Research On Permanent Magnetic Synchronous Motor Drived By Multi-level Inverter

Suppresion of current harmonics of permanent magnetic synchronous motor(PMSM)is important to the reduction of torque ripple and iron loss.Current harmonics can be reduced sharply when the multilevel inverter is applied.This paper deals with the diode-clamped neutral point clamped(NPC)three-level inverter.The neutral point potential balance,current ripple prediction and variable switching frequency PWM(VSFPWM)based on the current ripple prediction are researched in this paper.Traditional short vector balancing coefficient algorithm has limited neutral point potential balancing ability due to the fixed voltage vector sequences.This paper proposes two balancing coefficient algorithms based on dynamic voltage vector sequences.The balancing abilities of different vector sequences are compared based on the neutral point potential,phase current and dwelling time of related vectors.Then the voltage vector sequence is optimized to strengthen the balancing ability under different power factors.The zero-order voltage injection method and virtual space voltage vector method are also researched to balance the neutral point potential.The equivalent relationship between the proposed balancing coefficient algorithm and the zero-order voltage injection method is analysed in this paper.This paper analyses the output current slopes of three-level inverters under different voltage vectors based on the equivalent Thevenin circuits.The current ripple peak values are predicted accurately based on the dc bus voltage,output inductance,duty cycles and the switching frequency.A variable switching frequency PWM(VSFPWM)method based on the predicted current ripple is developed in this paper.The switching frequency is regulated dynamically with the current ripple requirement to be satisfied.The average switching frequency and the switching losses are reduced with VSFPWM.Meanwhile,the current spectrum is dispersed and the EMI noise is mitigated consequently.This paper sets up the MATLAB/Simulink model of the whole system and verified the proposed neutral point potential balancing algorithm,current ripple prediction method and three-level VSFPWM.A new PMSM driving system with three-level inverter is maded.Experimental results show that the two proposed balancing coefficient algorithms have stronger balancing abilities than traditional algorithms.Besides,Zero-order voltage injection method and virtual space vector method are verified by experiments.The current ripple is predicted accurately in experiments.When VSFPWM is applied,experimental results show that the switching losses are reduced greatly.Meanwhile,the currenr spectrum is dispersed and the EMI noise is mitigated.PMSM is drived by a three-level inverter with sensorless algorithm and its advantage of less current harmonic is verified.