Page 165 - Programmable Logic Controllers, Fifth Edition

a cumulative overpressure condition for 60 s. At that
point, a horn is sounded automatically to call attention • The bearings also receive oil when the machine is
running.
to the malfunction. When they are alerted, maintenance • When the operator turns SW off to stop the
personnel can silence the alarm by switching the key machine, the oil pump continues to supply oil for
switch S1 to the reset (contact closed) position. After 15 s.
the problem has been corrected, the alarm system can be • A retentive timer is used to track the total running
reactivated by switching the key switch to open contact time of the pump. When the total running time is
position. 3 hours, the motor is shut down and a pilot light is
Figure 7-30 shows a practical application that uses the turned on to indicate that the filter and oil need to be
on-delay, off-delay, and retentive on-delay instructions in changed.
the same program. In this industrial application, there is • A reset button is provided to reset the process after
a machine with a large steel shaft supported by babbitted the filter and oil have been changed.
bearings. This shaft is coupled to a large electric motor.
The bearings need lubrication, which is supplied by an oil Retentive timers do not have to be timed out com-
pump driven by a small electric motor. The operation of pletely to be reset. Rather, such a timer can be reset at
the program can be summarized as follows: any time during its operation. Note that the reset input
to the timer will override the control input of the timer
• To start the machine, the operator turns SW on. even though the control input to the timer has logic
• Before the motor shaft starts to turn, the bearings continuity.
are supplied with oil by the pump for 10 seconds.

Inputs Ladder logic program Outputs
Motor starting time delay
L1 SW T4:2 TON L2
TIMER ON DELAY EN
Timer T4:0
DN Time base 1.0 OL
Preset 10 DN Pump M1
Accumulated 0
SW Pump O time delay OL
TOF Motor M2
TIMER OFF DELAY EN
Timer T4:1
Reset
Time base 1.0 DN PL
Preset 15
Accumulated 0
T4:1 Pump
DN
T4:0 Motor

DN
Reset T4:2
RES
Pump running time
Pump RTO
RETENTIVE TIMER ON
Timer T4:2 EN
Time base 1.0
Preset 10800 DN
Accumulated 0
T4:2 PL

DN
Figure 7-30 Bearing lubrication program.

146 Chapter 7 Programming Timers

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