A Simple Method to Estimate Operating Characteristics of Variable-Speed Induction Motors Using a Digital Power Meter

This paper proposes an online method for estimating the operating characteristics of variable-speed induction motors, such as their rotational speed, torque, and efficiency, using voltage, current, input power, and frequency measured by a digital power meter. The technical contribution of this paper is its ability to easily estimate motor losses (iron loss taking into account voltage and frequency dependence, mechanical loss taking into account rotational speed, and stray load loss (no need for the assumed value standardized by IEC or JEC) taking into account frequency and load level) in real time during operation at any V/f ratio. Furthermore, the resistance of the primary and secondary windings, which is an obstacle in online estimation, are calculated using a simple approximate formula that considers temperature dependence due to thermal equilibrium. In addition, an approximate slip calculation formula based on the T-II equivalent circuit model is derived as a rotational speed estimation method suitable for this technology. The effectiveness of the proposed method is demonstrated through a case study using a 0.75kW IE3-class induction motor.