An interior Permanent magnet synchronous motor (IPMSM) drive employing sensorless control strategies offers to simplify the design of servo systems, reduce costs, and improve reliability, thus attracting significant research attention from both academic and industrial sectors for decades. The straightforward configuration and decreased application prerequisites make model-based sensorless control approaches highly popular. Notably, the conventional first-order integrator flux observer technique exhibits remarkable robustness owing to its minimal reliance on motor parameters. However, the conventional first-order integrator experiences a DC drift and harmonics in the estimated rotor flux as a result of non-ideal factors, such as detection errors, integral initial value, converter nonlinearities, and parameter mismatches. In this paper, an improved ADRC-based integral flux observer capable of eliminating drift is developed to achieve high-accuracy flux estimation. The efficiency of the proposed technique in eliminating the drift from the estimated flux, as evidenced by theoretical analysis, has no detrimental effect on the amplitude or phase angle of the fundamental waveform. The validity of the proposed improved ADRC-based integral flux observer is verified by sensorless vector control of a 7.5 [kW] three-phase IPMSM motor via extensive numerical simulation.