User Manual Owner's manual

266 Rockwell Automation Publication 280E-UM001B-EN-P - July 2012
Appendix A Applying More Than One ArmorStart Motor Controller in a Single Branch Circuit on Industrial Machinery
rated output current of 1.5 amperes. The controller shown on the left is intended
for installation in individual-motor branch circuits. The controller shown on the
right is the ArmorStart LT Bulletin 294 controller that must be listed for group
installation to be installed, as intended, in multiple-motor branch circuits. For
this example, assume all testing is done with fuses of the same class.
The UL investigation of both controllers is done according to UL 508C, Power
Conversion Equipment. The controllers are connected to the test supply through
the three-phase conductors and equipment grounding conductor and then
covered with cotton in areas that are likely to vent hot gases and sparks during
the tests. During the test, electrical faults are impressed on the output of, and
internal to, these variable-frequency AC drive-based controllers. Increasing the
ampere rating of the fuses increases the magnitude of the fault currents that flow
through, and damage, the controller before the fuses open. Afterwards, the
damage to the controller is evaluated to determine whether a potential shock or
fire hazard exists when protected by fuses having this ampere rating. One
criterion of the evaluation is the examination of the equipment grounding
conductor that must not open during the test as this could leave exposed
conductive parts in an energized state (shock hazard). Another criterion is that
the cotton must not ignite as this indicates the expulsion from the controller
of hot gases or molten metal fragments (fire hazard).
Referring to the controller on the left, UL 508C permits the individual-motor
testing to be performed with the maximum rating of fuse that can be used to
protect an individual-motor branch circuit. According to both NFPA 70 and
NFPA 79, this is 400 percent of the full-load current rating of the largest motor
that the controller can supply. In UL 508C, this is taken to be 400 percent of the
rated output current of the controller, or 6 amperes.
Referring to the controller on the right, UL 508C permits the group installation
testing to be performed with the maximum rating of fuse that can be used to
protect a multiple-motor branch circuit. According to both NFPA 70
(430.53(C)) and NFPA 79 (7.2.10.4(3)), this is 250 amperes. This value, derived
from the installation requirements of 430.53(C) and 430.53(D) of NFPA 70, is
determined by the largest size of power conductor that the ArmorStart LT
controller can accept, 10 AWG. Because the UL 508C test covers all possibilities
in NFPA 70 and NFPA 79, it permits the maximum value of 250 amperes. This
covers 7.2.10.4(2), which permits only 100 amperes. However, in this case, the
manufacturer, Rockwell Automation, chose to test and mark with the lower value
of 45 amperes. This value was chosen as the tradeoff between the maximum
number and type of controllers in the branch circuit – limited by the maximum
fuse rating - and the electrical and mechanical robustness engineered into each
controller.
Therefore, to make its use in the multiple-motor branch circuit of Figure 95
practical, the ½ horsepower Bulletin 294 controller was engineered to be robust
enough to safely contain the damage when protected by a fuse having a rating
of 45 amperes, rather than just 6 amperes.