Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - Classic Control - Manual (Page 22)

Overview of Drive 1-5
7000 “B” Frame 7000-UM150I-EN-P – June 2013
The small integral AC line reactor (see Fig. 1.4) provides additional
filtering and current limiting features to a line side short circuit fault.
The rectifier input current, the rectifier terminal voltage and the line
current and voltage waveforms are shown in Figure 1.4. The line
current THD is approximately 4.5%, while line-to-line voltage THD
is approximately 1.5%. (THD of line voltage is a function of system
impedance.) Input power factor with the AFE rectifier is equal to or
greater than 0.98 for the entire speed and load range when applied to
variable torque loads.
Figure 1.4 – Active rectifier (PWM) and its input current/voltage waveforms
a) Line current
b) Line-to-line voltage at PCC
“Direct-to-Drive” Technology
The PowerFlex 7000 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 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.
a)
b)