The exact equations defining steady-state operation of a controlled current induction motor drive system are derived by solving the system state equations in the stationary reference frame. These equations, which assume ideal current filtering, eliminate the difficulties involved in taking derivatives of discontinuous currents by defining a pair of pseudocurrent variables. Effects of saturation are included by using the slope ratio method. Electromagnetic torque and current pulsations are computed for various load conditions, and experimental confirmation of the calculated results is made. Similarities and differences to voltage controlled characteristics are presented. It is shown that normal open-loop operation occurs on the unstable side of the torque-slip characteristic necessitating the use of feedback control for stable operation.

State-Variable Steady-State Analysis of a Controlled Current Induction Motor Drive

This paper presents the design of an induction machine current controller that is entirely implemented in digital hardware. A hardware current controller allows high switching frequencies with only modest processing power, as well as simplified controller hardware and software. The paper briefly presents the concepts of the algorithm implemented, and then outlines the changes that are made to make the digital implementation even more efficient. It then discusses the architecture used for the hardware design. Experimental results are presented to demonstrate the algorithm's performance.

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Digital hardware implementation of a current controller for IM variable speed drives

The usual techniques for implementing current control for hard switched inverters usually involve PI control or hysteresis based control. PI strategies suffer from poor transient performance, and the latter requires complex analog circuitry to give a constant switching frequency. This paper describes a new current control algorithm suitable for implementation in software or digital hardware. A novel feature of the algorithm is that it is able to identify the required machine parameters online. Furthermore there is no tuning of control parameters required from the user. The algorithm output is the switching times for the inverter switches. Comprehensive simulation results are presented, and issues related to hardware digital implementation are presented.

A digital current controller for three phase voltage source inverters

The principle of operation of a dual current source converter is briefly explained. The combination of two single current source converters (SCSC) to form a ``dual (duplex) current source converter'' (DCSC) is proposed. The DCSC is shown to have the following merits: 1) it retains all the advantages of the SCSC; 2) it reduces the harmonic content of the current waveform considerably; and 3) since the load current is shared equally between two current source converters, ratings of the individual components employed in the circuit are considerably lowered. A DCSC can be an attractive choice for sophisticated large horsepower drives where a good performance of the drive rather than cost is a prime factor. An open-loop control scheme employing the DCSC for an ac motor drive has been successfully implemented in the laboratory. Oscillograms of the improved load current waveforms are shown.

Principle of a Dual Current Source Converter for AC Motor Drives

An induction machine model in which the order of the electrical differential equations is one half of the conventional real variable model is obtained by using the complex time variables introduced by Ku and Lyon in the 1950's. Since the stepped excitation waveforms associated with polyphase inverters are also well suited to complex representation, the method provides a powerful means of exploiting the symmetry inherent in inverter-machine systems. After a brief introduction to the concept of complex time variables, the steady state analysis of six-step voltage and current source inverter driven induction machines is presented. Closed form solutions for the instantaneous voltages, currents and torques are derived. For small slip the current fed machine solutions are reduced to simple expressions involving rotor frequency sinusoidal functions and the relationships between these waveforms and the fundamental component equivalent circuit solutions are demonstrated. The solutions for the voltage fed machine are more complicated, however, hand computation of peak values is feasible or extremely simple computer programs can be written to yield the instantaneous current and torque waveforms.

Steady state performance of inverter fed induction machines by means of time domain complex variables

The paper shows that, if a cage induction motor is fed with a quasi-square-wave current, the motor terminal voltage is approximately sinusoidal under normal operating conditions. A simple analytical approach, based on the assumption that the motor resistances and inductances do not change with frequency, is shown to describe adequately the motor-terminal voltage. The quasi-square-wave-current supply is derived by modification of a known variable d.c.-link-voltage invertor. These modifications result in numerous advantages. The modified invertor is capable of regeneration back into the mains supply, has only 12 noninvertor-grade thyristors, and is undamaged by short-circuiting of the output terminals or by misfire of the output thyristors. Overall, the invertor lends itself to a simpler electrical and mechanical arrangement.

W. Farrer

Papers

Quasi-sine-wave fully regenerative invertor