Page 120 - Programmable Logic Controllers, Fifth Edition Mobile 2

Line side

Load side Line side
Stationary Load side
contacts

Movable
contacts

Coil

Movable iron
armature or
plunger

Line side

Contacts
Coil
Load side
Wiring symbol
Figure 6-5 Three-pole magnetic contactor.
Source: Image Courtesy of Rockwell Automation, Inc.

Pump
6.3 Motor Starters
Contactor power A motor starter is designed to provide power to motors.
contacts The motor starter is made up of a contactor with an
overload relay attached physically and electrically to it
as illustrated in Figure 6-7. The function of the overload
L2
relay can be summarized as follows:
High-current
wiring • Overload relays are designed to meet the special
L1 protective needs of motor control circuits.
• They allow harmless temporary overloads that occur
when a motor starts.
Coil
terminals • The overload relay will trip and disconnect power to
the motor if an overload condition persists.
• Overload relays can be reset after the overload con-
dition has been corrected.
Low-current Figure 6-8 shows the diagram for a typical three-phase,
wiring
magnetic motor starter. The operation of the circuit can be
L2 L1 summarized as follows:

• When the START button is pressed coil M is ener-
gized closing all normally open M contacts.
PLC output
module • The M contacts in series with the motor close to
Figure 6-6 Contactor used in conjunction with a PLC output. complete the current path to the motor. These con-
Source: This material and associated copyrights are proprietary to, and used tacts are part of the power circuit and must be de-
with the permission of Schneider Electric. signed to handle the full load current of the motor.

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 101

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