An active front end (AFE) rectifier, suitable for the PowerFlex® 7000 topology, is also commonly referred to as a PWM rectifier. This approach is particularly attractive for applications involving new motors, as it does not require an isolation transformer to comply with IEEE-519 requirements (refer to Specification 80001-004, Requirements for Medium Voltage Motors Without Common Mode Voltage Elimination).

In contrast, most medium-voltage drive technologies currently available rely on multi-winding isolation transformers to mitigate undesirable harmonic distortion through phase-shifted secondary windings. Depending on the topology, such transformers may incorporate up to 15 secondary windings. Eliminating the isolation transformer reduces capital and installation costs, conserves valuable floor space, and improves overall system efficiency.

The AFE rectifier requires a switching pattern that follows principles similar to those used in the inverter. In the example shown in Figure 1.4, a 42-pulse selective harmonic elimination (SHE) switching pattern is employed, effectively eliminating the 5th, 7th, and 11th harmonics. Integral input capacitors are incorporated to attenuate higher-order current harmonics.

A filter transfer function design approach is used to position the filter break frequency in a range where no significant harmonics are present, thereby preventing the excitation of system harmonic resonances. Additional design considerations include input power factor performance and compliance with Total Harmonic Distortion (THD) requirements for both input current and voltage waveforms.