The voltage/current model in field-oriented AC drives at very low flux frequencies

For field-oriented control of an induction machine it is necessary to determine the angle of the magnetic flux vector inside the induction machine with respect to the stator frame. When this is to be done without a flux sensor and without a rotor speed/position sensor, the voltage/current model, or uti-model, is the means of determining this flux angle. The uti-model experiences drift problems, which are problematic especially at low flux frequencies. By analyzing the uti-model in field-oriented coordinates instead of fixed stator coordinates, the drift problem is transformed into a stability problem. The stability improvement is done by two feedback methods inside the uti-model: flux magnitude feedback and flux derivative feedback. The uti-model is first analyzed apart from the induction machine (basic analysis) and finally in a closed loop with the induction machine. With flux magnitude feedback a steady-state angle and magnitude error will occur. This error increases with decreasing flux frequency. It can be eliminated by utilizing the flux command value in the feedback loop. The steady-state error does not occur with flux derivative feedback. For high flux frequencies, flux derivative feedback with constant gain is the best solution. At low frequencies flux magnitude feedback with constant gain should be used. At very low frequencies however, the flux magnitude feedback must be extended by an additional feedback gain, which results in vector instead of scalar feedback. This extension considerably improves the steady state behaviour and the stability of the uti-model. Finally, at very low frequencies the two flux magnitude feedback gains must be taken in function of the speed and the load of the induction machine. The two internal feedback methods still leave an operation area to be solved: the area around zero flux frequency. In this small frequency range the high-frequency magnetizing current injection method can provide a way of determining the flux angle, even at zero flux frequency. This method is based upon saturation effects inside the induction machine and its basic functioning is introduced in this thesis. Further research should contain the complete theoretical background and prove and optimize its practical utilization. An important issue that is not fully studied in this thesis is the effect of parameter detuning, especially stator resistance detuning. The negative effects (in steady state and dynamically) of stator resistance detuning seem to be minimized by flux magnitude feedback with proper feedback gains. This phenomenon is still to be studied in more detail.

[1]  Werner Leonhard.  30 Years Space Vectors, 20 Years Field Orientation, 10 Years Digital Signal Processing with Controlled AC-Drives, a Review (Part 1) , 1991 .

[2]  D.W. Novotny,et al.  The universal field oriented controller , 1988, Conference Record of the 1988 IEEE Industry Applications Society Annual Meeting.

[3]  Aja André Vandenput,et al.  Regeltechnieken voor draaiveldmachines , 1996 .

[4]  Robert D. Lorenz,et al.  A physically insightful approach to the design and accuracy assessment of flux observers for field oriented induction machine drives , 1992, Conference Record of the 1992 IEEE Industry Applications Society Annual Meeting.

[5]  Pasi Pohjalainen,et al.  The Next Generation Motor Control Method: Direct Torque Control (DTC) , 1995 .

[6]  Michele Pastorelli,et al.  Comparison of universal field oriented (UFO) controllers in different reference frames , 1995 .

[7]  Werner Leonhard,et al.  Control of Electrical Drives , 1990 .

[8]  Thomas A. Lipo,et al.  Direct field orientation controller using the stator phase voltage third harmonic , 1992, Conference Record of the 1992 IEEE Industry Applications Society Annual Meeting.

[9]  Tsugutoshi Ohtani.  Reduction of motor parameter sensitivity in vector-controlled induction motor without shaft encoder , 1990 .

[10]  Thomas A. Lipo,et al.  Field oriented control of induction machines employing rotor end ring current detection , 1994 .

[11]  Jorge L. Duarte,et al.  A multi-DSP system for real-time simulation and control of electric drive systems , 1995 .

[12]  T. Ohtani,et al.  Vector control of induction motor without shaft encoder , 1989, Conference Record of the IEEE Industry Applications Society Annual Meeting,.

[13]  M. Depenbrock,et al.  Direct self control of inverter-fed induction machine, a basis for speed control without speed-measurement , 1989, Conference Record of the IEEE Industry Applications Society Annual Meeting,.

[14]  Toshihiko Noguchi,et al.  A New Quick-Response and High-Efficiency Control Strategy of an Induction Motor , 1986, IEEE Transactions on Industry Applications.

[15]  A.J.A. Vandenput,et al.  Stability analysis of a new voltage/current model in open loop operation in direct field-oriented control , 1995 .

[16]  A. Veltman.  The fish method: Interaction between AC-machines and switching power converters , 1994 .

[17]  M. Depenbrock,et al.  Direct self-control (DSC) of inverter-fed induction machine , 1988 .

[18]  P. Vas.  Vector control of AC machines , 1990 .

[19]  Longya Xu,et al.  Robust, parameter insensitive position sensorless field orientation control of the induction machine , 1994, Proceedings of 1994 Power Electronics Specialist Conference - PESC'94.

[20]  Peter Vas,et al.  Electrical Machines and Drives: A Space-Vector Theory Approach , 1993 .