Page 306 - Programmable Logic Controllers, Fifth Edition - Mobile version

13.5 Voltage Variations and Surges
The power supply section of the PLC system is built to L1 PB L2
sustain line fluctuations and still allow the system to func-
tion within its operating range. If voltage fluctuations ex-
ceed this range, then a system shutdown will occur. In
areas where excessive line voltage variation or extended L1
brownouts are anticipated, installing a constant voltage M
(CV) transformer may be required to minimize nuisance C R
shutdowns of the PLC.
Isolation transformers are used in some PLC systems
to isolate the PLC from electrical disturbances generated L2
by other equipment connected to the distribution sys-
tem. Although the PLC is designed to operate in harsh Output
environments, other equipment may generate consider- module
able amounts of interference that may result in intermit- Figure 13-12 RC snubber circuit connected to suppress
tent disturbances in normal operation. A normal practice AC loads.
is to place the PLC power supply and I/O devices on a
separate transformer that may also serve as a step-down
transformer to reduce the incoming voltage to the desired loads. The operation of the circuit can be summarized
level. as follows:
When current in an inductive load is interrupted or • The diode is connected in reverse-bias across the
turned off, a very high voltage spike is generated. This solenoid load.
high voltage can be reduced or eliminated through sup- • In normal operation, the electric current can’t
pression techniques which absorb the inductive induced flow through the diode, so it flows through the sole-
voltage. Generally, output modules designed to drive noid coil.
inductive loads include suppression networks built in as
part of the module circuit. • When voltage to the solenoid is switched off a
An additional external suppression device is recom- voltage opposite in polarity to the original applied
mended if an output module is used to control devices voltage is generated by the collapsing magnetic
such as relays, solenoids, motor starters, or motors. The field.
suppression device is wired in parallel (directly across) • The induced voltage creates a current flow
and as close as possible to the load device. The sup- through the diode bleeding off the high-voltage
pression components must be rated appropriately to spike.
suppress the switching transient characteristic of the Figure 13-12 illustrates how an RC (resistor/capacitor)
particular inductive device. Figure 13-11 illustrates snubber circuit is connected for suppressing AC load de-
how a diode is connected to suppress DC inductive vices. The operation of the circuit can be summarized as
follows:
• The voltage peak, which occurs at the instant the
L1(+) L2(–) current path to the coil is opened, is safely short-
circuited by the RC network.
Output module • The resistor and capacitor connected in series slows
the rate of rise of the transient voltage.
Solenoid coil
(+)
• The voltage across the capacitor cannot change
instantaneously, so a decreasing transient current
will flow through it for a small fraction of a second,
Reversed-bias allowing the voltage to increase more slowly when
diode the circuit is opened.

The metal oxide varistor (MOV) surge suppressor,
Figure 13-11 Diode connected to suppress DC inductive shown in Figure 13-13, is the most popular surge protec-
loads. tion device. It functions in a manner similar to two zener

PLC Installation Practices, Editing, and Troubleshooting Chapter 13 287

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