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

1-6 Overview of Drive
7000L-UM302B-EN-P – June 2013 7000 “C” Frame - Marine
“Direct-to-Drive” Technology
The PowerFlex 7000L with “Direct-to-Drive” technology allows you to:
 connect supply power directly to the drive without an Isolation
Transformer
 connect a new or existing motor directly to the drive without
extra motor filtering.
Most Medium Voltage Drive Manufacturers use multi-winding
isolation transformers to mitigate unwanted harmonics by phase
shifting the transformer secondary windings. Depending on the
topology, the transformer can have up to 15 sets of secondary
windings. The disadvantages to this method are the high degree of
drive and transformer complexity, a very high component count and
many interconnecting cables and connection points. This leads to
much higher maintenance requirements and lower reliability.
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.