| Due to high power density of the permanent magnet synchronous machine(PMSM),it has been increasingly employed in industries such as the hybrid and the pure electrical vehicles,electric propulsion ships,and electric aircrafts.However,compared with the asynchronous machines and the current-excited synchronous machines,the PMSM has worse damping characteristics and hence is more sensitive to external disturbance which causes electromagnetic oscillations in the machine and even makes the system unstable at high speed.So far,most current controllers for the PMSM is designed mainly foused on the command tracking,and much less attention is paid on the disturbance rejection of the current controller.To improve both the command tracking and the disturbance rejection of the current loop and obtain current controller with better performance,the PMSM is firstly modeled mathematically.In continuous domain,the surface-mouted and the interior PMSMs are modeled using the complex vector notation and the state-space form,respectively.In discrete domain,their zero-order-hold(ZOH)equivalent models is derived considering the ZOH of the inverter in stator coordinates and the digital one-step computational delay.The damping characteristics of the PMSM are also analyzed using the eigenvalues and the torque coefficients.After reviewing the state feedback decoupling and the internal model decoupling,the active resistor is introduced into the current controller to improve the disturbance rejection of the current loop.The delays in the current feedback loop and their influences are then analyzed.To suppress the sensitivity of the current controller with the active resistor on feedback delays,two current controllers based on an improved PI and a pure integrator are proposed,respectively.Using the two proposed controllers,the current oscillations caused by feedback delays are reduced significantly,especially by the pure integral type controller.The active resistor concept is then extended to the active impedance.A current controller for PMSM employing active impedance is proposed to improve the disturbance rejection further.The feedback delays are compensated for via the Pade approximation.The stability boundary is then analyzed and the stable gain selection region is derived.The current oscillations due to feedback delays and large active resistance are suppressed effectively.Then the active resistor is extended from the continuous domain to the discrete domain.Theoretical analyses with the surface-mounted PMSM in discrete domain show that the active resistance value has an upper limit which is only related to the ratio between the machine time constant and the sampling period.To achieve better command tracking and disturbance rejection,a new discrete current controller based on complex vector deoupling and pole-zero cancellation is proposed.The rapidity and the robustness of the proposed controller is compromised with given inductance error level.Theoretical anslyses reveal that the active resistor is essentially the second degree of freedom of the current controller.Then a two-degree-of-freedom discrete current controller which can place the poles and zeros arbitrarily in the z-plane is proposed for the surface-mounted PMSM.And the command tracking and disturbance rejection of the current loop is further improved.To manipulate the parameter errors,a self-tuning algorithm based on the projection algorithm is developed,which significantly improved the dynamic current responses in case of inaccurate initial machine parameters.For the interior PMSM,a state feedback current controller using the state-space method is developed based on its ZOH-equivalent discrete model considering the one-step computational delay.Poles can be placed arbitrarily in the z-plane using this controller and the calculation of the control parameters are argmented.Compared with the classical discrete current controller,the proposed controller can increase both the command tracking and the disturbance rejection of the current loop,which is of certain theoretical and practical value. |
