Research On Reducing Adverse Effects By DTC In PWM Drive System

Inverter is an important aspect of AC drive system.However, some notable adverse effects are found in the wide applications of PWM inverters. Firstly, voltage source PWM inverters generate high frequency common-mode voltage, which induces high shaft voltage, leads to bearing current, and results in premature bearing failure; Moreover, high speed switching power electronic devices generate strong EMI. Conducted EMI and radiated EMI will disturb other control systems or electronic equipment, even result in malfunction. These damages due to the adverse effects maybe larger than the cost of the AC variable frequency drive system. Therefore, investigating the adverse effects of PWM inverters and its solutions has important significance and practical value.This paper investigated the mechanism of the adverse effects produced by the AC variable frequency drive system, and illustrated common-mode voltage is the essence of the adverse effects. For common-mode voltage output by the two-level inverter is only reduced by a filter, common-mode voltage reduction strategy in three-level inverter is investigated,which come to the conclusion, common-mode voltage can be eliminated in three-level inverter.On the basis,this paper analyzed and discussed Three-level Direct Torque Control. In Matlab, the new eliminated common-mode voltage DTC model is constructed with the seven effective zero-common-mode voltage switching states in three-level inverter, which combines Direct Torque Control technology. The simulation result verifies the effectiveness and advantages of the control strategy.By analying and improving the eliminated common-mode voltage DTC model, the simulation result verifies the referred strategy has the ability to completely eliminate common-mode voltage.