Parker H Series, Installation & Service Instructions Manual

Page: 106 / 344

Industrial Automation Wiring and Grounding Guidelines E115P
11
Common Power Source for I/O
Unless each I/O of a module or block is individually isolated,
multiple I/O within the block or module share a common
terminal for one side of the power source. All I/O sharing
a common terminal must share a common power source
(i.e., from the same pole of a disconnect or from the same
transformer tap).
If a module or block has multiple commons, each common
and its I/O may be isolated from the other commons. In
that case, each common and its I/O can have a separate
power source. If each I/O is individually isolated, each I/O
can have a separate power source. If a module or block
has individually isolated I/O or multiple isolated commons
and multiple power sources are used, be certain that the
difference in potential between any two power sources
does not exceed the specified maximum continuous voltage
that can be applied between the channels.
Under-Voltage Shutdown
Each power supply with under-voltage shut-down
protection generates a shut-down signal on the backplane
when the AC line voltage drops below its lower voltage
limit. The power supply removes the shut-down signal
when the line voltage comes back above the lower voltage
limit. This shut-down is to guard against invalid data
being stored in memory.
Because a capacitive-input power supply converting AC
to DC draws power only from the peak of the AC voltage
wave-form, the external transformer load (in VA) of each
power supply is 2.5 times its real power dissipation (in
Watts). If the transformer is too small, the peaks of the
sine wave are clipped. Even if the voltage is still above
the lower voltage limit, the power supply senses the clipped
wave as low voltage and sends the shut-down signal.
Sizing the Transformer
To determine the required rating of the transformer add
the external-transformer load of the power supply and all
other power requirements (input circuits, output circuits).
The power requirements must take into consideration the
surge currents of devices controlled by the processor.
Choose a transformer with the closest standard
transformer rating above the calculated requirements.
For example, the external-transformer load of a power-
supply module at maximum backplane load current is
140VA (2.5 x 56W = 140). A 140VA transformer could be
used if a 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 power-supply module.
Second Transformer
Certain manufacturer's 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
Notes:
1 To minimize transient EMI generation when power is interrupted
by the interrupt switch, connect a suppressor across the primary
of the transformer.
To Output Circuits
Isolation/
Constant-Voltage
Transformer
To Power Supplies
and Input Circuits
Step-Down
Transformer
1FU
L1
2FU
L2
3FU
L3
L1
L2
L3
Disc.
H
1
H
3
H
2
H
4
Suppressor
1
Incoming
AC
To Motor
Starters
X
1
X
2
H
1
H
3
H
2
H
4
X
1
X
2