Rockwell Automation 7000 PowerFlex Medium Voltage Air-Cooled (B Frame) Installation - ForGe Control - Manual (Page 12)

12 Rockwell Automation Publication 7000-IN007E-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 (Active Front End [AFE] rectifier with integral line
reactor and Common Mode Choke)
• AFE rectifier with separate isolation transformer
• 18 Pulse 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 Symmetrical Gate Commutated Thyristors (SGCTs). Unlike the diodes
used in VSI rectifier bridges, SGCTs are turn ed on and off by a gating signal. A
Pulse Width Modulation (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 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.