Research On The Control Technologies Of Seven-Phase Induction Motor

Compared with traditional three-phase AC speed regulation system, multiphase motor system has some advantages, such as high power with low voltage, high reliability and smaller torque ripple, especially under the condition where voltage level of power supply is limited. Except for the five-phase induction motor(the multiphase motor which has the most simple structure) and six-phase induction motor(the typical multiphase motor which has N three-phase windings), the seven-phase IM (Induction Motor) is another kind of important multiphase motor which has a prime number of phases. And the seven-phase motor shows the more obvious characteristics and advantages of multiphase motor, such as various PWM modulation methods with 128 voltage vectors, complicated control strategies under normal condition or under open-phase fault, and so on.With the seven-phase IM as the main research object, the motor model is built up firstly according to the space decomposition transformation theory. Aiming at the shortcoming of big harmonic stator currents, the cause and characteristics of the harmonic current and harmonic torque are analyzed.With regard to the PWM of seven-phase system, big harmonic currents are produced by traditional seven-phase SVPWM. According to the concept of reducing the 3rd harmonic subspace compositive voltage vector, the SVPWM with 3rd harmonic elimination is proposed, by which a small amount of computation and a small switch loss, but a bigger amount of 5th harmonic component of current are acquired. Using the SVPWM with 3rd and 5th harmonic elimination, which reduces either 3rd or 5th harmonic subspace compositive voltage vector to be zero, the harmonic currents are reduced remarkably, but this method costs more complicated computation and the position sector which reference vector lies must be judged in order to select the corresponding voltage vectors. So a novel PWM, seven-phase carried wave UVM(Unified Voltage Module), is put forward, with which the same harmonic elimination efficiency as the SVPWM with 3rd and 5th harmonic elimination is acquired without judging the position sector.As far as the vector control of seven-phase IM is concerned, it is important that the flux and torque could be controlled decoupling and the stator harmonic currents could be reduced as far as possible at the same time. Using the phase current hysteresis control method based on rotor field orientation, the control effect depends on the hysteresis width, which is limited by the power devices parameter restrictions due to the unfixed switching frequency. Therefore three kinds of vector control strategies of seven-phase IM are presented, which could control the flux and torque of the fundamental subspace decoupling, and could also reduce the harmonic currents at the same time, with open-loop SVPWM A, SVPWM B or close-loop UVM C respectively.In addition to the classic vector control strategy, the FCSMPC (Finite Control Set Current Model Predictive Control) is introduced firstly in the seven phase system. In the dissertation the predictive model of the seven-phase inverter with the resistance and inductance load are designed, and the effect of main parameters are also analyzed by some simulation and experiment results. Combined with the FCSMPC, the rotor field oriented vector control of seven-phase induction motor is simulated, and the effect of the key parameters are also analyzed, including the sampling frequency, the finite control set, the weighted factor of objective function, etc.The multiphase motor could keep running under open-phase condition through fault tolerant control strategies, which is an important advantage of seven-phase IM. In the dissertation, the motor model and voltage vectors distribution of seven-phase under all 12 kinds open-phase circumstance (including 1,2,3 or 4 phases opened) are constructed. Among the control algorithms under the open-phase fault, the current hysteresis control method is most simple and commonly-used whose object is maintaining the stator MMF unchanged, but it is shown that the hysteresis control strategy could cause an unfixed switching frequency and a certain torque ripple. The second control algorithm is traditional DTC (Direct Torque Control) whose object is also maintaining the stator MMF unchanged before and after the open-phase fault. The voltage vector selection table must be built up individually according to the different type of phases-opened fault. More seriously, the stator current are distorted remarkably and a large torque ripple is acquired, since only vectors with unequal magnitudes in fundamental subspace are considered, which leads to the poor load capability at the same time. Therefore, a new vector control of seven-phase IM with rotor field oriented under open-phase fault is proposed adapting modified UVM and fuzzy PI controller. Using the simple model of seven-phase induction motor under open-phase fault, a round shape rotor flux and minimized harmonic component of stator currents are guaranteed. And the load capability is also improved compared with the former two algorithms.At last the seven phase inverter experiment platform was constructed with TMS320F28335 DSP chip as the controlling core. With the seven-phase IM-DC generator or the symmetrical resistance and inductance as load, some experiments were carried out to verify the conclusions proposed in the dissertation.