An Electronically Isolated 12-Pulse Autotransformer Rectification Scheme to Improve Input Power Factor and Lower Harmonic Distortion in Variable-Frequency Drives

Diode rectifier front-ends with large dc bus capacitors used in variable-frequency drives draw discontinuous current from the power system, resulting in a poor input power factor and high input current distortion. The 12-pulse and 18-pulse techniques used to reduce input current harmonics are popular because of their simplicity but are bulky and expensive. Autotransformer techniques require additional external magnetic components that are bulky and add to the cost of the system. A new electronically isolated 12-pulse autotransformer topology that does not need bulky external magnetic components is proposed in this paper. The proposed topology employs active switches to achieve electronic isolation between the two rectifiers, resulting in continuous input current with reduced harmonics. The proposed nonregenerative active scheme achieves less than 5% total harmonic distortion at a much lower cost compared with other passive or active topologies currently in the market. Experimental results from a 240-V 75-hp system are given to prove the concept.