Dec. 16, 2024
Measurement & Analysis Instruments
Choosing the Right Power Measurement Instrument
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In order to empower development teams to fulfill their objectives across the development cycle, it is important to consider a whole solution approach toward instrument selection. Aside from satisfying unique needs for power accuracy, waveform analysis, or data acquisition, the technology must also be supported by appropriate training, hardware, and software. This maximizes the value of the investment. Regardless of the electromechanical phenomena to be measured, the computation capabilities, the level of accuracy, and measurement technologies must be reliable over the long term. They must offer support in hardware, software, and services in order to help engineers and manufacturers take their products from concept through production with greater quality in shorter time frames.
Instruments
There is a variety of instruments on the market that can potentially meet power measurement needs. Depending on the circumstances, one may need the waveform analysis of an oscilloscope, the high accuracy of a power analyzer, or a hybrid combination of the two with the flexibility of a data acquisition added into the mix. There are three main categories of instruments to consider when making power measurements. While there are exceptions, the categories below represent most instruments on the market and their core functionality.
Yokogawa has defined two instruments that expand capabilities beyond the standard instruments: Power Scope and the ScopeCorder.
The following will highlight some key considerations to weigh when choosing the best suited instrument.
Measurement Technology
The underlying power measurement principle behind each of these instruments is essentially the same: sampling the voltage and current waveforms simultaneously, multiplying them together after acquisition, integrating the resultant instantaneous power readings over a whole number of fundamental waveform cycles, and then dividing by the time. However, depending on the resolution of the analog-to-digital converter and the sampling rate, there are two broad category types of power measurement instruments: continuous streaming and digital storage.
Continuous Streaming Instruments
These include the traditional power meters and power analyzers. Streaming instruments use high resolution on the analog-to-digital conversion stage and instantaneously compute/integrate the voltage, current, and power values in order to achieve continuous measurements and high accuracies. This architecture allows instantaneous computations without deadtime or gaps by continuous flushing of the memory buffer after each acquisition.
A continuous streaming instrument will acquire every cycle and calculate power based on a time, t, defined by the measurement instrument. This equation is true for any wave shape, including AC, DC, or distorted.
Digital Storage Instruments
A digital storage instrument such as an oscilloscope, acquires data at a high sampling rate, stores it in acquisition memory, and then processes it for output. During processing of the sampled data, there is "dead time" when the instrument is not reading the input waveform, thus missing the data points for continuous measurements.
A digital storage instrument triggers on waveform data used in the power calculation. Because of internal rearm times and data movement to memory, dead times are introduced where the instrument is not recording large portions of data. Since these dead times could easily be greater than 90% of the waveform acquisition time, these instruments are not ideal for power measurements.
The high sampling rate in a digital storage-type instrument allows for a better representation of the input waveform, making it ideal for analyzing single shot or transient events. However, oscilloscopes are not designed for stability and manufacturers do not specify AC uncertainty. Therefore, when high accuracy is needed, particularly in compliance testing, a streaming-type power analyzer is usually the better choice as it can achieve accuracies up to 0.01% of reading. Hybrid instruments such as the Power Scope can combine waveform analysis together with high accuracy measurement, as shown below.
Figure 1. A power analyzer, Power Scope, and ScopeCorder can stream data continuously while oscilloscopes and data acquisition products are subject to large dead times.
Key Considerations
Job Functions
The various stages of product development each demand different capabilities and levels of accuracy. For example, isolated tests of individual components in early development stages may only need waveform analysis at limited accuracy, but when a multicomponent system needs to be tested, the objective is to optimize the system rather than a single component. This calls for sophisticated multichannel, multi-parameter measurements. Requirements for compliance and adherence to standards become stricter as the project nears production testing. Given the diverse operating conditions and objectives among development phases, one needs to be mindful that measurement needs can vary or evolve across the development cycle.
Design
During this stage, individual components may need to be measured. Engineers must deal with fast switching speeds (in inverters, power supplies, and electronic circuitry), dynamic behaviors at high frequencies, and frequent overshoots on pulses. High sampling rates are, therefore, needed to capture waveforms faithfully, making the oscilloscope the go-to instrument for such scenarios. Oscilloscopes offer automated measurement of voltage and current waveforms providing peak, average, and root mean square (rms).
