Answer / kishore kumar

Distance is the main concideration, as distance increases
voltage drop will be increased where current remains same

Answer / rakesh kumar n

There won't be any current drop in mA, but when you consider
Voltage there will be Voltage drop.

-->main Reasons for Voltage drop

#Distance between the Field and Control Room(more the
Distance and more will be the Voltage drop)

#Consider if the Distance is small between the Field and
Control room,then also there will be Voltage drop,because it
is milli Volt

#Theory Sum of the incoming current and Out going current
is always Zero 20mA=20mA by KCL, if you find any thing
wrong,you can check it in Transmitter Outlet and the DCS
Inlet will have the same milli Amp.

Thank you
Rakesh Kumar.N
INSTRUMENTATION ENGINEER

Answer / mahesh soni

Distance is the main concideration, as distance increases
voltage drop will be increased where current remains same
due to ohms law there is

I=V/R SO CURRENT VWILL SAME any time or no lossed by
wire resistance so v can lay cables long root laying in
plant& all instruments draw curren not voltage

Answer / mohammed abbas

As length of the cable increses the line resistence also
increse, but the current remain same, suppose if your
using the long cable assume 50mtr the line resistance
assume 2 ohm for 50mtr, for 100mtr the line resistance will
be double 100=4 ohm. as per ohmns law. if you are measuring
the flow parameter for 50 mtr and another 100 mtr for same
process condition the reading will be diffrent, suppose if
your are using millivolt means but milli amps will remain
same.
the volatge will increse or decerse as the lenghth incerese
of decerses, but current remain same that is what milli
amps will be used

Answer / pankaj kumar (iert)

* voltage pickup more noise as compare to current.
* if we use voltage signal then chance of its drop.
*

Answer / vijay vaidya

ya i m agree wid mr. kishore

Answer / mohammed

because we cannot lay a cable for more than 50 m to carry mV
signal whereas we can connect current signal cable because it
is less diameter, will not affect by noise and not costly in
addition can be connected for long distances

Answer / delta_v

That's due to voltage drop :)
As simple as that...

Answer / shankar prasad p

MV if transmitted over long distance in wires, voltage
drops occurs leading to variation in actual value.where as
if we transmit mA signal over cable voltage drop not takes
place so we can get actual readings.

Answer / ashutosh

in series the current remain same so the loss of current
due to wire res is not into the consideration in mA but it
is there in mV

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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