A novel autotransformer based 18-pulse rectifier circuit

A popular harmonic current reduction method specifically geared towards diode-bridge rectifier loads like motor drives is to increase the number of rectification pulses. For applications where the demand for harmonic current reduction is stringent, the 18-pulse rectification method is the preferred method. This is more economical than the 24-pulse rectifier while being more effective than the 12-pulse version. For applications where line-voltage isolation is not required, several autotransformer based multi-pulse rectification schemes have been suggested. This paper presents a novel autotransformer suitable for 18-pulse rectification. It differs from existing schemes by achieving harmonic current reduction without the need for equal diode-bridge current sharing. The need for additional magnetic components like line-impedance matching inductors is also eliminated. The paper presents the principle of operation of the circuit and guidelines for designing the autotransformer. Operation under constant DC current conditions is investigated and equations determining the power ratings of the transformer and diode bridges are derived. Finally, the paper presents experimental results obtained from a proof-of-concept prototype operating on a 3-O, 230 V/15 hp motor drive load. The line current has a total harmonic distortion (THD) of 6.9% at rated load, further validating the concept.