Rockwell Automation 7000A PowerFlex Medium Voltage Drive (A-Frame) - Classic Control - Manual (Page 314)

6-10 Component Definition and Maintenance
7000A-UM150F-EN-P –June 2013 7000 “A” Frame
Transient Suppression
Network – TSN
Description
The Transient Suppression Network Module consists of an assembly of
suppressors that are connected to each of the three phase input lines and
the structure‟s ground bus.
A transient voltage spike in excess of the semiconductor rating will
destroy or shorten the lifespan of the device. The Transient Suppression
Network Module provides suppresion of transient overvoltages appearing
on the input of the drive and is a standard feature of the drive. The two
basic blocks of the TSN module are the MOV suppressor and the MOV
fuse.
MOV Suppressor
The transient suppressors used in the module are heavy-duty metal oxide
varistors or MOVs. Varistors are voltage dependent, nonlinear resistors.
They have symmetrical voltage/current characteristics similar to back-
to-back connected Zener Diodes. The varistor has very high resistance
below its voltage rating and appears as an open circuit.
The leakage current through the device would be very small in this region.
When a voltage transient occurs in which the voltage exceeds the „knee‟
in the curve, the varistor resistance changes from its high state by several
orders of magnitude to a very low level. The voltage will be essentially
clamped for a change in current of several orders of magnitude. This can
be seen in Figure 6.9.
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
CURRENT (AMPERES) -
VOLTAGE
(VOLTS)
10
-8
log scale
lo
g
s
c
a
le
High
Resistance
Region
Voltage Clamping Region
Short
Circuit
Region
Figure 6.9 – Typical MOV V-I Characteristic Curve