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Current regulated pulse width modulation controller impact on low speed performances of industrial encoderless adjustable speed drives under sliding mode variable structures

Although adjustable-speed drives (ASDs) with induction machines (IMs) have already evolved as a mature technology, accurate encoderless operation at very low speed range remains to be problematic for all existing control methods, which include nonlinearity compensation, model-reference-adaptive-system (MRAS), various types of observers, high frequency injection leveraging saliency effects, artificial intelligence (AI) or neural network techniques. These encoderless solutions are integrated into either direct torque control (DTC), or stator current regulated pulse-width-modulated (CRPWM) controllers. The DTC solution can have better dynamic responses, while the CRPWM regulator provides superior steady state performances. In the category of observer formulations without signal injection, one promising dual frame sliding mode observer (DFSMO) can achieve a published performance of below 6 revolutions per minute (rpm) (0.2Hz) at full load under a DTC scheme. DFSMO does not need the rotor position or rotor flux angle, thus it increases the estimation accuracy and is tolerable to IM parameter variations and disturbances. Even though DTC schemes have been researched more in recent years, the rotor flux orientated vector control (RFOVC) architecture still dominates industrial applications. In this paper, instead of DTC, a DFSMO is developed under the RFOVC architecture which cannot be found in published literatures to demonstrate its transient and steady-state low speed performances at both motoring and regenerating mode. Analytical, simulation studies are presented and verified through a 3hp (2.2kW), 380V, 50Hz IM ASD experimental system.