Rockwell Automation 7000A PowerFlex Medium Voltage Drive (A-Frame) - Classic Control - Manual (Page 315)

Component Definition and Maintenance 6-11
7000 “A” Frame 7000A-UM150F-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
6.12 , these fuses are located on the assembly. The fuses are
mounted on the assembly (on the Line Terminal Module).
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.