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Ask the Experts

Dec 1, 2003 12:00 PM, By Mike Lowenstein, Harmonics Limited

Understanding the meaning of total effective current or voltage can be tricky. This month Mike Lowenstein, president of Harmonics Limited, helps us set the record straight on these oft misunderstood topics.

Q. When a power meter or instrument is indicated by the manufacturer to be “true rms,” does it necessarily follow that the instrument readings will, in the case of measuring circuit current or voltage, reflect the “total effective” current or voltage in the circuit, as defined and discussed in the November 2002 PQ Corner article “Harmonic Current and Voltage Distortion”?

Lowenstein's answer: In the article you reference, the expression “total effective” as used with current or voltage relates to the calculation of power losses through a system resistance. Fig. 1 at right illustrates the measurement of voltage in a system that powers a nonlinear load. Using this system, it's easy to determine the power loss due to the voltage drop across system impedance. First calculate the power loss for the 60 Hz or fundamental current, then do the same for the rms current. You'll find the presence of harmonic currents results in increased power losses.

For example, consider the system data as shown in Fig. 2 at right. In this example, the harmonic currents increase power loss by 132W, or 12.6%. As you can see, the rms values, when used to calculate losses, give the real losses in watts, and the rms currents indicate the real load on circuit components. Any true rms meter from a reputable manufacturer should give the desired values. The total effective voltage and current values are also useful in determining loading on thermal circuit breakers.

That said, you should be aware of a few subtle differences between meters. When selecting a meter to be used for measuring current or voltage in a system that serves loads with switched-mode power supplies, the largest crest factor, or the ratio of peak to rms values, that the meter can be exposed to and still read correctly will determine whether the true rms value actually captures all the harmonics. Some inexpensive meters state they can handle crest factors of 2 or 2.5. Since the crest factor of computer power supplies is usually between 3.5 and 4, this type of meter won't provide readings that represent the true rms value.

In general, if you select a true rms meter from a reputable manufacturer and make sure that the meter can handle high enough crest factors, you can feel confident that, for power calculations and loading values, the meter will show you the “total effective” current or voltage.

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