Rockwell Automation 7000L PowerFlex 7000 Medium Voltage AC Drive C Frame - Marine - Manual (Page 182)

4-6 Component Definition and Maintenance
7000L-UM302B-EN-P – June 2013 7000 ―C‖ Frame - Marine
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-7
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-6
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-5
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-4
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-2
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-1
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0
10 10
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CURRENT (AMPERES) -
VOLTAGE
(VOLTS)
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-8
log scale
lo
g
s
c
a
le
High
Resistance
Region
Voltage Clamping Region
Short
Circuit
Region
Figure 4.4 – Typical MOV V-I Characteristic Curve
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 4.5, 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.
Transient Suppression
Network – TSN (cont.)