Page 119 - Programmable Logic Controllers, Fifth Edition

Normally closed Normally open
(NC) contacts (NO) contacts
Open Closed

Stationary
contact Energized
Armature Coil Armature Coil

Movable contacts
Coil de-energized Coil energized

M M CR CR

Coil Associated Coil Associated
contacts contacts

Figure 6-3 Relay normally open and normally closed contacts.
Source: Photo courtesy Eaton Corporation, www.eaton.com.

• With the switch closed, the coil is energized. Control relay coils and contacts have separate ratings.
• The normally open contact closes to switch the red Coils are rated for the type of operating current (DC or
pilot light on. AC) and normal operating voltage. Contacts are rated in
• At the same time, the normally closed contact opens terms of the maximum amount of current the contacts
to switch the green pilot light off. are capable of handling at a specified voltage level and
type (AC or DC). Control relay contacts generally are
not designed to carry heavy currents or high voltages.
L1 L2 The contacts are usually rated between 5 and 10 Amp,
S with the most common rating for the coil voltage being
CR
120 VAC.
O
CR
R 6.2 Contactors
A contactor is a special type of relay designed to handle
CR On heavy power loads that are beyond the capability of con-
G
trol relays. Figure 6-5 shows a three-pole magnetic con-
Switch open—coil de-energized tactor. Unlike relays, contactors are designed to make and
break higher powered circuits without being damaged.
Such loads include lights, heaters, transformers, capaci-
L1 L2
S tors, and electric motors for which overload protection is
CR provided separately or not required.
Programmable controllers normally have an output ca-
On
Single-pole CR pacity capable of operating a contactor coil, but not that
double-throw relay R
needed to operate heavy power loads directly. Figure 6-6
illustrates the application of a PLC used in conjunction
CR O
G with a contactor to switch power on and off to a pump.
The output module is connected in series with the coil to
Switch closed—coil energized form a low-current switching circuit. The contacts of the
Figure 6-4 Control relay used to control two pilot lights. contactor are connected in series with the pump motor to
Source: Photo courtesy Digi-Key Corporation, www.digikey.com. form a high-current switching circuit.

100 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

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