Research On Low-torque Ripple Direct Torque Control For Salient PM Brushless DC Motor

Permanent magnet brushless DC motor is a kind of motor with a relatively rapid development.It has the characteristics of simple structure,high output and high efficiency,and has been widely applying in robot,industrial process control,precision machine tools,automotive electronics,home appliances and office automation.To achieve a good torque characteristic,direct torque control is used in PM-BLDCM with two-phase 120° conduction.However,there are problems in conventional direct torque control with torque bang-bang,such as larger ripple of torque and current.So,an improved low torque-ripple direct torque control strategy is searched.In this thesis,the mathematical model of the salient permanent magnet brushless DC motor was constructed,and the theoretical basis of the direct torque control is analyzed.Two-phase conduction DTC strategy adopting a single hysteresis-controlled torque loop is used to drive PM-BLDCM.The two-phase conduction voltage vectors make the stator flux vector rotate rapidly,achieving the control of torque.The strategy has a simple structure,and is convenient to control.But DTC using the hysteresis-controlled torque loop has problems of torque and current ripple,witch lead the obvious torque ripple,since the bus voltage is fully applied in the stator windings in a control period.When the control period is relatively long,the torque ripple is evident.In order to overcome the disadvantages of traditional direct torque control strategy of torque ripple,the voltage space vector modulation(SVM)which is usually used in three-phase conduction is applied in the two-phase conduction DTC of PM-BLDCM.The torque angle increment ?? is obtained according to the torque error.Combined with the stator flux linkage vector ?s in this control period,the expected flux increment ??s is calculated.Considering resistance voltage,the given voltage vector is obtained.And according to the principle of two-phase conduction,the duty cycle of conduction phase is obtained,and precise control of torque can be achieved.Experiments show that compared with the traditional direct torque control,torque ripple obviously decreases using the direct torque control of two-phase SVM.In the commutation process of two-phase conduction,the change rate of currents in the upcoming phase and outgoing phase are not equal,which cause the current fluctuations in the non-commutation phase,resulting in the torque ripple.In high speed,the direct torque control of two-phase SVM can not restrain the torque drop caused by the sharp decrease of the outgoing phase current.Therefore,in the process of commutation of high speed,the duty cycle of the outgoing phase is adjusted according to the size of the torque drop in the delay time,to achieve the suppression of the non commutation phase current drop.And the torque ripple is restrained.The effectiveness of the proposed control strategy is verified by experiments.In order to control the stator flux,a preliminary study of permanent magnet three-phase conduction mode DTC forbrushless DC motor was done.The flux is divided into two parts:fundamental and harmonic,and equations of flux and torque were reconstructed.The defined torque ripple witch was a part of electromagnetic torque was compensated as disturbance.And the permanent magnet brushless DC motor was controlled in the similar way of permanent magnet synchronous motor.The simulation result proves the feasibility of the control strategy.At last the thesis introduces the design of the drive system of the permanent magnet brushless DC motor witch use the TMS320F2812DSP as the core and the IGBT as the switching device of the full bridge inverter as well as the software process.