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Speed estimation schemes based on the closed-loop synchronization (CLS) methods for speed-sensorless control of motor drives attract much popularity due to several advantages, e.g., easy implementation, high flexibility, and acceptable performance. However, most of the existing CLS-based estimation schemes may suffer from performance degradation during frequency ramps. Considering this, an attempt of the type-3 phase-locked loop (PLL)-based scheme is made. This solution, however, may adversely affect the system dynamics and stability margin. To address these issues, an open-loop synchronization (OLS) method is proposed for speed-sensorless control of induction motor drives in this paper. In the proposed scheme, the estimated speed is obtained according to the sinusoidal signals and their time-delay signals, rather than increasing the system order. With this, system dynamics and stability margin are maintained. In practice, the disturbance of DC offsets is of concern in induction motor drives. Thus, a closed-loop flux observer is adopted to guarantee the estimation performance under DC offsets. The performance of the proposed OLS scheme is investigated and compared with that of the CLS schemes and the type-3 PLL scheme through experimental tests.Index Terms-Open-loop synchronization (OLS), speed estimation scheme, closed-loop flux observer, induction motor drives I. INTRODUCTIONWith strong demands of reliability enhancement and cost reduction, speed-sensorless control becomes increasingly popular in induction motor drives. The procedure involves estimating the speed information properly, and accordingly, removing speed sensors. This task is mostly carried out by using various speed estimation schemes, which can be roughly categorized into non-ideal-phenomena-based and model-based schemes [1], [2]. Poor flexibility and high complexity are two main barriers of the widespread application of the non-idealphenomena-based estimation schemes [3], [4].

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This paper deals with I-f control of surface mounted permanent magnet synchronous machine (SPMSM) drives. Conventional open-loop I-f control is poorly damped and can only operate stably with positive d-axis current. An I-f control with a new current compensation loop is proposed to achieve stable i d = 0 operation in this work. The proposed method can be used for startup with a seamless transition to sensorless field-orientedcontrol (FOC), since i d = 0 operation is already achieved before switching to FOC. In addition, as the I-f control can now maintain stable i d = 0 operation against 100% rated load step disturbance from 10% to 100% rated speed, it could be a standalone control scheme for wide-speed-range operations. The proposed method is supported with detailed stability analysis and its performance is verified experimentally on a 2.7-kW SPMSM drive platform.

This paper deals with stability analysis of openloop I-f control for permanent magnet synchronous machine (PMSM) drives. The nonlinear system dynamics is linearized at steady state working point for analyzing the local stability of open-loop I-f control for PMSM. The analysis shows that the open-loop I-f control is poorly damped but can achieve stable operation in a wide speed range without the mid-frequency instability issue that commonly exists for the open-loop V/f control. In fact, the open-loop I-f control can operate stably from zero to rated speed under different load conditions. Extensive simulation results are presented to verify the theoretical analysis.