Rockwell Automation 1770 Industrial Automation Wiring and Grounding Guidelines - Manual (Page 13)

Industrial Automation Wiring and Grounding Guidelines 13
Publication 1770-4.1 – February 1998
For example, the external-transformer load of a 1771-P4S
power-supply module at maximum backplane load current is 140VA
(2.5 x 56W = 140). A 140VA transformer could be used if a
1771-P4S power-supply module were the only load. A 500VA
transformer should be used if there were 360VA of load in addition
to that of the 1771-P4S power-supply module.
Second Transformer
Allen-Bradley power supplies have circuits that suppress
electromagnetic interference from other equipment. However,
isolate output circuits from power supplies and input circuits to help
prevent output transients from being induced into inputs and power
supplies. In many applications, power is provided to the input
circuits and power supplies through a second transformer (Figure 9).
Figure 9
Power Supplies and Input Circuits Receiving Power through
a Separate Transformer
To output circuits
19242
1FU
L1
2FU
L2
3FU
L3
L1
L2
L3
To Motor Incoming
Disc.
Starters AC
H
1
H
2
H
3
H
4
X
2
X
1
H
1
H
2
H
3
H
4
X
2
X
1
Notes:
1 To minimize transient emi generation when power is interrupted by the interrupt switch,
connect a suppressor across the primary of the transformer. Refer to Figure 11 and
Table C for suppressors to use.
Suppressor 1
Step-down
Transformer
Isolation/
Constant-Voltage
Transformer
To power supplies
and input circuits