What is True RMS? And Why is it Important in a Phase Monitor?

Posted: 4/14/14 | Three-Phase Monitor Relays

RMS stands for Root Mean Square. True RMS is a mathematical formula that is used to calculate the effective value of an AC wave shape. To understand why this is important, it is helpful to first discuss what a phase monitor does. A phase monitor measures voltage to confirm the presence of a desired voltage; it also detects errors or fault conditions that can cause hazardous conditions or damage equipment.

Voltage is commonly graphically represented by a sine wave like the image below. This is also referred to as a sinusoid.

Voltage is commonly graphically represented by a sine wave.

A common way to monitor voltage is to measure the peak of the sine wave then calculate the average to determine the voltage. This is very effective and works great when the sine wave is a smooth repetitive oscillation as shown in the graph above because this method makes an assumption that the sine wave is consistent and sinusoidal in shape.

Often the voltage is not smooth and linear. Equipment such as adjustable speed motors, solid state motor drives or heating controls can have distorted sine waves. This is also referred to as non-sinusoidal. See the examples of non-sinusoidal graphs below.

Often the voltage is not smooth and linear. This is also referred to as non-sinusoidal.Example of non-sinusoidal graph

Peak measuring and averaging will not provide an accurate measurement of these voltage conditions. Complex voltages such as these can be under-calculated anywhere from 5 to 40% when using the peak measurement method. This type of error can mean that the phase monitor is not providing the protection that it is intended to do and potentially damaging voltage conditions are going undetected.

This is where True RMS is a benefit. The true RMS value is proportional to the square-root of the average of the square of the curve. This means that True RMS is measuring the actual sine wave and not assuming the shape and filling in the blanks as is the case with the peak averaging method. This is a more complex calculation and it is more accurate. The end result is far greater accuracy and a true measurement of voltage.