Abstract
This paper proposes a novel multifunctional topology for a grid-connected voltage source inverter to control the speed and power flow of a squirrel-cage induction motor. For high-inertia loads during startup, the issues faced include a large transient current and high heat generation. However, the solutions proposed by existing startup methods are inadequate. The topology presented in this study not only addresses the problems related to these methods, i.e., creating a smooth startup, but also presents a flexible alternative for power factor correction using capacitor banks. First, the proposed technique accelerates the motor smoothly to its operating point through the sinusoidal voltage provided by an inverter with an LC filter in its output. In the second step of the control method, after achieving stability in the desired operating point, a converter with an LC filter is assigned the task of power factor correction. Thus, the proposed topology achieves a smooth startup and unity power factor. It includes a new control strategy in which the rotor field-oriented control method is employed for the speed control mode. Finally, the validity of the proposed theory is verified.
Highlights
I NDUCTION motors (IMs), squirrel-cage induction motors (SQIMs), are interesting for industrial applications as they are compact, economical, and mechanically robust for operation under rush conditions [1]-[3]
Direct across-the-line starting is associated with a large initial current, known as the inrush current, which is typically six to seven times the full load current and is only used if the line disturbances can be tolerated, and the driven load can withstand the shock of an instantaneous application of full voltage to the motor
The proposed control algorithm, in the whole offline mode, adopts an efficient current-based active damping technique to damp out the resonance frequency oscillations and to stabilize the unstable operating point added by the LC filter to the closed-loop system through the generation of vid and viq signals for the current control loops (Fig. 3)
Summary
I NDUCTION motors (IMs), squirrel-cage induction motors (SQIMs), are interesting for industrial applications as they are compact, economical, and mechanically robust for operation under rush conditions [1]-[3]. By considering factors such as core saturation and skin effect, a reasonable linear reduction for the drawn current can be expected with decreasing voltage These methods still present some challenges; auto-transformer starting, for example, causes a current spike at the transient instant and the resultant transient torque may cause problems for the driven equipment. VFDs control an IM such that it operates on the right side of the breakdown on the speed–torque curve at a low slip (near-synchronous speed) through supplying a variable frequency voltage Using this method, a high torque can be achieved across the entire speed range, implying that the motor can rotate the load as slowly as desired and maintain maximum torque.
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