Rockwell Automation 294D ArmorStart LT Distributed Motor Controller DeviceNet Version - User Manual - Manual (Page 201)

201 Rockwell Automation Publication 290D-UM001A-EN-P - June 2012 201
Applying More Than One ArmorStart LT Motor Controller in a Single Branch Circuit on Industrial Machinery Appendix A
Input and Output Conductors
of Bulletin 290D and 291D
Controllers (a) For Bulletin 290D and 291D controllers, which use an electromechanical
contactor to control the motor, the input current, like the output current, is just
the current to the motor. Therefore, the minimum conductor ampacity for both
input and output conductors is 125 percent of the motor full-load current rating,
as specified in the text of 12.5.3 (a).
Referring to Figure 44 , the full-load current rating of a three-phase, 460 V, 5 Hp
induction motor is 7.6 amperes. Using this value, both the input and output
conductors must have an ampacity that is not less than 125% of 7.6 A or 9.5 A.
Input and Output Conductors
of Bulletin 294D Controllers
(b) The Bulletin 294D controllers use a variable-frequency AC drive to control the
motor. These drives use a power conversion method that generates input currents
that are larger than the output currents. The input currents are larger because,
unlike the output currents to the motor, they are not sinusoidal. Consequently,
when determining the minimum ampacity of the input conductors, the
requirement of 12.5.3 must be based on the rated input current of the controller,
rather than the full-load current rating of the motor. Therefore, the minimum
ampacity of the input conductors must be 125% of the controller rated input
current, while that of the output conductors must be 125% of the motor full-load
current rating.
Referring to Figure 44 , the 1 Hp Bulletin 294D controller has a rated input
current of 3.0 amperes. Using the rated input current, the conductors from the
combined load conductors to the controllers must have an ampacity of 125%
of 3.0 A or 3.75 A. The output conductors must have an ampacity of 125% of
2.1 A or 2.6 A.
Combined Load Conductors
(c) The requirement for the minimum ampacity of the combined load conductors
is given by 12.5.4. When the combined load conductors supply one or more
Bulletin 294D controllers, the minimum ampacity calculation of 12.5.4 must
be made by substituting the rated input current of the Bulletin 294D controllers
for the full-load current rating of the motors that these controllers supply.
In Figure 44 , the currents I1, I2, I3, I4 and I5 are the input currents to each
controller. I3 and I4 are the full-load current ratings of the 5 Hp motors. I1, I2
and I5 are the rated input currents of the Bulletin 294D controllers. Referring to
the explanatory text (c) in
Figure 44 , the method for calculating the minimum
ampacity of the combined load conductors follows: first, multiply the largest
input current to any controller – Bulletin 290D, 291D or 294D - by 125%. In
this case, the input currents to the Bulletin 290D and 291D controllers, I3 and
I4, are the largest, 7.6 A. Because they are the same, either can be used. Choose I3
to calculate 125% of the maximum. 125% of 7.6 A is 9.5 A. Second, sum the
remaining input currents (I1, I2, I4, I5) for a total of 17.9 A. Third, add the result
from the first step to the result from the second for a total of 27.4 A. Finally, the
minimum ampacity of the combined load conductors is 27.4 A.