the current of each phase, while the position angles s and R in space can be realized by controlling the instantaneous phase of the current of each phase. Therefore, as long as the instantaneous control of each phase current (IA, IB, IC) of asynchronous motor stator can be realized, the effective control of asynchronous motor can be realized.

How to realize the instantaneous control of the stator current of the asynchronous motor by using the vector transformation control method? When the symmetrical three-phase sinusoidal alternating current IA, IB, IC is introduced into the three-phase symmetrical stator winding of the asynchronous motor, the fundamental wave resultant rotating magnetic potential is formed, and the corresponding rotating magnetic field ABC is established by him, and the rotating angular speed is equal to the angular frequency s of the stator current. However, it is not necessary for three-phase winding to generate rotating magnetic field. In addition to single-phase, any number of phase-to-phase windings can generate rotating magnetic field by introducing multiple phase-to-phase sinusoidal current. When two-phase symmetrical sinusoidal currents I and I are introduced, a rotating magnetic field is generated. If the magnitude, rotation speed and direction of the rotating magnetic field are exactly the same as the rotating magnetic field produced by the three-phase alternating current winding, the two sets of alternating current winding can be regarded as equivalent. It can be seen that the three-phase fixed symmetrical AC winding in the three-phase static coordinate system can be equivalent to the two-phase symmetrical fixed AC winding according to the principle of generating the same rotating magnetic field, and there is a certain transformation relationship between the three-phase symmetrical sinusoidal AC IA, IB, IC and the two opposite sinusoidal AC currents I and I in the three-phase AC winding. 

4 AC vector control simulation

According to the basic idea of vector control, Matlab / Simulink software can be used to simulate the motor AC control system.

In this system model, the most important is the current regulator. The function of current regulator is to provide driving signal for IGBT three-phase bridge. It compares the feedback current signal with the current given value and gets the difference of current signal. The difference value outputs high and low level signals through a relay. Each signal is connected to the upper and lower arms of the same bridge directly or through an inverted door. In this way, a complementary driving signal is added to the upper and lower arms of the same bridge. This avoids the simultaneous conduction of the two IGBTs on the same bridge, prevents the generation of the through phenomenon, avoids the overheating and burning of the three-phase bridge, and also enables the effective conduction of each device. Thus, the electromagnetic torque waveform can be obtained. 

Through the simulation experiment, the above simulation curve can be seen that the simulation model is basically consistent with the actual speed control system movement process. The correctness of the simulation model based on the vector transformation mathematical model of induction motor and Matlab / Simulink is fully verified. From this, we can get the following conclusions：

(1) The common mechanical characteristics of AC motor and DC motor are the essence and key of vector control of AC motor. 

(2) In physics, coordinate transformation must follow the principle of rotating magnetic field equivalence and power invariance. In mathematics, it is through the similar transformation of system state to achieve the purpose of state reconstruction and parameter reconstruction, so that the mathematical model becomes simple and easy to solve. 

(3) The difficulty of vector control is recognized by the observation of flux linkage. Because there are some problems in direct measurement, such as pulsation component and so on, most of them are indirect measurement now, but there are still some problems such as its accuracy is easily affected by the change of parameters. 

(4) Using Matlab / Simulink simulation, we can see that vector control AC motor can get very good dynamic performance.