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AP05311002E For more information visit: www.eaton.com

Measuring Linear and Non-linear (Distorted) AC

Waveforms with CurrentWatch™Application Note

New InformationMay 2007

Introduction

Variable frequency drives (VFDs) improve control andconserve energy by regulating motor speed. The mostcommon types are PWM (pulse width modulation) and Six

Step. Both technologies are based on high speed switching,which distorts the AC sine wave and can make accuratecurrent sensing difficult.

Measuring Current

The two principal methods used to measure linear andnon-linear waveforms are ”Average Responding“ and ”TrueRMS.” Understanding these two methods can help youspecify the right current sensor for your applications.

Most current sensors have ”Average Responding“ outputvalues, meaning they work on the assumption the measuredcurrent is a pure sine wave (linear). Their circuitry rectifiesand filters the current waveform to obtain an ”average“signal. Using the peak value divided by the square root

of two, this ”average“ signal is scaled and converted to4 – 20 mA or 0 – 5V DC. This method provides fast responseat a moderate cost, but only works on linear waveforms.

However, VFD outputs are simulated sine waves (non-linear)with peaks significantly greater than the true current. The

relative sizes of these peaks change as the output frequencychanges. An ”Average Responding“ sensor that is calibratedfor sinusoidal (linear) waveforms could accurately measurethe VFD output at 20 Hz, but be 20 percent high at 30 Hz and10 percent low at 40 Hz.

The True RMS Solution

The best way to accurately measure non-sinusoidal (non-linear) waveforms is to use the Root Mean Square (RMS)algorithm. A True RMS measurement is obtained by firstsquaring the signal, then averaging the squared signal andfinally taking the square root of that average (right side ofFigure 1

 

).

The result is the true power (heating value) of the waveform.This allows very different waveforms to be compared toeach other and to the equivalent DC (heating) value.

How do you know if you have a True RMS sensor? If theproduct is described as ”True RMS on sinusoidal wave-forms,“ you have an ”average responding“ sensor with aclever but misleading description. A True RMS sensor willbe described as ”True RMS on all waveforms“ and will”accurately“ measure current from waveforms found withthe use of VFDs or SCRs and on linear loads in ”noisy"

power environments.For non-linear load applications, choose the CurrentWatchEACR or EPRMR Series, as both have the capability to senseTrue RMS.

 

Figure 1. Measuring Waveforms of Equal Power (Heating) Valve

An “Average Responding” sensor performs the equivalent oftaking the signal peak and then dividing the peak average valueby 1.414. This is accurate on pure sinusoidal (linear) loads only.

A True RMS sensor applies the “Root Mean Square” calculation(See Formula). This produces an accurate measurement of allwaveforms, including the distorted VFD output shown above.

Cause of Error for“Average Responding”

Sensors

Pure Sine Wave

I=

Actual VFD Waveform (PWM Type)

IP

IPIP

2IRMS= Avg (i2)

i

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Application Note

 

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Effective: May 2007

 

Measuring Linear andNon-linear (Distorted)AC Waveforms withCurrentWatch™

 

Eaton Corporation

Electrical Group1000 Cherrington Parkway

Moon Township, PA 15108

United States

877-ETN CARE (877-386-2273)

Eaton.com

© 2007 Eaton CorporationAll Rights ReservedPrinted in USAPublication No. AP05311002E/CPGMay 2007

 

Other Applications for True RMS

True RMS sensors can be used in any application where adevice distorts the current waveform — not just VFD applica-tions. Another common use for True RMS sensors involvesapplications with dimmer circuits, frequently used to control

lighting. As the dimmer is adjusted, the waveform changesshape and becomes non-sinusoidal. An “Average Respond-ing“ sensor, always assuming a sinusoidal waveform, wouldreport inaccurate current measurements.

Understanding the difference between “Average Respond-ing“ and True RMS sensing technologies will help you tochoose the right current sensor for your application.

For application assistance or for help with technical issuesregarding current sensors or any other sensor in Eaton’sportfolio, contact our Sensor Application Engineers (800)426-9184 option 2.