Answer / ranjith_asp

As the name suggests, these instruments will have 4 wires
connected to its Terminal Block. Take a Pressure Transmitter
for example
- 2 wires are for Power Supply. Typically 24VDC. This is to
power the instrument to make it work. These 2 wires will be
routd via a JB and then connected to a power supply module
in the PLC cabinet.
- remaining 2 wires are for 4-20mA signal transmission. This
output signal will be proportional to the measured Pressure.
These 2 signal wires will be connected to the AI card in the
PLC cabinet.

Answer / hadi

4 wire connection sensor have two methods:
1-sensor's power is provided from card
2-sensor's power is provided from field
in first method the power is +24VDC and the current limit
exist but in second method the power is every thing and
transmitter feed from external power supply.
in two methods the signal lines and power lines are different.

Answer / ramaswamy and karuppusami

Usually in 2wire transmission, 24v supply is given and o/p
in mA are taken through single pair cable. e.g: Transmitters.

But in 4wire system, supply is provided in separate single
pair cable and o/p is taken through another cable. e.g :
local speed indicator device: i/p = 230v and o/p = 4-20
mA(Both are in separate cables ).

Why we are using 4-20 mA as Analog signal transmission other than 3 to 19 or 2 to 18 or 6 to 22 like wise ? What is the reason behind using 4 & 20 mA? Can anyone give proper technical reply ?

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A PLC is programmed to drive three motors A, B and C as follows : After running motor A for an hour, motor B should get ON and motor A should get OFF. And after running motor B for an hour, motor C should get ON and motor B should get OFF. And after running motor C for an hour, motor A should get ON and motor C should get OFF. (That is, a cyclic repetition with time period one hour) Now, the second condition is: If any motor gone faulty and is tripped, the consecutive motor should get ON and the faulty (tripped) motor should be eliminated from cyclic repetition. The remaining two motors should bear the cyclic repetition of one hour each. (For healthy motor take logic as high and for tripped motor take logic as low) Now, the third condition is: When motor A load(Amps) exceeds 60% of its rated load, motor B should get ON in parallel to motor A. And when both motors A & B are running in parallel, and if their load exceeds 60% of their rated load, motor C should also start in parallel to A & B. [Here the cyclic repetition is eliminated, until their loads become normal(40%)] [Take logic high(1) for load at 60% and logic low(0) for load at 40%] Now draw a flow chart or a ladder diagram to fulfill the above condition.

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