DVM as PQ Troubleshooting Tool: What a digital volt meter can and cannot do

By Richard P. Bingham | Jul 15, 2021
iStock / Yevhenii Orlov




There have been a million or more power quality instruments sold by dozens of manufacturers since they were introduced in the 1970s. However, a typical instrument that can measure and capture the array of PQ phenomena defined in standards such as IEEE 1159 remain priced as a capital equipment purchase for most users.

Larger electrical contracting companies may have several units available for use, but a one-person company is unlikely to be able to afford the cost of an instrument. However, nearly every electrician today has a digital volt meter (DVM) with them on every job. Since a DVM measures voltage, it can be useful for certain troubleshooting applications, though without substantial memory, it doesn’t have much use in benchmark or audit surveys.

Transients—Voltage transients are very short-duration disturbances to the normal fundamental frequency sinusoidal waveform. They often last less than a millisecond, and they can be positive (increase the voltage waveform at that instant) or negative (decrease the voltage waveform). Depending on the source of the transient, they may occur randomly in time as well as where on the sinusoidal waveform they occur.

Verdict: A DVM is not likely to be useful except in limited applications if it has a peak, max capture function with a response time fast enough for positive transients that exceed the normal peak of the voltage sine wave.

Short-duration RMS variations—Voltage sags and swells are a reduction and increase, respectively, of the cycle-by-cycle RMS value of the waveform. They can last a half-cycle (8 milliseconds at 60 hertz [Hz]) or up to a half-second for instantaneous variations, up to 3 seconds for momentary or 1 minute for temporary variations. The RMS value changes from nominal to 10%–90% of nominal for a sag and 110%–180% for a swell. Like transients, they usually occur randomly and unpredictably, except for sags caused by large motors that start up on a predictable schedule.

Verdict: While a DVM can measure the temporary variations, the odds of seeing the number on the display that reflects the sag or swell value aren’t high unless the meter has a min/max function. This would allow capture of one such event. In the case of a large motor start, this may be a useful application to see how low the voltage goes when the motor’s inrush current peaks.

Long-duration RMS variations—These are similar to short-duration RMS variations, but last longer than 1 minute.

Verdict: A DVM isn’t needed for an interruption (or outage), but it can be useful for sustained under- or overvoltage conditions, both of which can damage equipment, if they last long enough for the meter to be connected in time.

Imbalance—Three-phase voltage systems, either wye or delta, should have each phase conductor at the same magnitude and phase angle with respect to the other phases and ground conductor, where applicable. NEMA MG1 recommends that a motor be derated by 10% for just a 1% imbalance.

Verdict: The DVM is a good application for measuring the magnitude of the voltage imbalance if it is a steady-state condition, but the phase angle is unlikely.

Waveform distortion—The steady-state waveform distortion is divided into five categories: DC offset, harmonics, interharmonics, noise and notching. Harmonics are signals with frequencies that are multiples of the fundamental power frequency (i.e., for 60 Hz systems, 120, 180, 240 Hz, etc.). Typically, up to the 50th harmonic is measured. Interharmonics are signals with frequencies between those of the harmonics. Noise is electrical signals, typically of very high frequencies (megahertz). Notching is repetitive negative transients, often caused by the commutation period of three-phase power converter circuitry.

Verdict: The typical lower-cost DVM isn’t of much use, except for DC offset. Some models do have total harmonic distortion parameters, and higher-end models have a limited number of harmonic values. However, interharmonics, noise and notching are beyond the scope of DVM design.

Voltage fluctuations (flicker)—The variation in the voltage waveform that results in flickering lights, particularly incandescents, requires a complex mathematical process on the digitized waveforms to calculate such. In addition, lighting types such as LEDs and CFLs have a different response than incandescents that needs to be accounted for.

Verdict: Even many PQ meters don’t do flicker calculations correctly, so it is beyond the realm of DVMs.

Power frequency variations—Changes in the fundamental power frequency, which aren’t that common on grid measurements, can be a factor in generators as the loading changes.

Verdict: There are DVMs that accurately measure such. They may be referred to as multimeters.

Though the capability of a DVM to capture PQ phenomena is limited, it is still the tool that I carry on every PQ troubleshooting job, as it is quick and easy to get preliminary data that may not require bringing out the PQ monitor, hooking up all the leads, programming it, and then wading through the data to see a simple answer.

About The Author

BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.





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