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A current I = 5 + 14.14 sin (314t + 45°) is passed through a centre zero PMMC, hot wire and moving iron instrument, the respective reading are
1. -5, 15, √125
2. 5, √125, √125
3. -5, √125, 19.14
4. 5, 10, 10

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Correct Answer - Option 2 : 5, √125, √125

Concept:

  1. Permanent Magnet Moving Coil (PMMC) is only used for DC measurements.
  2. Rectifier-type instruments are used for both AC & DC measurements.
  3. Induction-type instruments are only used for AC measurements.
  4. Thermocouple meters can be used for both DC as well as AC quantities.
  5. Moving Iron (MI) type instruments and Hotwire instruments can be used for both AC & DC measurements.
     

RMS value: 

RMS current is that value of steady current which when flowing through a given resistance for a given period of time, produces the same quantity of heat like that of

An alternating current flowing through the same resistance and for the same time period.

It is also known as the effective value of alternating current.

The RMS current Irms is related to the peak current I0 as:

 \(I_{rms} = \frac{I_0}{√2}\)

Calculation:

Given:

 I = 5 + 14.14 sin (314t + 45°) 

When PMMC is connected, it measures the average value of current

Reading of PMMC instrument = 5 A

When Hotwire or moving iron instruments are connected, it measures the AC values i.e RMS value of current

Reading of respective instruments = ​\(\sqrt{5^2\ +(\frac{14.14}{√2})^2}\)

√125 A 

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