Question

Compare an electro-magnetic relay system with an electronic logic or PLC system that controls a 3-ph contactor. Include appropriate commercial specifications, drawings, and their advantages/disadvantages in your explanation.

Answer 1

1. Relays are electro mechanical switches which has two contacts and coil generally electromagnetic coil two contacts are NO and NC where as P L C (Programmable logic controller is a programmable computer which makes decision based on the program and its input and output .

2. P L C will perform many functions with same hard wiring where as we need different hard wirings in case of relay .

3. Basic function of the both Relays and P L C' s is to control the electro mechanical system in industries .

4. P L C's can monitored and control mechanical system where as relays can only control the system

5.We can program the P L C according to our requirement which is not possible in case of relays .

6.Generally P L C's are operated on digital systems and Relays are operated on Analog systems

7. As P L C is a programmable device it consist of more programming functions like timer counter etc and we can modify the P L C designing circuit . Relay has only one function that is fault detection and modifying the relay circuit is more difficult when compare P L C

8.Reliability and flexibility of P L C's are more when compare to relays because we can easily find the fault using the program in P L C.

9.Time response of P L C's are 50 ms and above where as time response of relays is less than 10 ms

Commercial specifications of relay

• Time Delay: 0.02 Seconds Operate Time
• Time Delay: 0.01 Seconds Release Time
• Duty Type: Continuous
• Load Current Rating At Maximum Rated Voltage: 25.0 Amperes Dc Lamp Load
• Load Current Rating At Maximum Rated Voltage: 50.0 Amperes Dc Resistive Load
• Contact Load Current Rating At Maximum Rated Voltage: 50.0 Amperes Dc Motor Load
• Contact Load Current Rating At Maximum Rated Voltage: 50.0 Amperes Dc Inductive Load
• Contact Maximum Voltage Rating In Volts: 28.0 Dc
• Arc Quenching Method: Air

Commercial specifications for P L C's

• Number of Inputs and outputs : User can specify both and neither of both i.e Atleast and not more than value
• Input : Input can be AC input ,DC Input ,Analog, Thermocouple, R T D, Transition, Interrupt and other input.
• Output : Output can be DC, AC, Relay, Analog, Frequency ,Pulse ,Transistor,Triac and other output
• P L C Power: We can choose 12 V , 24 V and other DC and 115 V , 230 V , and other AC.