In various applications that utilize maximum torque per ampere (MTPA) operation of interior permanent magnet synchronous machines (IPMSMs), there exist unexpected perturbations in electrical parameters and operating environment, which deteriorate the accuracy and efficiency of the MTPA operation. This paper establishes a step-signal-injection-based robust MTPA operation strategy for the IPMSM drives. The method works by injecting a step signal into the current vector angle and observing the response in the current magnitude, followed by a proportional-integral controller which returns the system to optimal operation. The stability of the proposed algorithm is established using the Lyapunov theory. A speed-servo control system of IPMSM is considered, where a disturbance detection unit is designed to switch operation between optimal current angle updating mode and steady-state MTPA operation mode. Since the optimal current angle updating is independent of machine parameters, the impact of system perturbations on the MTPA operation can be effectively suppressed. Extensive experimental results for IPMSM and hardware-in-the-loop-based machine are presented to validate the effectiveness and robustness of the proposed method.