Abstract
Variable voltage and frequency are required to govern the torque-speed characteristics of many industrial drive systems. Traditionally, this is achieved with a power converting system implemented with multistage converters. This technology is based on rectifying AC power into DC and then DC into AC with an inverter circuit. The power quality concerns of both conversion stages are tackled by selecting high switching frequency PWM control and harmonics mitigation filters. Also, using a bulky DC-link capacitor is one of the big sources of low system reliability, so this approach increases the conversion losses, circuit, and control complications. The frequency step-down conversion is very attractive with direct AC-AC converters as it has a simple control and circuit structure, but these converters face poor power quality challenges once the output frequency is decreased with respect to an input. In these converters, the total harmonic distortion (THD) of the output voltage becomes very poor once the output frequency is reduced. The problem of high THD of the output is addressed in the power converting circuits implemented with line frequency multi-winding transformers. The required number of output winding and switching devices (diodes and thyristors) increases once the value of the output frequency is decreased. This will increase the overall volume, cost, and losses. The use of a bulky and costly line frequency transformer may be eliminated if AC voltage controllers have non-inverted and inverted voltage buck capabilities, such existing topologies either have complex control schemes or require a large number of operating devices. Therefore, in this research article, a new transformerless frequency step-down converter employing fewer devices is proposed. This approach is realized with a high-frequency controlled rectifier for the required voltage stabilization and a low-frequency inverter bridge for frequency control. Its validation is supported by the results attained from Simulink and practical-based prototypes.
Highlights
The electric power supplied by the utility grid to an end-user has a constant voltage and frequency
Her line frequency harmonic (50 Hz) has significant magnitude, and it is a major cause of the high total harmonic distortion (THD) of the output voltage
This study was focused on the development of an improved direct AC-AC frequency controller having a low value of the THD of the output voltage
Summary
The electric power supplied by the utility grid to an end-user has a constant voltage (rms value) and frequency. The conversion losses of thyristors-based operating topologies are high as they have high internal resistance and voltage These schemes are mostly employed to reduce the input or grid frequency at the output. The stabilization of the frequency at the output may be attained with the highfrequency operation of the semiconductor devices This target is achieved using PWM control that may invert or non-invert the input with voltage regulation features. One thyristor and one diode turn on to obtain the required form of the output Their low-frequency on and off behavior is the main source of high conduction losses.
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