Page 134 - Programmable Logic Controllers, Fifth Edition

L1
L1 Hardwired relay circuit L2
Motor 1
Motor 1 Start PB
Stop Start Stop PB
L2 OL
M1

M1-1
L1 L2 I1 I2
Inputs
Motor 2
I2 I1 Q1 Motor 2 Start PB
Stop PB
OL
M2
M1-2
Q1
Outputs M2-1
Q1 Q2 Q3 Q3
Motor 3
Motor 3 Start PB
Stop PB
Motor OL
M M3
starter coil
M2-2
Figure 6-47 Motor seal-in circuit implemented using an
Allen-Bradley Pico controller. M3-1
Figure 6-48 Sequential hardwired three motor relay
released. When this circuit is programmed into a PLC, control circuit.
both the start and stop buttons are examined for a closed
condition because both buttons must be closed to cause
the motor starter to operate. • Motor 2 has to be operating before Motor 3 can be
Figure 6-47 shows a PLC wiring diagram of the motor started.
seal-in circuit implemented using an Allen-Bradley Pico
controller. The controller is programmed using ladder • The NO auxiliary interlocking contact M2-2 is used
logic. Each programming element can be entered directly for this purpose.
via the Pico display. This controller also lets you program Figure 6-49 shows a PLC program equivalent of the
the circuit from a personal computer using PicoSoft pro- hardwired circuit.
gramming software. Pushbutton interlocking is one of the methods of
preventing two loads from being energized simultane-
ously. The hardwired pushbutton interlocking circuit of
6.9 Electrical Interlocking Circuits Figure 6-50 is designed to prevent solenoids SOL-A and
An electrical interlocking circuit is used to prevent a piece SOL-B from being energized at the same time. The in-
of equipment from operating under certain potentially terlocking feature of the circuit can be summarized as
hazardous or undesirable conditions. Figure 6-48 shows follows:
a three motor hardwired relay control circuit electrically • Each pushbutton is equipped with a set of momen-
interlocked to prevent the motors from accidently oper- tarily normally open (NO) and normally closed
ating in an order other than their proper sequence. The (NC) contacts mechanically connected together.
interlocking feature of the circuit can be summarized as • The NC contact of SOL-A pushbutton is con-
follows:
nected in series with the NO contact of SOL-B
• Motor 1 has to be operating before Motor 2 can be pushbutton.
started. • The NO contact of SOL-A pushbutton is con-
• The NO auxiliary interlocking contact M1-2 is used nected in series with the NC contact of SOL-B
for this purpose. pushbutton.

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 115

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