Motor ON OFF Logic in PLC

Develop ladder logic program for Motor ON OFF according to the logic given below,

  1. The start button is pressed to start the conveyor motor
  2. Cases move past the proximity switch and increment the counter’s accumulator value.
  3. After the count of 3, the conveyor motor stops automatically and the counter’s accumulator value is reset to 0. Restart button is used to start the process from the beginning.
  4. The conveyor motor can be stopped and started manually at any time without loss of the accumulator count.
  5. Accumulated count of the counter can be reset manually at any time by means of the count reset button.

PLC Motor Logic

Motor ON OFF Logic in PLC

Motor Logic Explanation

Rung 0000:

Start/Stop PB latched with memory B3:0/0.

Rung 0001:

B3:0/0 enabled to turn ON conveyor motor  (O:0/0 ). B3:0/1 is connected in series with normally closed contact to break the line when counter accumulator value reached 3.

Rung 0002: 

B3:0/0 and proximity sensor (I:0/2) are connected in series to input the counter presented at 3 with one shot coil.

Rung 0003:

B3:0/0 and comparator block is used to enable B3:0/1 to turn OFF conveyor motor. It is also latched to avoid to restrict the motor to turn ON again. Restart button is pressed to start the process from the beginning.

Rung 0004:

Two ways are used to reset the counter, using manual reset switch and B3:0/1  when the counter accumulator reached preset value.

Program Output:

When Start PB is pressed

Start Pushbutton is Pressed in Motor PLC Logic

When three cases are passed

Motor Logic Passed

When the Restart button is pressed

Reset Pushbutton is Pressed in Motor PLC Logic

When Manual Restart is pressed

Manual Button Pressed in Motor PLC Logic


We can use this example to understand the programming logic in Allen Bradley Programmable Logic Controllers (PLC).

Author: Hema Sundaresan

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