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Which of the following instrument is undesirable for measurement of AC values?

  • The permanent magnet moving coil type instrument is undesirable for measurement of AC values.
  • The permanent magnet moving coil (PMMC) instrument is a type of DC ammeter or voltmeter that uses a permanent magnet to produce a magnetic field and a moving coil to measure the current or voltage.
  • In a PMMC instrument, the deflection torque is proportional to the average value of the square of the current flowing through the coil.
  • For DC, this is a constant value, and the instrument can give accurate readings.
  • However, for AC, the current continuously changes direction and magnitude, and the average value of the square of the current over a complete cycle is not a constant. Therefore, the PMMC instrument is not suitable for measuring AC values.
  • To measure AC values, specialized instruments such as moving iron, dynamometer, or electrodynamic instruments are used.
  • These instruments are designed to respond to the average value or the RMS value of the AC quantity and can give accurate readings for both sinusoidal and non-sinusoidal waveforms.