A New Integrator for Voltage Model Flux Estimation in a Digital DTC System

This paper presents a new solution, which can completely remove the DC offset and compensate the steady-state error for voltage model flux estimation in an induction motor (IM) direct torque control (DTC) system. The voltage model flux estimator has been paid more attention for its clear concept and simple signal processing, however, the integration drift associated with the inherent initial value error and DC offset input, especially in low speed region, can not be completely eliminated by the conventional low-pass-filter (LPF), in addition, LPF also brings steady-state amplitude and phase errors. Based on Bode plots analysis of some existing integrators, a new integrator with a first- order LPF and a high-pass-filter (HPF) in series is proposed to replace the pure integrator. The filters have adaptive cutoff frequency so as to suit various conditions. With the time phasor method, an error compensation formula is derived, which makes it simpler and easier for the new integrator to be applied to the digital control system. The new integrator with error compensation not only completely eliminates the integration drift, but also realizes errorless flux estimation in steady state. Both the simulation and experimental results on a prototype of 4 kW IM DTC system show that the performance of the system especially in low speed region has been improved remarkably. This also verifies the effectiveness of the new integrator