Monday, March 18, 2013

Motor Start Stop Time Sequence Electrical Control Circuit Using PLC Ladder Program

The following is a sample tutorial of a typical timing sequence involving a start and stop function of a common DOL (Direct On Line) motor control circuit using PLC (Programmable Logic Controller) ladder program.

The electrical components comprising the hardware parts of the control circuit includes:
  1. First stage start push button switch PBW (normally-open contact) - 1 piece.
  2. Third stage start push button switch PBY (normally-open contact) - 1 piece.
  3. Stop push button switch PBG (normally-open contact) - 1 piece.
  4. PLC with at least 3 available digital inputs and 3 available digital outputs.
The Control Circuit Diagram of the PLC Ladder Program:

PLC Program Control Circuit for Motor Start Stop with Time Sequence
Fig.1 PLC Ladder Program Control Circuit for Motor Start Stop with Time Sequence

Circuit description:
The circuit created with a PLC ladder programming software shown in Figure 1 above is designed to provide a switching selection for running three motors in three stage sequence. There are two rungs in the circuit of the PLC program, the first rung (rung 0) in the circuit which is initiated by pushing the start push button switch PBW will run the first two motors (motor 1 and motor 2) simultaneously for the first stage sequence, leaving motor 3 on stand-by.

The second stage of the sequence occurs once the 10 seconds time delay of the first timer (Timer 1) is reached, this causes motor 1 to stop while motor 2 remains running. When the third stage start push button switch PBY is pushed, the first rung (rung 0) is completely deactivated to stop motor 2 from running, while at the same time transferring the command to the second rung (rung 1) of the circuit which contains the third stage of the sequence.

This third sequence runs motor 3 continuously until the 5 seconds time delay of the second timer (Timer 2) is reached. Once timer 2 is up, both rung 0 and rung 1 of the circuit is deactivated awaiting another initial sequence to start over beginning with the start push button PBW for the first stage of the sequence. This third sequence in rung 1 of the PLC ladder program can be stopped at any time using the stop push button switch PBG.

This type of circuit can be applied for water pumps wherein two motors are running simultaneously to fill-up an empty tank (first stage), then once the tank is halfway full after a specified period of time, only one motor is left running (second stage) to reduce the pumping of water to the tank in order to prevent the overflowing of water. When the tank is full, all pumping is completely stopped, which in turn signals another pumping station (third stage) to start pumping water out of the full tank for distribution to another water tank station. Then once the primary tank is already empty, another signal is sent to activate the first stage sequence, then the second stage, then the third stage, continuously following the sequence of operation as commanded by the PLC program.

Circuit procedural sequence of operation:
  • Figure 2 below shows the first stage start push button switch PBW (white button) is pushed to activate the first stage sequence in rung 0 of the PLC program as shown on the HMI (human machine interface) panel with corresponding bit number 0.01 in the ladder logic.
Fig.2 PLC Ladder Program showing PBW push button ON

  • After releasing push button PBW, the circuit is maintained energized by means of the internal output 1 contact bit 10.00 connected in parallel across PBW bit 0.01, which also maintains both motor 1 (bit 10.01) and motor 2 (bit 10.02) in running condition while also permitting Timer 1 to start incrementing to count up to its set time delay value of 10 seconds as shown in Figure 3 below. Note that both motor 1 and motor 2 are both active as represented by their red indication lamp in the HMI panel. 

Fig.3 First stage rung ladder program maintained energized after release of push button PBW 

  • Figure 4 below shows circuit condition after Timer 1 set value of 10 seconds has elapsed, which stops motor 1 (blank color) and leaving only motor 2 running (red color) as indicated on the HMI panel. The circuit would now appear dormant at this stage while it is waiting for the activation of the next rung to open internal output 2 (bit 10.03) which is intended to release internal output 1 coil in order to completely deactivate rung 0 containing stage 1 and stage 2 of the operation sequence.
  • Fig.4 Timer 1 expires after 10 seconds to stop motor 1 and leaving motor 2 running
    • Push button PBY (yellow button) is switched ON to energize internal output 2 (bit 10.03), which in turn opens the normally-closed contact to deactivate the coil of internal output 1 (bit 10.00) to release its corresponding holding contact connected in parallel across bit 0.01 of push button PBW as shown in Figure 5 below. This step would also command motor 2 to stop running, which concludes the operation of motor 1 and motor 2 for stage 1 and stage 2 as indicated on the HMI panel.
      Fig.5 Motor 1 and Motor 2 stops which concludes the first two stages of the operation sequence
    • The energizing of internal output 2 (bit 10.03) resulting from the actuation of push button PBY (yellow button) activates the next rung in the PLC ladder program as shown in Figure 6 below. The holding contact (bit 10.03) across PBY (bit 0.00) locks the circuit to run motor 3 continuously represented by its red color indication on HMI panel, the circuit is maintained until Timer 2 reaches its set value of 5 seconds to stop motor 3.
      Fig.6 showing Motor 3 maintained in run condition waiting for Timer 2 to reach set value 

    • Once Timer 2 increments up to its 5 seconds set value, the third stage sequence that runs motor 3 is deactivated. This is caused by the opening of the normally-closed contact of Timer 2 connected in series with the coil of the holding contact bit 10.03 (internal output 2). Refer to Figure 7 below which shows an inactive ladder program denoted by the absence of the green circuit flow path in the ladder logic program with the corresponding absence of red color run indication of any motors.
    Fig.7 showing inactive ladder circuit and stop indication of motors on the HMI panel

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