Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - Classic Control - Manual (Page 297)

Component Definition and Maintenance 6-7
7000 “B” Frame 7000-UM150I-EN-P – June 2013
When the MOV is clipping the voltage transient, the energy of the
transient is being absorbed by the MOV. The varistor has a limited
energy absorbing capability and generally there is not enough time
for the heat generated to be conducted out of the device. The MOV
is sized based on the steady-state voltage rating, the energy in the
transient, and the repetition rate of the transients. A critical element
in the MOV selection and protection offered is the impedance in the
line supplying the transient. This impedance will be mainly that
provided by the Isolation Transformer or the AC Line Reactor on the
input of the drive. That is why an impedance level is specified for
these input devices.
MOV Fuse
In series with each of the Phase MOVs is a medium voltage fuse. As
seen in Figure
6.7 , these fuses may be located on the assembly or
remote from the assembly (on the Line Terminal Module). Check the
part number on your module and the information in this documentation
to determine which assembly you should have in your case.
The fuses provide overload protection for the conductors feeding the
suppression network (and overcurrent protection if a short circuit
occurs on the downstream side of the fuse.) These conductors will
normally have a much smaller current carrying capacity than the
drive input conductors and thus will not be protected by the drive
input fuses. The fuses also serve to isolate a failed MOV. Varistors
initially fail in a short-circuited condition. The high follow-through
current will open the fuse and remove the MOV from the circuit.
The fuses used are E-rated current limiting fuses with a high
interrupting rating. Because they are current limiting, they will limit
both the magintude and duration of fault currents. They are small
dimension, ferrule-type fuses with a fiberglass body, and mount in
standard fuse clips.
I M P O R T A N T I M P O R T A N T The fuses sent with the Transient Suppression
Network have been selected based on their
characteristics (including internal resistance).
This is necessary for optimum MOV performance
and protection. Do not substitute other fuses
without contacting the factory first.
Note: Voltage sensing occurs after the MOV fuse, and as a result
open fuses will be detected in drive control as a Master or Slave
Undervoltage or Unbalance.