In this article, a new frequency adaptive second-order disturbance observer (FASODO) for back electromotive force (EMF) and active flux estimation is proposed using the active flux model of interior permanent magnet synchronous motors. FASODO is a frequency adaptive approach and eliminates the phase delay compensator demand of the traditional sliding mode observer-based back EMF estimator. Besides, FASODO estimates the active flux and the back EMF simultaneously. As a result, a quadrature phase-locked loop (Q-PLL) using either the back EMF or the active flux and a tan inverse approach can be used for speed-position identification. Meanwhile, an integrally compensated Q-PLL is designed to overcome the traditional Q-PLL position estimation error during ramp frequency tracking. The design guideline of FASODO and the integrally compensated Q-PLL are provided. A comparative analysis with traditional SMO shows that the proposed approach can deliver a better sensorless control performance. Furthermore, a comparison with a frequency adaptive complex coefficient filter enhanced SMO shows a comparable result, while the proposed approach additionally delivers the active flux. The approach is verified using the Texas Digital Signal Processor (TMS320F28335) and RT-LAB Real-Time Simulator, taking into account different operating conditions.