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| 'Manufacturers of Digital Power Analyzers, Digital Wattmeters, Harmonic Power Analyzers, Wideband Spectrum Power Analyzers, ATC Ohmmeters, Micro-Ohmmeters, Milliohm-Meters, Digital Ohmmeters, Igniter Testers, Current Calibrators, AC-DC Active Current Shunts, Hipot Testers, Body Comp Scales and more…' |
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Fundamentals of Power Measurement
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RMS, AVERAGE and
PEAK AMPLITUDE MEASUREMENTS When describing the amplitudes of electronic devices, terms such as "volts RMS" or "amperes RMS" are used. The RMS (root mean square) of a sinewave produces the same "heating effect" as an equivalent DC voltage level. (i.e. 5 VAC RMS = 5 VDC). Since a given AC RMS amplitude is equal to the same "DC heating level" the term is useful for describing amplitudes of irregular shaped or distorted current or voltage waveforms. Two waveforms with different shapes but a similar RMS value will produce the same amount of heat. Valhalla power analyzers directly measure and display the True RMS voltage and current levels for a wide variety of loads, sources and power supplies. The RMS voltage (E rms) may also be shown to be comprised of the following components: |
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| The RMS, Average and Peak amplitude values for a sinewave have a mathematically constant relationship. |
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| Examine the relationship current and voltage play in defining a power or watts measurement. |
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| In an AC power measurement, the true power (P) is the product of the voltage (E) and current (I) modified by power factor which is the cosine of the phase shift angle between the voltage and current. For Sine wave loads: P = EI cosθ or True Watts = Volts x Amps x cosθ With a pure resistive load, both the current and voltage are in phase, and the phase shift angle θ is 0 degrees. If θ = 0° = 1 (unity) = P.F. . As loads vary from pure inductive through resistive to capacitive, the phase angle varies from -90° to 0° to +90° and the power factor varies from zero to unity. With an ideal resistive load, the sinusoidal current and voltage are in phase (angle = 0°), the cosine is 1 and the true power (watts) will be the voltage times the current. |
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| Power Measurements in Inductive
Circuits |
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| The AC power drawn from the source (e.g. line - 120VAC @ 60Hz 220VAC @ 50Hz) is the integral over one cycle of the instantaneous watts values. As shown in figure C, during a portion of each cycle power is used by the inductive device (e.g. electric motor), while during other portions of the line cycle, power is actually given back by the inductive device. The portion of the cycle where power is given back by the inductive device is called "negative power". In sinusoidal applications, Power = EI cos θ can be seen in figure C above to reflect the true watts during a complete line cycle (360°). All Valhalla Digital Power Analyzers can accommodate negative power portions of a circuit's power curve and perform accurate power measurements regardless of waveform differences between voltage and current. Valhalla's four-quadrant multiplier design determines instantaneous watts by multiplying voltage and current in real time. The "instantaneous watts" levels (both positive and negative) are accumulated and the integrated average value (True Watts) is displayed by the power analyzer. The four-quadrant multiplier overcomes problem VA phase offsets, accurately measures power for virtually any non-sinusoidal wave shape and works at low power factors. Valhalla offers a wide variety of digital power analyzers ideally suited for what may be normally a problem power measurement ( i.e. circuit power levels). Low current versions (option 20mA) are available to provide optimized, full-scale resolution and accuracy's at lower power levels often associated with modern circuitry. Valhalla's Digital Power Analyzers can be an excellent research and development resource for today's engineers designing the most energy efficient products ever. |
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| WATTS CONVERTERS |
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| For Distorted Waveshapes of All Types |
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| The success of the Valhalla 2100 series wattmeter design is centered around a four quadrant multiplier. The significance of the four quadrant theory is to provide a proper computation for watts under any phase relation between voltage and current to insure that indeed EI cos θ or true power consumption is being measured. That is, the four quadrant multiplier takes the instantaneous product of both E and I (instantaneous watts). Continuous integration of instantaneous watts provides a true power measurement regardless of waveshapes within the bandwidth limits of the converter. Valhalla uses a proprietary wideband VLSI multiplier design to capture high order harmonics and thereby ensure an accurate watts product. Once the multiplicative watts product is achieved, it is in turn filtered, passed through the A to D (Analog to Digital converter) and displayed. |
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| AC-DC Current Shunts versus Current
Transformers |
 Although
all Valhalla wattmeters are compatible with general purpose
current transformers, there are distinct accuracy and
bandwidth advantages to using a current shunt when making a
power measurement. Current shunts generally provide greater
accuracy over a broader frequency range than current
transformers (CT's). Rather than measure actual
in-series current flow the indirect inductive magnetive
field principle employed by current transformers is subject
to a variety of potential error sources. The inductive,
magnetic field sensing technique employed for a CT
measurement is often subject to inherent phase errors.
Handheld clamp-on CT's in particular have centering
(placement) problems, stray magnetic field
(proximity-based) interactions, bandwidth limitations and
CT turns ration errors. Phase offset errors are of no
significance when measuring simply the current level,
however, if used for a true power watts (EI cos θ)
measurement, then CT phase errors significantly magnify the
true power watts uncertainty. Measuring current and power
using a current shunt basically employs a voltage
measurement made across a known resistive shunt. The
voltage potential across the shunt is directly proportional
to the current flow. Valhalla current shunts in
particular are designed to maintain a stable resistance
with minimal heat rise, as well as have a flat frequency
response. A magnetic field cancellation effect is achieved
specifically by the shunt for greater frequency
response. Valhalla wattmeter shunts use a proprietary,
low-temperature coefficient resistive alloy, lined with a
high-thermal conductive, electrically-insulating layer
(Kevlar) in a high efficiency heat dissipation package.
Using Valhalla's temperature-stabilized shunt design allows
a 100 ampere shunt to withstand 150 amperes without
shifting the calibrated value of the shunt (no
overheating) |
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Power Factor, Real
Power and Apparent Power Apparent Power
is the product of the rms voltage times rms current that
is: Apparent Power = E-rms x I-rms <---- also called (VA) The single phase (θ) power factor of a load is a ratio of real or true power (EI cos θ) to the apparent power (EI or volt-amperes). In sinusoidal applications, power factor is related to the phase angle between voltage and current as: |
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| You may also examine the vector relationship Power Factor has with harmonics in the Advanced Power measurement topics chapter of this handbook. Valhalla's 2110 Power Factor Wattmeter series directly provides the operator with a display of power factor from zero to unity with 0.001 resolution. In addition, all Valhalla Digital Wattmeters provide a true watts (EI cos θ) direct measurement, and simultaneously display selectable volt-ampere parameters to allow the calculation of power factor . Power factor can range in value from 1 to zero depending upon load circuitry being resistive, inductive or capacitive. In non-sinusoidal volt-ampere waveform applications, power factor values provide more insight into the nature of the load circuit than does phase angle (since phase angle primarily describes sinewave relationships of the same frequency). |
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APPLICATIONS |
