Page 120 - Programmable Logic Controllers, Fifth Edition

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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