Rockwell Automation 1560E SMC Flex Smart Motor Controller - Manual (Page 64)

2-16 Installation 
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Rockwell Automation highly recommends that motor surge capacitors and/or
motor surge arresters not be located on the load side of the SMC. The
issues that warrant this are:
• Motor and system inductance limits the rate at which the current can
change through the SMC. If capacitance is added at the motor, the
inductance is negated. The surge capacitors downstream of the SMC
represent a near zero impedance when presented with a step voltage
from the turn on of the SCRs near the line voltage peak. This causes a
high level of di/dt to occur, due to the fact that the motor cables are
generally short in length. There is very little impedance between the
capacitor and SCR to limit the di/dt of this capacitor charging current.
This can result in damage to the power semiconductors (SCRs) in the
SMC.
• It is essential to understand the clamping voltage of the arresters and
type of grounding used in the distribution system. The switching of the
SCRs generates slightly higher than nominal peak line to ground
voltages. The typical peaks are 1.5 times the nominal line to ground
peak voltages. These may cause the arresters to conduct, which could
affect the operation of the SMC and result in faults. Depending upon
the instance at which the arresters conduct, this could also result in
SCR damage.
• The capacitance in combination with the line and motor impedance
could also be excited by the voltage steps from SCR switching to
create resonant voltage swings which could exceed the device voltage
withstanding ratings or surge arrester rating, or cause distorted voltage
signals which may be misinterpreted by the MV SMC-Flex control
system.
Surge Arrestor Protection
Devices