Rockwell Automation MV SMC Flex Motor Controller (for Series K or later) - Manual (Page 62)

52 Rockwell Automation Publication 1560E-UM051F-EN-P - June 2013
Chapter 2 Installation
Figure 34 - Typical One-Line Diagram
(Showing 2 Different Styles of Power Factor Capacitor Connections)
Surge Arrestor Protection
Devices 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.
Bypass
Contactor
di/dt Inductor
PFCC
Contactor
Fuse
Isolation Switch
Power Bus
Isolation Switch
Fuse
Isolation Contactor
di/dt Inductor
PFCC
(Power Factor
Correction Capacitor)
Contactor
Capacitor
Motor