Page 121 - Programmable Logic Controllers, Fifth Edition - Mobile version

Magnetic Motor
Contactor PLC
starter

Pushbuttons
L1 Inputs Output L2
Start Starter
PLC Program coil
Start Stop OL M M
Stop
Overload
relay M
OL

Figure 6-9 PLC control of a motor.

Figure 6-7 Motor starter is a contactor with an attached an overload current is sensed to de-energize the M
overload relay. coil and stop the motor.
Source: Image Courtesy of Rockwell Automation, Inc.
Motor starters are available in various standard Na-
• Control contact M (across START button) closes tional Electric Manufacturers Association (NEMA) sizes
to seal in the coil circuit when the START button is and ratings. When a PLC needs to control a large motor,
released. This contact is part of the control circuit it must work in conjunction with a starter as illustrated in
and, as such, is only required to handle the small Figure 6-9. The power requirements for the starter coil
amount of current needed to energize the coil. must be within the power rating of the output module of
• An overload (OL) relay is provided to protect the the PLC. Note that the control logic is determined and
motor against current overloads. The normally executed by the program within the PLC and not by the
closed relay contact OL opens automatically when hardwired arrangement of the input control devices.
6.4 Manually Operated Switches
Start
Stop OL
M Manually operated switches are controlled by hand.
Magnetic starter These include toggle switches, pushbutton switches, knife
M
switches, and selector switches.
Pushbutton switches are the most common form of
Low-current manual control. A pushbutton operates by opening or clos-
control circuit
ing contacts when pressed. Figure 6-10 shows commonly
used types of pushbutton switches, which include:
M OL
L1 • Normally open (NO) pushbutton, which makes a
T1
M OL T2 Three- circuit when it is pressed and returns to its open po-
L2 phase sition when the button is released.
motor • Normally closed (NC) pushbutton, which opens the
M OL T3
L3 circuit when it is pressed and returns to the closed
High-current position when the button is released.
power circuit • Break-before-make pushbutton in which the top
Figure 6-8 Three-phase magnetic motor starter. section contacts are NC and the bottom section con-
Source: This material and associated copyrights are proprietary to, and used tacts are NO. When the button is pressed, the top
with the permission of Schneider Electric. contacts open before the bottom contacts are closed.

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

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