Ingeniería de Sistemas y Autmática

Vector Control of AC Drives. Details

Control of multiple motors from a single inverter

Control of several motors from a single inverter and with a reduced number of sensors is often found in the strands in rolling mills, as shown in Figure 2-left. The utilisation of an inverter for each motor is not a solution because of the increase in the final cost, the added complexity in the hardware what also results in a decreased reliability, additional cabling, etc. AC drives installed in such applications work based on a volts/Hz control, with the limitation in the performance intrinsic to this technique. The goal of this project was to obtain performances closer to field oriented controlled AC drives without adding any additional hardware. The inverter developed for this project is shown in Figure 2-right, the control stage being based on a TMS320C30, a 32 bits, floating point DSP by Texas Instruments.
 

Figure 2: Left: Partial view of the strand. Right: View of an AC drive prototype developed for the control of a rolling desk, consisting of six induction motors fed from a single inverter. It includes a three phase inverter Skiip module by Semikron.

Surface defects in the slabs are for instance introduced during the cutting process (Figure 3-left). These defects will produce an unbalanced torque distribution among the motors of the desk. Volts/Hz controlled drives operating the rolling desk previous to the stamping machine (Figure 3-right) are unable to keep the slab at stand still during the stamping process. A field oriented controlled drive succeeded implementing an effective position control for such application.
 

Figure 3: Left: Cutting process. Right: Stamping machine.

Carrier injection based sensorless control

In its most common implementation, field oriented control of ac machines produces torque by injecting the torque producing component of the stator current so that it is always orthogonal to the rotor flux in the machine. To implement this class of field oriented controller knowledge of the rotor flux position is necessary. For full range operation of field oriented machines, rotor flux position is generally estimated using rotor position measurement. In addition to this, the measurement or estimation of position and/or velocity will be necessary for motion control.

The estimation of flux, position and velocity using the terminal properties of the motor (voltages and/or currents) has been a very active area of research for the last years, the reason being the cost and reliability problems associated with the mechanical sensors (usually encoders and resolvers) and cabling. The techniques proposed so far to accomplish this goal can be categorised in two major groups:

  1. Techniques based on tracking the back-emf.
  2. Techniques based on tracking of spatial saliencies (asymmetries).
Despite of being more popular, techniques based on tracking the back-emf present a major limitation, since the back-emf is the effect of a revolving field, they can not work at very low or zero speed.

Techniques based on tracking saliencies can potentially be used to estimate both the flux of the rotor position. A potential implementation of these techniques is shown in Figure 4. A high frequency carrier voltage (several hundred Hertz) is injected to the motor in addition to the fundamental voltage. The induced high frequency current will contain information relative to the position of saliencies (asymmetries) present in the motor. This information is extracted by means of some advanced digital filtering. The major advantage of this technique is that it doesn’t depend on the fundamental excitation, providing a wide bandwidth estimation even at low or zero speed.

Figure 4: Injection of a Carrier Signal Voltage Excitation for the Estimation of Rotor Position or Flux Angle

Saliencies are intrinsic to any motor design (e.g. due rotor and stator slotting). They are also produced due to the saturation of the flux paths, what can be used to estimate the main flux position. Rotor position dependent saliencies can also be introduced during the machine design or in a further modification to provide position feedback. An example of this is shown in Figure 5. The rotor slots are opened following a sinusoidal modulation, the induced carrier current containing information about the position of the saliency, and therefore of the rotor. Figure 6. shows the estimated position and the estimation error using this technique.
 

Figure 5: Rotor of a 0.75 kW induction machine modified to create a spatial dependent saliency by opening the rotor slots



 
 
 
 
(Mech.Deg.)
Error (Mech.Deg.)

