TEST & MEASUREMENT HANDBOOK
Basics of monitoring vs. testing in current, voltage and power WILL DELSMAN | NK TECHNOLOGIES
Industrial measurements often must take place via specialized transducers sized specifically for the current and voltage swings involved.
OLDER AMMETERS worked great when the current flowed at a steady 50 or 60 Hz. Consider the antique GE model
Dealing with distorted waveforms Pure sine wave
8AK1A1AF. I have one that remains accurate after decades of use and
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misuse. The nameplate shows “60 Hertz,” denoting its limited scope of applications. Try to get an accurate reading when there is much distortion to the current sine wave shape or if the frequency is anything
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other than 60 hertz and you are in for a frustrating experience. Back in the dark ages, test equipment like this old beast utilized a current transformer design based on a toroid. The metal jaws surrounding the conductor were wrapped with many turns of thin copper wire. The magnetic field produced by the ac current caused a voltage to be generated in the wraps, and this small voltage would drive the pointer to display the amount of current present. There was no simple method to detect or measure dc current beyond current shunts. Present-day ammeters are most often now designed to use hall-effect elements rather than toroids and can accurately measure ac or dc current. The output from the hall element is conditioned to produce an output proportional to the RMS current regardless of the amount of distortion or harmonic component in the monitored circuit.
TEST VS . PERMANENT MONITORING Test equipment like handheld ammeters and voltmeters are essential tools, but few have the ability to transmit the readings to a remote location or to store the measurement data for later review. And these abilities are usually necessary for industrial data collection. Usually it is a permanently installed device, known to the trade as a transducer or transmitter, that monitors industrial parameters and produces an output which can be stored at the site or uploaded to a cloud service. These sensors produce an output which is typically connected to and read by a panel meter, HMI, programmable controller, or data acquisition system. Generally, industrial transducers must be installed in a cabinet for protection against the surrounding environment conditions while also providing a degree of safety against electrical shock and arc flash. These transducers are designed to be as compact as possible and have
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DESIGN WORLD — EE NETWORK
6 • 2020
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Actual VFD waveform
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Average responding transducers are adequate for the measurement of pure sine waves. But the accurate measurement of distorted waveforms as, for example, from variable-frequency motor drives, requires a true RMS transducer.
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