Research On High Performance Control Of Induction Motor At Low Switching Frequency

Medium-or-high power converters used for induction machine drives operate at very low switching frequency,such as only several hundred Hertz,in order to reducing the switching losses of the power semiconductors for increasing the system efficiency.However,there are typically two problems when lowering the switching frequency:one is the increasing of the harmonic distortion of the machine current;the other is the poor dynamic performance of the motor control.For the first distortion problem,some optimized PWM techniques such as selective harmonic elimination(SHEPWM)or synchronous optimal pulsewidth modulation(SOPWM),can be employed to suppress the low order harmonic distortion due to its symmetrical pulse pattern and its synchrony between the switching frequency and the fundamental frequency.But the optimization is done offline and the steady-state operation must be assumed.It is not valid for transient and may cause PWM disorder or overcurrent fault when directly used in high performance control method such as vector control.In other hand,for the second sluggish dynamic performance problem,the system control bandwidth is narrow as the switching frequency reduced,the dynamic performance is poor.The delay of the digital control system and the PWM process further worsen the transient performance.So the key aspect of the high performance motor control at low switching frequency is the combination of the optimized PWM strategy and the fast dynamic control method.This topic is covered in this paper and the main content of the research are as follows:Firstly,the principle of the vector control(VC),also called field-oriented control(FOC),is introduced based on the model of induction motor in conventional synchronous rotating reference frame(d-q).In order to fully understand the coupling effects of the machine model or the nonlinear influence of the inverter and its control system,the machine model using complex vector notation is also established.Based on complex vector model,this paper analyzes a full-order machine model decoupling control method,which is coping with the digital control delay and PWM delay and can achieve excellent dynamic performance even in very low switching frequency.But,the whole control system is very complex due to full-order machine model and not easy for industrial implementation.This paper proposes a simplified method to reduce the order of the complex vector controller and the validity of the proposed method is confirmed by the simulation and experimental results.Among high performance control methods of induction machine,vector control has become the industrial standard method.The indirect vector control is quite popular because of its good dynamic response and relative ease of implementation.Hower,performance of indirect vector control depends on good knowledge of motor parameters,especially the rotor resistance.It is also influenced by the digital control delay,PWM process and the dead-time effects of the inverter.This paper proposes a novel simplified rotor resistance adaptation for induction machine indirect vector control.It is used the q component of the rotor flux to index the field orientation based on steady-state voltage model equation of the machine and the system features a simple and effective online scheme.The nonlinear effects such as digital control delay,PWM process delay and dead-time effect of the inverter,which are more pronounced at low switching frequency,are also investigated and are given effective compensation.Simulation and experimental results show good dynamic and steady-state performance.To combinate the optimized PWM strategy and the high performance vector control,a stator flux trajectory tracking control of the induction machine is investigated in this paper.By controlling the stator flux space vector to coincide with its optimal trajectory with pulse pattern modification algorithm,high dynamic performance in transient state is achieved while low harmonic distortion output currents in steady state are maintained.The whole control system is very complex due to its self-machine operation and fundamental variable estimation.This paper proposes a feed-forward simplification of the stator flux trajectory tracking control.It uses the relative clean feed-forward voltage reference to drive the PWM modulator instead of the fundamental stator voltage estimation and there's no need of self-machine operation.The proposed system is very simple to implementation while maintains good dynamic performance and desired steady state harmonic features and the validity of the method is verified by simulation results in MATLAB/Simulink environment.Another choice for high dynamic control of induction machine at low switching frequency is the model predictive control(MPC)method.MPC features high capability of coping with multi-objective optimization.The dynamic performance and the steady state low harmonic constrains can be addressed both in its cost function.This paper proposes a feed-forward stator flux trajectory tracking control with finite-control-set method.It used the MPC cost function to calculate and choose the optimal switching state and drive the inverter directly by tracking the optimized stator flux trajectory reference,which is re-construction by the PWM pluses actual in use.The transient response is fast and the algorithm is simple to implementation while the steady state harmonic performance is compromised.For precise tracking control of the stator flux trajectory,a continuous-control-set method is introduced in this paper with the pulse pattern modification algorithm.The dynamic performance is maintained while the steady state total harmonic distortion(THD)level is similar with the typical SHEPWM.The proposed method is verified by simulation and experimental results.