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The aim of this paper is to study and realize a universal control scheme for a pulse width modulated drive system using the capability of digital computation in real time. Two microprocessor-systems and additional digital and analog circuits are implemented in order to fulfill that control scheme. The system speed would be dynamically changed for a wide range, starting from stand still up to twice the nominal speed in the field weakening range. Also at the same time, the maximum source voltages and currents are fully used. The implementation of the circuit to produce the firing pattern for the pulse width modulated inverter depends on voltage signals corresponding to the phase voltage instead of phase currents in order to eliminate the torque and field errors which will occur at high frequencies due to the inequality of the sinusoidal desired and actual currents. The control system is tested using a 2.5 KW squirrel cage motor and the obtained results prove that the system is stable and suitable to be implemented in industrial applications. FIFTH A.S.A.T CONFERENCE 4-6 May 1993, CAIRO 1 EE-315361 List of Symbols Signals Current; Current component; Current vector I Current in vector diagram; effective value Voltage e Induced voltage Rotating frequency (i'mech; e'm Mechanical rotating speed calculated for one pole pairs Angle; Phase-angle P Flux Te; Induced torque; load torque Machine Parameters R Resistance L; M Inductance; Mutual inductance J Inertia cr Total leakage factor K11 Factor of the inductance inverse matrix p Number of pole pairs To Calculated short-circuit time constant on stator side Other Notations T Time constant; switching time G Transfer function Kp Proportional amplification factor of a controller KI Integration factor of a controller A Relative error WMC; NC Speed controller IC Current controller VVC Voltage to voltage converter CVC Current to voltage converter PWI Pulse-width-inverter FC Field calculating circuit GC Transformation angle calculating circuit CRT counter AC Analog calculation circuit DAC Digital to analog converter Indices r a; b; c a; R x; y d a m Stator values Rotor values Values of the three-phase stator-fixed coordinates Values of the two orthogonal stator-fixed coordinates Values of the rotating frame Desired values Actual values Mechanical Values t Total value the rarely used notations are clarified where used., AC constant frequency DC PWI Pulse 0-• Voltage Vdc Width Source Inverter i AC variable frequency FIFTH A.S.A.T CONFERENCE I EE-31537 4-6 May 1993, CAIRO Introduction Since long time, the squirrel cage induction machine is used as a drive motor due to its simplified and robust construction. Today, drive systems of wide variable speed ranges are constructed by linking that type of induction machines with the modern semi-conductor elements. According to the control scheme and the used power electronics circuits the characteristics of the machine becomes, day after day, as of dc motors. Many types of power circuits are designed to fulfill such characteristics. One of them is the pulse width modulated drive system which will be studied in this work. The general construction of a PWM drive system consists as shown in Fig.1 of a dc voltage source, a pulse width modulated inverter (PWI), and a squirrel cage asynchronous machine (ASM). The voltage source may be a battery bank or a dc-converter followed by a suitable filter. According to the power of the machine, the inverter may be a transistorized or a thyristorized inverter. The implemented system consists of a three-phase dual-converter coupled with a three-phase transistorized inverter through an L/C filter. Fig.! A PWM Drive System Machine Modeling Generally, the design of any control scheme depends on the characteristic of the different elements of the drive system. The main part of the system is the induction machine and therefore a suitable mathematical model for it will be obtained. Thereafter the control strategy and its implementation by a micro-computing system is performed. The mathematical model of the ASM in the two orthogonal axis theorem has been studied in many works before as in /1, 2/ by using the following assumptions. 1. The iron losses and the magnetic saturation are neglected. 2. All parameters as resistors and inductors are constants. 3. The air-gap flux is sinusoidal. 4. The zero conductor is not connected. By micro-computing system control, as by the performed system, the non-linearity of the machine is taken into consideration and hence, the first two assumptions are overcome. Here, for the implemented control scheme, the stator current is and the rotor flux yr are chosen as state variables and hence the state-equations of the machine system are created after transforming the rotor flux yr into the following form; FIFTH A.S.A.T CONFERENCE I EE-31 5381 4-6 May 1993, CAIRO

Computer Control Of A PWM Induction Machine Drive System