User Manual Owner's manual

Rockwell Automation Publication 280E-UM001B-EN-P - July 2012 265
Applying More Than One ArmorStart Motor Controller in a Single Branch Circuit on Industrial Machinery Appendix A
controllers use a variable-frequency AC drive to stop, start and vary the speed of
the motor. This appendix refers to the Bulletin 290, 291 and 294 products as
either motor controllers or just controllers.
Each ArmorStart LT motor controller incorporates an integrated overload relay
and motor disconnecting means. The Underwriters Laboratories’ (UL) listing for
each motor controller confirms that the motor controller – including its integral
overload relay and motor disconnecting means — is suitable for motor group
installation.
The suitability of each ArmorStart LT motor controller for installation according
to either NFPA 79 or NFPA 70 depends on the means of connecting the power
circuit wiring. All of the controllers are suitable for installation in multiple-motor
branch circuits on industrial machinery according to 7.2.10.4 of NFPA 79. The
controllers that have the Conduit Entrance Gland Plate Option are also suitable
for installation in multiple-motor branch circuits according to 430.53(C) and
430.53(D) of NFPA 70 (NEC). The controllers that have the Power Media
Gland Plate Option are suitable for installation only on industrial machinery.
These versions are limited to industrial machinery because the UL listing for the
power media connectors themselves and their matching cable assemblies covers
installation only on industrial machinery.
Multiple-Motor Branch
Circuits and Motor
Controllers Listed for Group
Installation – General
Multiple-motor branch circuits, like that shown in Figure 95, have this
fundamental tradeoff: protecting more than one controller with a single set
of fuses requires more electrical and mechanical robustness in each controller.
In exchange for eliminating the cost and space necessary for a dedicated set of
fuses in front of each controller, the construction of each controller itself must be
more robust. For the circuit configuration shown in Figure 95
to be practical,
the ampere rating of the fuse must be large enough to operate all of the motors,
without opening, under normal starting and running conditions. This rating
of fuse must be larger than the rating permitted to protect a circuit that supplies
only a single motor and its controller. In general, as the rating of the fuse
increases, so does the magnitude of fault currents that flow until the fuse opens.
This higher magnitude of fault current results in more damage to the controller.
Therefore, the additional controller robustness is necessary to withstand these
higher fault currents, without controller damage, that could result in a shock
or fire hazard.
Consequently, to the controller, being listed for group installation mostly means
the UL testing is performed with fuses that have this practical, and higher,
ampere rating. This testing verifies that it is safe to apply this controller in a
multiple-motor branch circuit, provided the fuse is of the same class and does
not have a rating exceeding that marked on the controller.
The example in Figure 96
, illustrates this increase in the maximum ampere rating
of fuse that is permitted to protect a controller. This example compares the rating
of the fuse used in the UL testing of two variable-frequency AC drive-based
motor controllers. Both controllers have a rated power of ½ horsepower and a