Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C-Frame) - ForGe Control - Manual (Page 271)

Rockwell Automation Publication 7000L-UM301D-EN-P - June 2014 271
Component Definition and Maintenance Chapter 5
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 are 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 suppression 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 224 .
Figure 224 - Typical MOV V-I Characteristic Curve
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
log s
cal
e
High
Resistance
Region
Voltage Clamping Region
Short
Circuit
Region