Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) Commissioning - ForGe Control - Manual (Page 12)

12 Rockwell Automation Publication 7000-IN012B-EN-P - June 2014
Chapter 2 PowerFlex 7000 Overview
Rectifier Designs Configurations
The PowerFlex 7000 offers three rectifier configurations for "B" Frame drives:
• Direct-to-Drive (AFE rectifier with integral line reactor and Common
Mode Choke)
• AFE rectifier with separate isolation transformer
• 18 Pulse rectifier (transformerless AFE rectifier) with separate isolation
transformer
Direct-to-Drive
Direct-to-Drive
TM
technology does not require an isolation transformer or
multiple rectifier bridges as in Voltage Source Inverter (VSI) topologies offered
by others. The approach is completely different. Instead of multiple
uncontrolled rectifiers, a single AFE rectifier bridge is supplied. The rectifier
semiconductors used are SGCTs. Unlike the diodes used in VSI rectifier bridges,
SGCTs are turned on and off by a gating signal. A PWM gating algorithm
controls the firing of the rectifier devices, very similar to the control philosophy
of the inverter. The gating algorithm uses a specific 42 pulse switching pattern
(
Figure 1 ) called Selective Harmonic Elimination (SHE) to mitigate the 5th, 7th,
and 11th harmonic orders.
Figure 1 - Typical PWM switching pattern, line voltage waveform
A small integral line reactor and capacitor addresses the high harmonic orders
(13th and above) and provides virtually sinusoidal input voltage and current
waveforms back to the distribution system. This delivers excellent line-side
harmonic and power factor performance to meet IEEE 519-1992 requirements
and other global harmonic standards, while still providing a simple, robust power
structure that maximizes uptime by minimizing the number of discrete
components and the number of interconnections required.
A Common Mode Choke (CMC) mitigates the common mode voltage seen at
the motor terminals, so standard (non-inverter duty rated) motors and motor
cables can be used, making this technology ideal for retrofitting existing motor
applications.