Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C Frame) - Classic Control - Manual (Page 350)

6-6 Component Definition and Maintenance
7000L-UM300I-EN-P – June 2013 7000 “C” 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. There is a separate assembly
for each set of three phase input voltages: three assemblies for an
18-pulse drive.
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 6.5.
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 6.5 – Typical MOV V-I Characteristic Curve