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Relay sequences control

This text book introduces basic knowledge you should be aware of for relay sequences control together with simple examples for first-time users of PLCs.

Learn PLC programming (relay sequences control)
Learn PLC programming (relay sequences control) (on photo: Mitsubishi Electric MEAU-L Series PLC)

Descriptions in this text book are primarily for the teaching material FX-I/O demonstration model that uses the micro PLC FX1S-14MR type.

Though the word “sequence control” may generally be unfamiliar to us, it is used very often around us, and everyone is likely to have seen or have had contact with something that is controlled by sequence control.

We can understand that “sequence” refers to a succession or order in which events occur.

Let’s consider an example

Example // Controlling the level of water in a tank

Details of sequence control

  1. When the operation switch is closed, the float switch limit switch 1 closes if the tank is empty, and the magnet switch MC is operated to drive the transfer pump motor. Relay MC is designed to hold its own state even if the water level is at the mid position.
  2. When the tank becomes full, float switch limit switch 2 opens, hold operation of the relay MC is canceled, and the transfer pump motor stops.
  3. When the water level reaches the empty level, the motor starts to operate automatically again.
Example - Controlling the level of water in a tank
Example – Controlling the level of water in a tank

With motor operation in this control example, the motor starts to operate at the mid position after an empty tank is detected. Motor operation stops at the mid position after a full tank is detected, and the motor operating state differs even at the mid position.

Details of sequence control
Details of sequence control

This kind of operation is called “hysteresis operation” and can reduce the number of times that motor operation is stopped.

Sequence diagram

Sequence diagram
Sequence diagram

The sequence in this section is generally replaced by a sequence program on a PLC.

About self-hold circuits
A “self-hold circuit” refers to an operation circuit that bypasses signals arriving from an external relay or other device by means of the contact of the relay itself. Self-hold circuits can also be said to have a memory function that allows them to operate continuously even if the circuit is cut by releasing a pressed pushbutton.

In this circuit, the contact of the relay (MC2) for driving the motor is connected in parallel to float switch limit 1 (LS1). This provides an example of a “self-hold switch.”

Though limit switch 2 (LS2) functions to cut the self-hold circuit to stop the transfer pump motor, when limit switch 2 (LS2) is used with a N.C. contact, the switch can also be turned OFF to stop transfer of water during a switch contact failure or wiring break.

Title:Learning PLC Programming by Relay Sequences – Your First PLC by Mitsubishi Electric
Size:4.30 MB
Download:Right here | Video Courses | Membership | Download Updates
Learning PLC Programming by Relay Sequences - Your First PLC by Mitsubishi Electric
Learning PLC Programming by Relay Sequences – Your First PLC by Mitsubishi Electric

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  1. wossen tessema
    Nov 10, 2022

    I am working in a medical equipment private company as technical adviser and expected to do several modification control system which requires the knowledge of PLC programing that can control different sequence of operations like autoclaves, laundry machines and on radio active decay tank control system. I have little background on PLC and ladder circuits and my field of specialization is on Electrical and Electronics technology. I want to start from scratch to be aligned with your methods of teaching
    Wossen Tessema

  2. niwat
    Dec 17, 2019

    thank you

  3. Donald Patan
    Oct 20, 2019

    I’m working in ship elect.and all of our PLC control is Melsec I don’t have enough knowledge of this PLC sequencer control system Thank you for sharing your knowledge it’s a big help for me.

  4. Saheed Hussain
    Jun 13, 2019

    I’m an heavy duty automobile specialist. I hope to use this book as a window to PLC control operation follwing my background in caterpillar control systems.

  5. Suyog Waghmare
    Jul 18, 2017

    I’ve to join PLC program course

  6. Abraham Tetteh
    Nov 01, 2016

    am a switched broad attendant i wish to get this book and know more about the control operation

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