Figure 6: Estimated Rotor Position and Estimated Rotor Position Error

Publications in International Conferences

  1. A. Diez, J.A. Cancelas, F. Mateos, J.A. Sirgo, F. Briz, "Design of Speed Digital Interface to ac Drives", Mediterranean Electrotechnical Conference MELECON, Ljubljana, Yugoslavia, May 1991, pp: 1281-1284, ISBN 0-87942-655-1
  2. F. Briz, J.A. Cancelas, A. Diez, J. Gomez, "Design of ac Drives with Position and Speed Dynamic Control", Workshop Motion Control for Intelligence Automation (IFAC), Perugia, Italy, October 1992, pp: 159-163
  3. F. Briz, J.A. Cancelas, A. Diez, "Design of ac Drives with Position and Speed Dynamic Control Using DSP", Internation Power Electronics Congress, CIEP, Cuernavaca, Mexico, August 1993, pp: 95-100, ISBN: 0-7803-1405-0
  4. F. Briz, J.A. Cancelas, A. Diez, "Speed Measurement Using Rotary Encoders for High Performance ac Drives", International Conference on Industrial Electronics, Control and Instrumentation IECON, Bolognia, Italia, September 1994, pp: 538-542, ISBN: 0-7803-1328-3
  5. F. Briz, F;J.C. Alvarez, A Diez, "Speed Estimation Using Torque Observer in FOC Systems", Workshop Motion Control (IFAC), Munich, Germany, October 1995, pp: 67-74
  6. Fernando Briz, M.W. Degner, R.D. Lorenz, "Analysis and Design of Current Regulators Using Complex Vectors", IEEE Industry Applications Society Annual Meeting, New Orleans, USA, October 1997, pp: 1504-1511, ISBN: 0-7803-4067-1
  7. Fernando Briz, A. Diez, M.W. Degner, R.D. Lorenz, "Current and Flux Regulation in Field Weakening Operation", IEEE Industry Applications Society Annual Meeting, Saint Louis, USA, October 1998, pp: 524-531, ISBN: 0-7803-4943-1
  8. Fernando Briz, M.W. Degner, R.D. Lorenz, "Dynamic Analysis of Current Regulators for AC Motors Using Complex Vectors", IEEE Industry Applications Society Annual Meeting, Saint Louis, USA, October 1998, pp: 1253-1260, ISBN: 0-7803-4943-1
  9. L.A. de Souza, M.W. Degner, Fernando Briz, R.D. Lorenz, "Compensating Carrier Frequency Current and Voltage Injection for the Estimation of Flux, Position and Velocity in Sensorless AC Drives", IEEE Industry Applications Society Annual Meeting, Saint Louis, USA, October 1998, pp: 452-459, ISBN: 0-7803-4943-1
  10. F. Cabanas M, G. Melero M., A. Orcajo G., Fernando Briz, Capolino G.A., "A New Methodology for Applying the FFT to Induction Motor On-Line Diagnosis", IEEE International Symposium on Diagnostics for Electrical Machines, Power Electronics and Drives, Gijón, Spain, September 1999, pp: 537-544, ISBN: 84-699-0977-0
  11. L.A. de Souza, M.W. Degner, Fernando Briz, R.D. Lorenz, "Using Carrier Frequency Current Injection for the Estimation of Flux, Position and Velocity Drives", 5th Brazilian Power Electronics Conference COBEP’99, Brazil, September 1999, pp: 673-680
  12. Fernando Briz, A. Diez, M.W. Degner, "Dynamic Operation of Carrier Based, Sensorless, Direct Field Oriented AC Drives", IEEE Industry Applications Society Annual Meeting, Phoenix, USA, October 1999, pp: 2313-2320, ISBN: 0-7803-5589-X
  13. Ignacio Díaz Blanco, Alberto B. Diez González, Abel A. Cuadrado Vega, José M. Enguita González, "RBF Approach for Trajectory Interpolaton in Self Organizing Map based Condition Monitoring", Emerging Technologies and Factory Automation ETFA'99, (UPC, Bacelona), October 18-21, 1999
  14. J.M. Guerrero, Fernando Briz, A. B. Diez, J.C Alvarez, "Design and Tuning of PI Velocity Regulators for High Performance AC Drives", IFAC Workshop on Digital Control PID’00, Terrassa, Spain, April 2000, pp: 107-112
  15. Fernando Briz, A. Diez, M.W. Degner, R.D. Lorenz, "Measuring, Modeling and Decoupling of Saturation Induced Saliencies in Sensorless, Vector Controlled, AC Drives", IEEE Industry Applications Society Annual Meeting, (to be held in Rome, Italy in October 2000).
Publications in International Journals
  1. Fernando Briz, Michael W. Degner, Robert D. Lorenz, "Dynamic Analysis of Current Regulators for AC Motors Using Complex Vectors", Transactions on Industry Applications, Vol. 35, nº 6. Nov./Dec. 1999, pp.1413-1424, ISSN: 0093-9994
  2. Fernando Briz, Michael W. Degner, Robert D. Lorenz, "Analysis and Design of Current Regulators Using Complex Vectors", Transactions on Industry Applications, Vol. 36, nº 3, May/June 2000, ISSN: 0093-9994
  3. Fernando Briz, Alberto B. Diez, Michael W. Degner, "Dynamic Operation of Carrier Signal Injection Based, Sensorless, Direct Field Controlled AC Drives", to be published in the Transactions on Industry Applications, Vol. 36, nº 5. Sept./Oct. 2000, ISSN: 0093-9994
  4. Fernando Briz, Alberto B. Diez, Michael W. Degner, R. D. Lorenz, "Current and Flux Regulation in Field Weakening Operation", accepted for its publication in the Transactions on Industry Applications, ISSN: 0093-9994
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