This paper deals with the performance optimization of medium/high-power induction motors during soft starting by eliminating the supply frequency torque pulsations, and by keeping the line current constant at the preset value. Starting torque pulsations are eliminated by triggering back-to-back-connected thyristors at proper points on the first supply voltage cycle. Line current during starting can be kept constant at any preset value by a simple strategy composed of successive cosinusoidal and constant function segments of the triggering angle. These strategies are implemented by the use of an 8-b microcontroller. Transient performance analyses of the system are carried out by means of a hybrid ABC/dq machine model which takes into account the three-phase, two-phase, and disconnected modes of operation in terms of actual stator variables. Experimental results obtained on a custom-design test bed are found to be in good agreement with the theoretical ones.

Performance optimization of induction motors during Voltage-controlled soft starting

The performance of a voltage-controlled large induction motor soft starter has been improved, resulting in nearly perfect current and torque profiles. The performance analysis of the soft-starter motor-load combination has been carried out in the dynamic state using a hybrid induction machine model which takes account of disconnected two-phase and three-phase operational modes of the machine. Some simple control strategies have been proposed to keep the current constant at any preset value during starting, and to eliminate supply-frequency starting torque pulsations. These are shown to be very effective in the elimination of reclosing transient torque and current components at any speed at the reconnection instant to the supply after an interruption, or at the instant of bus transfer. The proposed strategies have been verified experimentally on a laboratory machine using a special torque measurement system in the dynamic state.

Soft starting of large induction motors at constant current with minimised starting torque pulsations

In order to mitigate the adverse effects of starting torque transients and high inrush currents in induction motors, a popular method is to use electronically controlled soft-starting voltages utilizing series-connected silicon-controlled rectifiers (SCRs). Investigation of semioptimum soft-starting voltage profiles was implemented using a flux linkage ABC frame of reference model of a soft-started three-phase induction motor. A state-space model of the soft-starter thyristor switching sequence for the motor and load was developed and implemented in a time-domain simulation to examine winding heating and shaft stress issues for different starting profiles. Simulation results of line starts and soft starts were compared with measured data through which validation of the model was established. In this paper, different induction machine soft-start profiles are shown, and comparisons of starting times, torque profiles, and heating losses are made. Discussion of these results and conclusions as to the near-optimum types of profiles are delineated based on peak torque, starting times, and winding heating criteria

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Soft started induction motor modeling and heating issues for different starting profiles using a flux linkage ABC-frame of reference

A unified and universally applicable method is proposed for simulating the dynamic behavior of a large, interconnected, nonlinear, time-varying physical system containing one or more three-phase induction machines as subsystems. The total system may also contain static power conversion, conditioning, or control equipment in which switching occurs. The procedure is simple yet accurate and independent of any specific system. A new time-domain static network model of a three-phase induction motor of wound rotor variety is presented. The model can account for unbalance on both sides of the airgap provided the structure of the windings remains unaltered. It has inherent capability for accounting iron losses as well. Similar network models can also be derived to represent a machine with asymmetrical internal faults that split one or more phases, on either or both sides of the airgap, into multiple sections with or without interconnections. A previously published model of a triac has also been extended. >

Digital computer simulation of three-phase induction machine dynamics-a generalized approach

A dynamic model based on instantaneous symmetrical components theory is presented for purposes of transient analysis of thyristor-controlled three-phase induction motors. The distinct advantages of this model over the phase-variable model have been highlighted. In-line switching constraints are incorporated and the simulation of all operating modes is accomplished without reference to rotor position. Typical transient response patterns are presented using the new model and comparisons are made with similar results obtained by earlier methods.

A New Approach to Dynamic Modeling and Transisent Analysis of SCR-Controlled Induction Motors

The problem of simulating the transient performance of thyristor controlled three-phase induction motors is discussed. A method of solution by digital simulation using phase variables has been developed and is presented. The method enables transient current, speed, and electromagnetic torque patterns to be predicted for run-up conditions and conditions of operating mode transition. Application of the method is illustrated and sample results are presented and discussed.

Gautam Nath

Papers

Transient Analysis of Three-Phase SCR Controlled Induction Motors