Understanding Rectifier Instrument Indication

A rectifier instrument is a type of electrical measuring instrument used primarily for measuring alternating current (AC) voltage or current. It achieves this by using a rectifier circuit to convert the AC signal into a pulsating direct current (DC) signal, which can then be measured by a standard DC measuring instrument, such as a Permanent Magnet Moving Coil (PMMC) meter.

The key principle here is how the DC measuring instrument responds to the converted signal:

• A PMMC meter is designed to respond to the average value of the current flowing through its coil.
• When the pulsating DC from the rectifier flows through the PMMC meter, the meter's pointer deflects based on the average value of this pulsating DC waveform.

Therefore, the fundamental electrical quantity that a rectifier instrument's measuring movement responds to and indicates is the average value of the rectified AC waveform.

Calibration and RMS Value:

It is important to note that although the instrument measures the average value of the rectified waveform, the scale of rectifier instruments is typically calibrated to show the Root Mean Square (RMS) value of the original AC signal. This calibration assumes that the input AC waveform is a pure sine wave. For a sinusoidal waveform, there is a known relationship between its RMS value and the average value of its full-rectified form:

Average value of full-rectified sine wave \(= \frac{2 \times \text{Peak Value}}{\pi}\)

RMS value of sine wave \(= \frac{\text{Peak Value}}{\sqrt{2}}\)

The instrument measures the average value but displays the RMS value by scaling the meter deflection based on the form factor (\(\frac{\text{RMS value}}{\text{Average value}}\)) for a sine wave, which is \(\frac{\pi}{2\sqrt{2}} \approx 1.11\).

However, if the input waveform is not a pure sine wave (e.g., square wave, triangle wave), the relationship between the average value of the rectified waveform and the RMS value of the original signal changes. In such cases, a rectifier instrument calibrated for sine waves will not accurately indicate the true RMS value of the non-sinusoidal signal.

In conclusion, despite being often calibrated to display RMS values for sinusoidal inputs, the core operational principle is based on measuring the average value of the rectified waveform.