SMECPLC and Relay - Advantages & Disadvantages

PLC and Relay – Advantages & Disadvantages


Why industries have chosen PLCs over the relay Systems?

PLC and relay, PLC can be seen in many industries basically it is an industrial computer. Many industrial processes can be controlled with the help of PLC. We can give data to a PLC through the inputs of the PLC and it would transmit the operating instructions through the output. In many automation processes, PLC is utilized a lot because we can use a PLC to control many complex operations, and due to this, the PLC has created a lot of changes in the industrial sector. PLCs can be programmed to do logic functions to control many industrial processes.

plc and relay

Why Relay was replaced by PLC in Industrial plants?

In earlier days relays were used instead of PLCs but it had several disadvantages. Industrial panels would have a lot of relay circuits if the relay is used instead of PLC.

In case there is any fault in the relay circuit which is used to do the industrial control, it can’t be easily detected. So to detect and troubleshooting the problems in a relay circuit was really difficult. Another problem was the expense to replace a relay circuit, relay circuits are really costly.

So in order to solve this problem PLC was developed, and we can perform many logical functions with the help of PLC. The major part of the PLC is a processor and it also has input and output sections. In a relay circuit, there could be a lot of complex wiring but in the case of the PLC there won’t be any complex wiring and in case any problem arises then we can easily troubleshoot it. The PLCs would have a lot of Input-output points when compared to the relay and they can be used to do a lot of complex operations. PLCs have played a major role in the development of the automation sector.

Advantages of PLC over relay

  • Doesn’t have complex wiring
  • It can act as a timer, comparator, and counter
  • It can handle the complex process
  • Hardware failure is very less when compared to a relay logic
  • In order to change the logic in relay there should be panel rewiring but in the case of PLC this is not necessary
  • Industrial process can be carried out securely by using PLC
  • PLCs require less hardware when compared to the relay and due to this, the complexity would be reduced.
  • The faults related to the complex hardware would be reduced due to the less wiring
  • PLC is really compact and due to this it would only take less space
  • PLCs have memory and better capabilities than the relay

Major difference between PLC and relay

Very less wiring when compared to relay logicIt requires a lot of wiring
Logic can be easily changed by changing the programRewiring is required to change the logic
Easily replacedRelay won’t handle the environmental conditions as well as the PLC
It can handle environmental conditionsRelay won’t handle the environmental conditions as good as the PLC
Compact sizeIt’s not compact and due to this it would take a lot of space
PLC’s have memoryRelays don’t have memory
Good reliabilityLess reliable due to the moving parts
Faults can be easily clearedIt would take a lot of time to find out the faults so faults can’t be easily cleared
Computational capability is highVery less computational capability
PLCs can handle vibrations, temperature, humidity etcRelays can’t handle humidity, vibrations, and temperature as good as PLC’s
PLC and Relay - Advantages & Disadvantages 4

What are the Major functions of a PLC and how does it work?

PLC can do the functions of a timer, comparator, counter, etc. In case you need to start several motors at different times automatically then it can be easily carried out with the help of a PLC. So a PLC is capable to do a lot of functions when compared to a relay.

The operation of the PLC is based on the input from the field device. So while receiving inputs from the field device PLC would compare it with the logic.

Then the PLC would provide an output based on the received input and also the logic. As the name of the PLC indicates it is a programmable logic device so it should be programmed first before the operation while programming we would give data related to the logic, based on which it would perform. PLC are programmed by using certain programming languages to do the industrial control.

The input from the field device would be connected to the PLCs input module. Then this input would be transmitted to the processor then it would be compared to the logic and output would be generated according to the logic after that it would be transmitted to the output module. Then from the output module, it would be transmitted to the field devices it could be motor, light, or any other device.

PLC Relay
PLC Relay

Classification of PLCs

Classification based on the Input/output

Small PLC

If the input and output are less than, a hundred then it is a small PLC

Medium PLC

The medium PLC would have less than ten thousand input-output points

Large PLC

If the input-output is more than ten thousand then it is a large PLC.

Classification of PLC based on its construction

Compact PLC

This would be a small PLC and it would have, and this PLC won’t be complex. The input-output of this PLC is fixed. We can’t use this PLC for any complex process.

Modular PLC

The major advantage of this PLC is that we can add input to this

Backplane PLC

This is a complex PLC, it has a busbar in it and we can connect input-output modules in it.

Why computers are not used instead of PLC for industrial process control?

Computers have better processing power and good memory compared to PLCs. Computers are not secure when compared to PLCs. Computers are prone to power spikes, surges, and crashes, but PLCs are really good against all of these problems. PLCs are really robust they can handle most environmental problems, they can handle environmental noise, field interference, and variable power. PLCs are less prone to crashes, bugs, etc. So in order to control industrial equipment, PLCs are a better choice than a computer. We need reliable control for industrial processes and PLCs are capable to do this.

What is the operating principle of relay?

A relay can be described as a switch that can be used to open or close a circuit. Relay is like an electromagnetic switch, it consists of a coil, and when a current pass through it then it would act as a magnet. If the relay coil is energized then it would make contact with the spring. So when a current pass through the coil then it would act as a magnet and makes contact with the spring. A relay could be normally open or normally closed according to the input.

How SMEClabs would help you to make a better career?

Nowadays a lot of industries are being automated so there could be a lot of chances for PLC programmers and also automation engineer. So in order to create a career in the automation sector, SMEClabs can help you a lot. SMEClabs would train you in industrial automation. There are a lot of opportunities for automation engineers in the Oil and gas industries and also in other industries such as chemical, automobile, etc.

So if you would join the Automation course in SMEC then you can learn a lot about, DCS, SCADA, PLC, PAC, Robotics, IoT, and a lot more. If you complete this course then you would get international certification. There would also be a lot of practical sections regarding industrial automation so you can improve your skills by attending these practical classes.

Author – Ashlin A J

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