Learn how a true-rms meter can accurately measure imperfect, nonsinusoidal waves compared to an average digital multimeter.
Most electronic loads have nonsinusoidal waves, which makes measuring with an average responding meter somewhat difficult. Anytime you’re measuring AC voltage or current signals that aren’t pure sinusoidal (sine) waves, you’ll want to use a true-RMS meter. Here’s why.
The shape of the waveform — pure sinusoidal vs. nonsinusoidal — can have a dramatic impact on a reading, depending on the type of multimeter used.
How Digital Multimeters Read Sine Waves
An average responding meter uses averaging formulas to accurately measure pure sinusoidal waves. While these meters can measure nonsinusoidal waves, they offer uncertain levels of accuracy.
An average responding meter assumes a sinusoidal wave and calculates the average of the highest and lowest parts of that wave, then multiplies it by 1.11, or by the conversion factor. If a waveform is distorted or uneven, this average measurement will be higher or lower than it should be — up to 40% low or 10% high depending on the waveform (see table).
Below are a few scenarios with nonsinusoidal waves — where current occurs in short pulses, rather than smooth waves. In these situations, you’ll want to measure with a true-RMS meter:
- Variable-frequency drives (VFDs).
- Electronic ballasts.
- HVAC control systems.
- Solid-state environments.
How a True-RMS Meter Works
A true-RMS meter can accurately measure those imperfect, nonsinusoidal waves, as well as perfect, sinusoidal waves. “RMS” stands for root-mean-square, which is a calculation used to determine the equivalent DC value of an AC waveform. For example, a 120VAC signal applied to a resistor and a 120VDC battery applied to the same resistor should both heat the resistor to the same temperature.
True-RMS meters use mathematical equations to convert the measurements and give an accurate measurement of both sinusoidal and nonsinusoidal waveforms.
V AC+DC = √ V AC 2 + V DC 2
V High Frequency = √ V Total 2 - V Low Pass 2
True-RMS Application in HVAC
One example of the use of true-rms meters is in HVAC. As heating and cooling technologies move toward energy efficiency, modulating systems are becoming more normal. If you’re servicing these newer HVAC units, you need to take your measurements with a true-RMS meter to get an accurate measurement. The computerized controls, solid-state VFDs and VFD-controlled motors all need true-RMS measurements to get accurate results to troubleshoot an issue.
Fluke Corp. is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™ program. Based in Everett, Washington, Fluke Corp. manufactures, distributes and services electronic test tools.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.