Research On Direct Torque Control For Electrically Excited Synchronous Motors

Electrically excited synchronous motor (ESM) has been widely applied in industry because of its advantages of adjustable power factor and high efficiency, and so it deserves researching high performance ESM drives. Recently, there are mainly two control schemes used by developing performance ESM drive systems: one is field orientation control (FOC), and the other is direct torque control (DTC). Although the ESM drive systems controlled by FOC have many advantages over the scalar controlled ESM systems in dynamic performance, the decoupling control in FOC ESM could not come true because of the salient pole of rotor structure and the large time constant of rotor winding circuit. The DTC considers the motor and inverter as a whole, and controls the stator flux linkage vector and electromagnetic torque directly by applying an optimal space voltage vector during each sampling period in the stator reference frame. If the DTC is implemented on ESM, the response time of torque can be much shorter. Although the ESM DTC was partly researched by ABB and Lappeenranta University of Switzerland in about 1998, the whole theory structure of ESM DTC could not be obtained from their few published papers. At home, the no-damper windings ESM DTC had been investigated by Nanjing University of Aeronautics and Astronautics partly from 1997 to 2001. So the ESM DTC lacks full and in-depth study, and the theory for ESM DTC has not been constructed. The main purpose of this thesis is to build up the theory construction as follows:After reviewing the DTC scheme with low torque and low flux linkage ripples for AC motors, the control methods of reducing the torque and flux linkage ripples are given as: improving the output of voltage vector in software, applying new inverter with multi-vector output, applying multi-phase motor, and applying modern control theory et. After reviewing the sensorless schemes of AC synchronous motors, the sensorless technologies are grouped under two kinds. One is based on the fundamental variables, and the other is based on external exciting signal. The estimation method of the initial rotor position can be classified as: basing on the calculation of stator inductance, alternative magnetize using high signal, high frequency signal injection, and the saturate effect of motor magnetic circuit et. There are few papers investigating the ESM initial rotor position estimation.The feature of air-gap flux linkage in ESM with damper windings has been analyzed, and the results show that the time constant of air-gap flux linkage is very large. The quick response of torque in ESM DTC can be gained by controlling the torque angular between the stator flux linkage vector and the air-gap flux linkage vector using the optimal space voltage vectors. The results of simulation and experiment show that the above analysis is correct.In this thesis the relationship between torque and torque angle in ESM of DTC has been deduced in detail, and the effect of damper windings on the dynamic performance of ESM based on DTC has been presented by digital simulation. The results of simulation show that the derivative of the main torque near the zero cross is positive with respect to the main torque angle. The q-axis damper winding can reduce the oscillation amplitude of the torque and speed significantly, and the d-axis damper winding improves the dynamic performance of the motor slightly and decreases the system's stability. So the d-axis damper winding of ESM operating at motor is unnecessary.The weakening control principle of ESM DTC systems has been analyzed and an optimal excitation control method for DTC-controlled ESM drives in the whole speed rang has been investigated in this thesis. The inner control method for excitation should been applied in the constant torque range, and the unit inner power factor is gained. The outer control method for excitation should been applied in the weakening range, and the unit outer power factor is achieved. The conversation between the two excitation methods can directly happens at the rated speed without transition. The results of simulation and experiment show the power factor of ESM is very high both in constant torque range and in field weakening range, and the conversation between the two excitation control methods is very smooth. A DTC scheme with space vector modulation (SVM) for ESM has been investigated in this thesis. It is based on the compensation of the stator flux linkage vector error using the space vector modulation in order to decrease the torque and flux linkage ripples. Compared with the basic DTC, the results of simulation and experiment show that the torque and flux linkage ripples are all much reduced, the maximum current value is decreased during the startup and the current distortion of steady state is much small in the SVM-DTC system. The SVM-DTC system is also simple and the switching frequency of inverter is approximately constant. The field-weakening control is incorporated into the SVM-DTC successfully.In this work, a proportional-plus-integral (PI) torque controller with variable proportion for direct torque controlled ESM drive based on the torque angle controlling the torque is proposed in order to ensure exact and quick control of torque. The experimental results verify the feasibility and effectiveness of the proposed PI torque controller. This PI torque controller is also suitable for the permanent synchronous motor drive system based on DTC.This thesis has proposed two methods of the initial rotor position estimation, i.e. closed loop estimation and open loop estimation, for ESM drives based on DTC without stator voltage sensors. The stator currents and rotor excitation current should be sensed under the condition of three-phase short-circuit and AC rotor current or DC pulsation rotor current in the two strategies. The results of simulation and experiment show that the estimated value error of initial rotor mechanical angle is less than±20, and SVM-DTC system can start up with given torque smoothly with the estimated initial rotor position.The effect of DC-link voltage, sampling period, and inductance of stator on the maximum start-up current has been analyzed on conventional DTC(CDTC) and SVM-DTC drive based on the two-level inverter. A modified CDTC scheme with time-controlled vectors has been proposed using torque and stator flux linkage hysteresis controller and switching table. A stator current-controlled SVM-DTC has been proposed based on the principle of stator flux linkage vector error compensation. The results of simulation and experiment show that the two proposed DTC schemes are effective to reduce the start-up current. The effect of multi-level inverter on decreasing the start-up current of ESM SVM-DTC has been researched by means of simulation. The results of simulation show that multi-level inverter can reduce the value of start-up current effectively because the magnitude of output voltage vector in multi-level inverter can be changed automatically according to the rotor speed.