Rockwell Automation 1336T Wiring and Grounding Guide, (PWM) AC Drives - Manual (Page 45)

Rockwell Automation Publication DRIVES-IN001M-EN-P - March 2014 45
Power Distribution Chapter 2
Surge Protection MOVs and
Common Mode Capacitors
MOV Circuitry
Most drives are designed to operate on three-phase supply systems with
symmetrical line voltages. To meet IEEE C62.41, these drives are equipped with
MOVs that provide voltage surge protection as well as phase-to-phase and phase-
to-ground protection. The MOV circuit is designed only for surge suppression
(transient line protection), not for continuous operation.
Figure 12 - Typical MOV Configuration
With ungrounded distribution systems, the phase-to-ground MOV connection
can become a continuous current path to ground. Exceeding the published
phase-to-phase voltage, phase-to-ground voltage, or energy ratings can damage
the MOV.
Suitable isolation is required for the drive when there is potential for abnormally
high phase-to-ground voltages (in excess of 125% of nominal line-to-line
voltage), or when the supply ground is tied to another system or equipment that
could cause the ground potential to vary with operation. We recommend an
isolation transformer when this condition exists.
Common Mode Capacitors
Many drives also contain common mode capacitors that are referenced to ground.
In installations with ungrounded or high resistive ground systems, the common
mode capacitors can capture high frequency common mode or ground fault
currents. This can cause bus overvoltage conditions that can cause damage or
drive faults. Systems that are ungrounded or have one phase grounded
(commonly called B-phase grounded) apply higher than normal voltage stresses
directly to the common mode capacitors and can lead to shortened drive life or
damage.
ATTENTION: When installing a drive on an ungrounded, high-resistance, or
B-phase grounded distribution system, disconnect the phase-to-ground MOV
circuit and the common mode capacitors from ground.
IMPORTANT In some drives, a single jumper connects both the phase-to-ground MOV and
the common mode capacitors to ground.
R
S
T
1 2 3 4
3-phase AC
Input
Ground
Phase-to-phase MOV rating includes
two phase-to-phase MOVs.
Phase-to-ground MOV rating includes
one phase-to-phase MOV and one
phase-to-ground MOV.