Sensor-Less Estimation of Rotor Position in a Doubly Fed Induction Machine

Abstract

For a precise power or speed control of a Doubly Fed Induction Machine (DFIM), with a good dynamic and steady state response, accurate information of rotor position and speed is essential. Thus, the signal processing stage should be provided with accurate speed/position information either through use of physically mounted shaft encoder or through robust sensor-less estimation. Usually, there is a large separation between the signal processor and the turbine shaft, such as in wind power generation. Hardwired shaft-mounted sensors involve ungainly cabling and require perfect mounting, periodic maintenance, and added cost. Sensor-less schemes involve estimation of speed and/or rotor position by means of a digital signal processor and a set of voltage and current sensors that are already part of the speed or power control system. Various studies are reported on sensor-less estimation of rotor position or speed. Typically, there are two broad categories: closed loop and open loop schemes. The open loop techniques usually involve computations based on the machine model while utilizing the measured voltage and current signals. The closed loop schemes are based on Model Reference Adaptive System (MRAS) techniques or observer-based techniques. In this chapter, an introduction to the operation of a DFIM , significance of rotor position, and an overview of the sensor-less rotor speed/position computation techniques for a DFIM are presented. Further, two typical schemes for a grid-connected DFIM are described in detail, out of which, one is an open loop scheme while the other is a closed loop scheme. The efficacy of the schemes is demonstrated through simulations and experimental results. In this chapter, a brief introduction about the grid-connected doubly fed induction machine is given in Sect. 5.1. The mathematical model of a DFIM and the basics of decoupled control of DFIM in generating mode along with the control aspects of the back-to-back converters (RSC and FEC) are presented in Sect. 5.2. The significance of rotor position and speed and a brief review of the sensor-less estimation techniques for the control of DFIM are discussed in Sect. 5.4. Section 5.5 presents the underlying theory and the mathematical formulation of the techniques for sensor-less estimation of rotor position. Further, one of the recently reported techniques under each category is considered for illustration in Sects. 5.5.1 and 5.5.2. Details of control implementation, test results, and comparison with other schemes are presented in Sect. 5.5.3. Section 5.6 presents the overall conclusions.