Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C-Frame) - ForGe Control - Manual (Page 20)

20 Rockwell Automation Publication 7000L-UM301D-EN-P - June 2014
Chapter 1 Overview of Drive
Manufacturers also use isolation transformers to protect motors from Common
Mode Voltage stress. When transformers are used they allow the motor neutral
point to be connected to ground, but with this method, the common mode
voltage that would otherwise be impressed on the motor is impressed on the
transformer. The disadvantage to this method is that increased transformer
insulation and increased cable insulation is required between the transformer and
the drive so it can withstand the common mode voltage stress.
Rather than use an Isolation Transformer, the “Direct-to-Drive” Active Front
End uses the semiconductor switching pattern to reduce line current harmonics
to levels that comply to the world’s most accepted harmonic standards. The
Active Front End is the best method of harmonic cancellation because it does not
suffer from complexity and high component count like multi-pulse drive
topologies do.
“Direct-to-Drive” technology produces virtually no common mode voltage so it
is suitable for new or existing motors and imposes no stress on the drive input.
The advantage of “Direct-to-Drive” technology over an Isolation Transformer is
that no extra insulation is required in the motor, in the motor cables or in the line
cables.
In addition to mitigating Common Mode Voltage, “Direct-to-Drive” technology
mitigates dv/dt or Reflected Wave Voltage Stress on motors.
The simplicity of its design results in a lower initial capital investment, lower
operating cost, lower installation cost and lower maintenance cost relative to
drives that require isolation transformers.
The PowerFlex 7000L with “Direct-to-Drive” technology is typically smaller and
lighter than drive technologies that use Isolation Transformers. Isolation
Transformers represent 30 to 50% of a drive system size and 50 to 70% of the
system’s weight. This means that there is no interwiring between drive and
transformer (for external transformer configurations). This makes the PowerFlex
7000L the simplest to install.