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Doctors Medicines MCQs Q&A

  1. Moving iron instrument
  2. Rectifier type instrument
  3. PMMC instrument
  4. Dynamometer type instrument
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1 Answers

Option 3 : PMMC instrument

PMMC:

  • Moving-coil permanent magnet instruments or PMMC are suitable for only DC measurements.
  • In PMMC type instruments, defection is directly proportional to the current (θ ∝ I) hence these instruments have uniformly divided scales.
  • In the PMMC meter, the current passes through control springs to the moving coil.
  • These coil springs are two in number so if one of them is broken then the current path to the moving coil is disrupted.
  • Hence no deflecting torque.
  • Since the other control spring is in good condition, it will hold the pointer at zero indication. Here control torque has become half of its original value.
  • The error due to the external magnetic field is called a stray magnetic field error. It is less in PMMC since a strong magnetic field is present inside.
  • The cost of PMMC instruments is very high.

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  • Moving Iron (MI) type instruments can be used for both AC & DC measurements.
  • Rectifier-type instruments are used for both AC & DC measurements.
  • Induction-type instruments are only used for AC measurements.
  • Thermocouple meters can be used for both DC as well as AC quantities.

 

Note: In terms of accuracy PMMC meter has the highest accuracy. The order of accuracy is given below.

Induction < Moving iron < PMMC instruments

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