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Power Monitoring: Dispelling Meter Myths

May 1, 2000 12:00 PM, By Dennis Stewart, P.E.

Understanding the capabilities of your monitor will help you interpret readings more accurately when investigating power quality problems.

Years ago, power quality concerns were primarily limited to the effects of voltage on computers and other microprocessor-controlled equipment. However, as electronics took over the business world, the equipment itself became its own worst enemy. In today's high-tech society, the majority of electronic equipment is not just susceptible to power quality problems; it frequently creates them. This reality continues to increase the need for better power monitoring.

What's your first step to troubleshooting power quality problems? First, you need a monitor that accurately measures voltage and current waveforms. Although most electrical professionals usually consider sample rate, most overlook the other design factors that affect a monitor's overall performance.

The figure (available in print version, page 16) demonstrates the fallacy of a common meter myth. It shows about 1.5 cycles of a voltage waveform. (Sample points shown are for reference only and represent a sample rate of 16 samples per cycle.) Although many people assume the sampling rate is a good indicator of the quality and accuracy of a meter, this is only partially true. We really know very little about a meter's accuracy if we only know its sampling rate. To accurately define a point on the waveform, you must establish an x and y value.

The sampling rate establishes the x value. That is, when we state the sampling rate we can determine the distance between samples on the x axis. However, when we know only the sample rate, (the x value), we do not know the y value.

The y value is the actual voltage measurement. This is the value that identifies the true applied voltage. Without knowing the y value, you really know nothing about the quality of the waveform you're measuring.

The circuitry of a digital meter enables the device to measure this value and report it to the processor. However, most meter users never ask "how" or "how well" the meter measures the y value. The y value is based on the quality and precision of the analog to digital (A/D) converter in the meter. The meter processes the value coming from the A/D converter that ultimately becomes the reported electrical value. Therefore, the accuracy and precision of a digital meter is a function of the sample rate, A/D converter, and quality of the calculations performed.

In modern electronic devices, sample rates vary. Some devices used for metering may sample at rates as low as four samples per cycle. On the other extreme, very accurate, high-quality meters may sample as fast as 256 samples per cycle.

The sample rate greatly affects the ability of a device to correctly identify variations in the wave shape. Low sample rates are not suitable for any measurement where you might require high accuracy or where there may be concerns about harmonics, low power factor, or any other type of power quality phenomenon.

The quality and performance level of the A/D converter also varies. Some meters use an 8-bit A/D converter. High-precision or high-accuracy meters may use up to 16-bit A/D converters. Although a 16-bit A/D converter is currently the best available technology, most modern power quality meters use only 12- or 13-bit A/D converters.

The number of bits determines the size of the divisions along the y axis. To determine the size of the division, you need to know the full-scale voltage range and the A/D converter bit rate.

A typical digital meter may have a full-scale voltage rating of 150V, which is based on the rms voltage. This results in a full-scale measurement range of approximately 424V. To get from rms voltage to the corresponding peak voltage, you multiply the rms voltage by the square root of 2 or 1.414. (150 x 1.414 = approximately 212V.) This is the peak value of the sine wave. To get the full peak to peak waveform you must multiply the single side peak times 2 because the sine wave goes above and below the zero point by the same amount: 212 x 2 = 424V. This is the peak to peak voltage for a 150V rms value.

The measurement problem is even more severe for current. Full-scale voltage is closely related to normal voltage. However, normal current is usually much less than the required full-scale current. A meter with a nominal 5A input must accommodate an actual full-scale input of at least 10A.

However, meters designed to record waveforms or fault currents must measure an rms full-scale reading of 50A or more; that's 10 times the nominal full scale. When the rms current is converted to peak-to-peak values, a 5A meter must accommodate a peak-to-peak reading of 140A or more.

As you can see, the rating on the A/D converter has a great impact on the meter's ability to correctly register the shape of the waveform. However, after you take a reading, you must process it.

Meters use a variety of processors to calculate the electrical quantities based on the readings. The processor should always be rated higher than the A/D converter. Some meters use 16-bit processors, however, high accuracy meters almost universally use 32-bit processors.

From this discussion, there's no doubt that sample rate, A/D converter rating, and microprocessor rating all affect the accuracy of a digital meter as well as its ability to provide accurate power quality measurements. For metering applications where you're concerned about power quality, harmonics measurement, voltagedisturbances, or fault currents, using a meter with a high sample rate and high-bit A/D converter is a wise choice.

Stewart is the Western Regional Manager for Electro Industries/Gaugetech, Boise, Idaho, and is a registered electrical engineer in the State of Idaho.

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