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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Answer Posted / t.naveen babu
The reasons for the 4-20 mA standard (and, before it, the
10-50 mA standard) have a lot to do with the original 3-15
psi/20-100 kPa air
signal. The origin of the suppressed or "live" zero in the
pneumatic system was two-fold - if you want to reduce a
passive pneumatic signal quickly, you need to maintain some
differential between the pressure and the "sink" - in this
case, atmospheric air. The basic characteristic of a
flapper-nozzle assembly also has a bit to do with it -
the "linear" range of the flapper pressure is about 3 - 15
psi for a 20 psi supply. Next question - why a 20psi
supply? Anyone else remember 6-30 psi equipment for
final actuators needing a bit more grunt??
The original electronic instruments were direct imitations
of the pneumatic systems with bellows replaced by magnet-
coil assemblies. The first I came across were used in a
geothermal power project (1958 vintage)where the pressure
at the power station was controlled by manipulating valves
in the steam field 1 1/2 miles or so away. The flapper-
nozzle was replaced by a
contact moving between positive and negative terminals - I
guess a bity of mains hum in the amplifier kept it
jittering. In the mid-70's Foxboro produced a range of
pressure transmitters using the magnetic principle which
used the same basic mechanism as their pneumatic equipment -
but the size of the coils made the assemblies a lot
bigger ...
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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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