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Applied Electricity Paper 2 , May/June 2009  
Questions:   1 2 3 4 5 6   Main
General Comments
Weakness/Remedies
Strength





















Question 4

(a) State one advantage and one disadvantage of a moving-coil instrument

(b)In fig.5, calculate the:
(i) voltage drop across the instrument;

(ii) resistance of shunt Rs required for a full scale deflection of 0.2A.
_____________________________________________________________________________________________________
Observation

The expected answers were:
                    (a)      Advantage
                             -    High sensitivity
                             -    Uniform scale
                             -    Well shielded from stray magnetic field
                             -    Low power consumption
                             Disadvantage

  • More expensive than moving-iron instrument
  • Only suitable for d.c. currents and voltages
  • Easily damaged/Not rugged

(b)(i)  The voltage drop across the instrument, Vm = Im rm
                        Vm = 0.015 x 3  =  0.045V  =  45mV
             (ii)   Voltage drop across shunt Rs = Voltage drop across the instrument
          The current through Rs, Is = (0.2 – 0.015)A ;  Is = 0.185ASince Vm = IsRs  =  45  x  10-3V

                         Rs  =  Vm / Is   =  45  x  10-3 / Is                            

                         =  45  x  10-3) / 0.185
                               Rs  =  0.2432Ω = 243.2mΩ

This question was on measuring instruments.  The question was popular.  Almost all the candidates attempted and got good marks in it.          

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