Encoderless Control of AC Drives

Field-oriented control of AC drives requires knowledge of the mechanical rotor position. Generally, it is detected by a position encoder. However, in cost sensitive applications where maximum performance is not required, for instance fan and pump applications or traction drives, it is desirable to save this expensive sensor. Besides the cost advantage, encoderless control convinces with high mechanical robustness, as no sensitive electronic components are present in the machine. There are two main methods for encoderless position detection, the first is based on additional signal injection, whereas the second is based on the evaluation of the back-EMF voltage. The back-EMF based methods typically detect the mechanical position by integration of the induced voltage, which is estimated with an asymptotic observer at nonzero speed. Around zero frequency, however, this method does not work in practice – here it is necessary to use another method using high frequency signals to detect the rotor position. The resulting amplitude modulated information in the HF current is used to find what is called the anisotropy position, from which the rotor position may be estimated. Research projects published recently indicate high frequency injection methods becoming more and more attractive. These methods generally interact with the saliencies of an electrical machine obtaining the desired rotor position by demodulation methods. Saliencies are well-known when using Interior Permanent Magnet Machines or Synchronous Reluctance Machines. The most recent progress in this area is the sensorless control of synchronous machines with surface mounted permanent magnets. When using encoderless control for synchronous machines, the rotor position must be known for start-up. The presentation will explain how the rotor position can be identified from start-up without any motion of the rotor. The impact of HF injection based encoderless control on the dynamics of the drive depends on the type of injection signal and the direction of injection. This will also be explained during the presentation.     «

Field-oriented control of AC drives requires knowledge of the mechanical rotor position. Generally, it is detected by a position encoder. However, in cost sensitive applications where maximum performance is not required, for instance fan and pump applications or traction drives, it is desirable to save this expensive sensor. Besides the cost advantage, encoderless control convinces with high mechanical robustness, as no sensitive electronic components are present in the machine. There are two m...     »