Validation and Verification
As control systems become more sophisticated, measurements are no longer limited to optimizing individual components. More I/O signals are needed and, consequently, faster sampling and higher bandwidth to circumvent noise from inverters or power supplies. More channels are needed to capture the dynamic behaviors of each component as part of an overall system. In an automotive powertrain, for example, together with the electrical parameters, physical parameters such as rotational speed, fuel injector pulse time, and crank angles are measured from sensor signals, rotary encoders, etc. For such multichannel measurements of dynamic system behavior, the combined features of an oscilloscope and a data-acquisition recorder are needed.
Compliance Testing
Industries today must meet several governmental and regulatory standards to ensure product efficiency, safety, comfort, and productivity for homes and businesses. Compliance to standards for standby power consumption (EN and IEC ) or harmonics and flicker (IEC/EN -3-2 and IEC -4-7) for different classes of electrical and electronic equipment affects both market validation (fit-for-use) and product differentiation for competitive advantage. A power analyzer that can guarantee its accuracy over specified operating conditions is the ideal solution for this stage. Oscilloscopes and data acquisition instruments are not rated highly for compliance testing due to the lack of guaranteed AC accuracy over the bandwidth of the instrument.
Production and Field Testing
Measurements in production line and field testing have different sets of requirements. It may be necessary to measure energy generated, converted, or consumed along with the effects of harmonics. It may also be required to troubleshoot power consumption in startup, standby, or operation modes, or use high crest factors at every measurement range for capturing inrush currents. This would be best served by power analyzers calibrated to minimize uncertainty within the specified operating ranges. ScopeCorders and DAQ may be useful in this stage if there is a need for collecting data over extended and unattended periods.
Measurements
Power measurements across instrument types can vary with internal architecture and signal processing capabilities. Choosing the best instrument can depend on the actual measurement(s) required. One consideration is connection to the signal under test. Instruments such as oscilloscopes require a probe for voltage and current connection, which adds additional impairments due to loading. Other instruments such as power analyzers and power scopes have direct connections for voltage and current, allowing for compensation due to thermal drift.
Efficiency
Efficiency measurements involve a comparison of the input to output power of a device. This is more complicated if the input and/ or output have more than one phase. Efficiency is a comparison measurement and to detect the smallest differences requires the most accurate measurement device possible. The power analyzer is the best instrument for this measurement because of the rated accuracy and the guaranteed accuracy over the entire bandwidth range. Additionally, if the output is mechanical power, the power analyzer includes motor inputs to measure a resolver or encoder with high precision.
To read an example of measuring inverter efficiency, click here.
Voltage
Voltage measurements are impacted by the method of connecting the signal to the measurement device. High voltage signals may be connected directly with safety rated cables, or by using a voltage probe, either single ended or differential. The best probe is one with infinite loading and unlimited bandwidth. It imparts the least amount of attenuation to the actual signal under test. While this probe does not exist, it is important to consider the bandwidth and loading effects of any probe or instrument front end. A power analyzer, Power Scope, and ScopeCorder have direct connections into the front end of the instrument, typically rated to V to eliminate the need to add a probe. Oscilloscopes and data acquisition instruments require the use of passive or active voltage probes, which can impact the magnitude and shape of the voltage waveform being digitized.
Current
Current is typically calculated by measuring the voltage dropped across a shunt resistor. Power analyzers and Power Scopes include integrated shunts for the most accurate current measurements. An internal integrated shunt allows for temperature compensation due to thermal effects.
Other common current measurement solutions include Hall effect clamps, external shunt resistors, Rogowski coils, AC transformer clamps, AC transformers, and fluxgate transformers. Each of these is an external device that can impair the accuracy of the current measurement. The high current measurement application guide highlights the tradeoffs with each solution. For details on using a fluxgate current transformer, click here.
Power
Power measurements are best made with an instrument with the highest resolution, lowest phase error, and guaranteed accuracy. There are some tradeoffs when measuring power with an oscilloscope due to the nature of the sampling architecture. Yokogawa uses the equation below to calculate real or true power by integrating the instantaneous voltage times the instantaneous current over time, T.
The phase relationships of the voltage and current waveforms are very important to capture accurately for the most precise power measurements. The instruments with high scores have integrated de-skew functions to eliminate skew inherent to external probes and long cable lengths. For more details on how a power analyzer calculates power, read the white paper Fundamentals of Electric Power Measurements.
