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.