40 Rockwell Automation Publication 7000-UM202D-EN-P - May 2018
Chapter 2 Power Component Definition and Maintenance
Figure 28 - Typical MOV V-I Characteristic Curve
When the MOV clips the voltage transient, the MOV absorbs the transient
energy. The varistor has a limited energy absorbing capability and there is
insufficient time to conduct heat out of the device. The MOV size depends on
the steady-state voltage rating, the energy in the transient, and the repetition
rate of the transients. A critical element in selecting a MOV for protection is
the impedance in the line supplying the transient. The isolation transformer or
the AC line reactor on the input of the drive provides this impedance, which is
why an impedance level is necessary for these input devices.
MOV Fuse
A medium voltage fuse is in series with each of the phase MOVs. As seen in
Figure 29 on page 41
, these fuses may reside on either 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 your drive requires.
The fuses provide overload protection for the conductors feeding the
suppression network (and overturned 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; they are not
protected by the drive input fuses. The fuses also 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.
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
10
2
10
3
10
4
10
5
10
-8
CURRENT (AMPERES) -log scale
VOLTAGE
(VOLTS)
log scale
High Resistance
Region
Voltage Clamping Region
Short Circuit
Region
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