Harmonics
Harmonic measurement is another area in which it is important to specify the accuracy in the context of the application. When left unaccounted for, harmonics can cause capacitance losses, undesired vibrations in motors, transformer losses in no-load conditions, heating losses in conductors at higher frequencies, premature melting of fuses when electronic breakers do not respond at designed levels, and many more. It is vital to equip an engineer with the ability to detect harmonics and assess their effects on components, systems, and subsystems within an application. When measuring harmonics, it is important to ensure the harmonic measurements have a guaranteed accuracy. Additionally, the technique to determine zero crossing and duty cycle is important since many instruments use incomplete cycle data in their harmonic calculations, resulting in spectral leakage. A technique using a phase-locked loop (PLL) can ensure proper zero crossing is used in the harmonic&#;s calculation. Highly accurate instruments can measure harmonics upwards of the 500th order.
Mechanical
Mechanical power is measured as the motor speed times the motor torque. Speed and torque sensors should be fitted to the motor dynamometer and integrated into the test system. These sensors typically provide an analog voltage output or a frequency style output. Modern power analyzers can accommodate both types and provide support for rotational position sensors such as encoders. The ScopeCorder can calculate the angle of rotation from a resolver.
Signal Type
Testing a device such as a motor powertrain could require measurement of a broad variety of inputs such as a combination of repetitive signals of voltage and current that are input into an inverter, high speed switching signals, control signals for the AC drive, temperature signals from the drive, and mechanical signals from the motor output. Capturing all signals with one instrument can be difficult.
Figure 2. A motor powertrain has many different test points with various signal characteristics that can make the use of one measurement instrument difficult.
Repetitive vs. Transient
The high sampling rate in a digital storage-type instrument allows for a better representation of the input waveform. It is ideal for analyzing single shot events. However, oscilloscopes are not designed for stability and manufacturers do not specify AC uncertainty. When high accuracy is needed, particularly in compliance testing, a streaming or averaging-type power analyzer is usually a better choice as they can achieve accuracies up to 0.01% of reading. Hybrid instruments like the Power Scope, however, can combine waveform analysis together with high accuracy measurement.
Serial / Automotive Bus (I2C, SPI, CAN)
While all these instruments can capture the signals for common serial buses, a combination of analog and digital channels, decode packages with built-in intelligence of the signal parameters, advanced triggers, and search capabilities make one better suited to the task. The oscilloscopes and ScopeCorder products are best suited to capturing and analyzing serial bus signals.
Figure 3. Typical oscilloscopes can display and decode multiple bus types.
Torque/Speed
Mechanical power is measured as the motor speed times the motor torque. There are many different types of speed and torque sensors that can allow integration into a dynamometer. These sensors can provide the speed and torque measurements necessary to calculate the mechanical power in the power analyzer. Calculations of motor speed and torque can be made directly on a power analyzer. The wiring for this measurement depends on the signal type for speed and torque, output as either a pulse or analog signal, or a three-phase encoder pulse represented by phases A, B, and Z. Additionally, the ScopeCorder can calculate the angle of rotation from a resolver.
Figure 4. Torque and motor wiring for a three-phase encoder on a power analyzer.
Sensors: Temp/Pressure/Flow
There are many sensors on a motor drive train, and it is not always cost effective to monitor them with an oscilloscope. A DAQ device will properly measure the sensor output, including thermocouples or thermistors for temperature. These systems will scale to the input requirements for a smaller investment than a new oscilloscope. A ScopeCorder and DAQ device will provide the most flexibility for these measurements.
Power Accuracy
Every measurement device has some degree of uncertainty, which is why accuracy is normally expressed as a range. Within this range, engineers consider power accuracy as the primary indicator of uncertainty for basic measurement parameters such as voltage, current, phase angle, and power (watts). These parameters may be presented using terms such as &#;guaranteed accuracy&#; and &#;typical accuracy.&#;
To learn more, please refer to &#;Accuracy Specifications: Reading it Right with Range.&#;
*The ScopeCorder has one module that has guaranteed AC accuracy, while the Yokogawa power analyzer and Power Scope has guaranteed accuracy over the entire bandwidth.
