Rockwell Automation Allen-Bradley 1305-AA02A, Installation Instructions Manual

Page: 39 / 102

Power Distribution 2-17
Surge Protection MOVs and
Common Mode Capacitors When installing a drive on an ungrounded or high-resistance ground
distribution system, disconnect the phase-to-ground MOV circuit and the
common mode capacitors from ground.
Note: In some drives, a single jumper connects both the phase-to-ground
MOV and the common mode capacitors to ground.
MOV Circuitry
Most drives are designed to operate on three-phase supply systems with
symmetrical line voltages. To meet IEEE 587, these drives are equipped
with MOVs that provides voltage surge protection as well as phase-to-phase
and phase-to-ground protection. The MOV circuit is designed for surge
suppression (transient line protection) only, not for continuous operation.
Figure 2.1 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, phase-to-ground voltage or energy ratings may
cause physical damage to the MOV.
Suitable isolation is required for the drive when there is potential for
abnormally high phase-to-ground voltages (in excess of 125% for nominal
line-to-ground voltage), or when the supply ground is tied to another system
or equipment that could cause the ground potential to vary with operation.
An isolation transformer is strongly recommended 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 grounded
systems, the common mode capacitors can capture high frequency common
mode or ground fault currents. This could cause over-voltage conditions,
which could lead to damage or drive faults.
THREE-PHASE
AC INPUT
GROUND
R
S
T
PHASE-TO-PHASE MOV RATING
Includes Two Phase-to-Phase MOV's
PHASE-TO-GROUND MOV RATING
Includes One Phase-to-Phase MOV
and One Phase-to-Ground MOV
1
2 3 4