Guaranteed vs. Typical
What does &#;typical&#; mean in this context in terms of watts? The term often is misleading. A typical value is usually a reference value based on a manufacturer&#;s expectation from its product. In practice, it can be translated as &#;usually but not always,&#; &#;maybe,&#; &#;perhaps,&#; or &#;possibly.&#; It is deliberately vague because typical accuracies are neither guaranteed nor traceable to a national calibration standard or accredited calibration laboratory standard. When selecting a power measurement instrument, the prospective user should be sure that the published accuracies are guaranteed rather than typical values.
AC Accuracy
In power measurement, not enough emphasis is placed on AC accuracy. Often, instruments will guarantee AC accuracy at 50-60 Hz, but more power applications measure signal content outside the 50-60 Hz range due to fundamental frequencies changing per application, and harmonic content at multiples of those frequencies. The ideal instrument will have AC accuracy specified through the entire bandwidth range.
Analog-to-Digital Converter (ADC) Resolution
In measurement terms, resolution is the smallest increment that the instrument can indicate or display. The more resolution an instrument has, the more it can resolve differences or details on waveforms. It can be expressed in a few different ways. Since we are working with time domain instruments, the most common is the number of bits. If the frequency domain is used for a Fast Fourier Transformation (FFT) for harmonics, the resolution can be expressed as signal-to-noise ratio (SNR). This number of bits is often provided by the manufacturer of the ADC. In an ideal condition, an instrument would have the resolution of the ADC, but noise and distortion will reduce the real resolution of the instrument. To account for this, instrument manufacturers use the term ENOB (effective number of bits) or SINAD (signal plus noise and distortion) to represent the resolution of the system. These are recommended metrics when determining the real resolution. Filters or techniques such as averaging can improve resolution, as well, but come at a tradeoff.
A power analyzer will provide the greatest resolution among measurement instruments at 18-bits, followed by the ScopeCorder at 16-bits. When power measurements require the best accuracy, a power analyzer is recommended.
Isolation
Isolating functional sections of electrical systems prevents current flow by removing a direct conduction path and eliminating ground loops. Isolation is used where two or more electrical circuits must communicate, but their grounds may be at different potentials. Benefits of isolation include signal-to-noise reduction and improved noise immunity.
Figure 5. Yokogawa's IsoPRO&#; uses optical isolation in their power analyzers, PowerScope, and ScopeCorder.
Common-Mode Rejection Ratio (CMRR)
The common-mode rejection ratio (CMRR) is the rejection by the device of unwanted input signals common to both leads of the voltage input. When two input terminals are connected to each other; the reference point is the device ground. Ideally, this should have no influence on the measurement result, but, in fact, leakage causes an interference voltage as a function of the symmetry of the input circuit. In practical terms, the noise voltage superimposed on the signal to be measured leads to measurement errors. It is important for the customer to consider this error in uncertainty calculations. Common-mode noise is especially present in inverter style applications because of the presence of high voltage potentials with high-frequency components to ground. Yokogawa specifies CMRR for power analyzers. The figure can be used while calculating uncertainties.
For instruments with poor CMRR, a differential probe can be used. With any probe, an additional network is placed in line between the device under test (DUT) and the measurement instrument. The resulting resistive and capacitive loading and potential bandwidth limiting introduce sources of uncertainty.
Banner Specifications
Are you interested in learning more about Industrial Power Quality Analyzer Manufacturing? Contact us today to secure an expert consultation!
Banner specifications are important to understand how well your test instrument can meet the basic setup requirements needed on a test stand. The most important specifications to note are bandwidth and input channels.
Bandwidth
Bandwidth is defined as the frequency at which the amplitude of the observed signal drops by -3dB, as shown in figure 6.
Figure 6. Bandwidth is defined as the frequency at which amplitude of the observed signal drops by -3dB.
Using bandwidth to select the proper instrument can be tricky because the recommendations of guidelines can depend on the signal under test, measurements being made, and other criteria.
Here are a few considerations:
Nyquist rate: In general, half of the sampling rate will define the maximum frequency signal that can be properly digitized without aliasing. For example, an instrument with 10MS/s should not be rated with a bandwidth higher than 5 MHz for time domain applications.
5x Rule: For digital signals, oscilloscopes should have enough bandwidth to capture up to the 5th harmonic to adequately show proper waveform details. Poor results lead to a slower rise time / slew rate, filtering out of high frequency details, and possibly distorted amplitude.
Rise time: If a measurement goal is to properly digitize a fast rise time, the equation of Bandwidth= (rise time)/k where the constant k is generally 0.35 for oscilloscopes with bandwidths lower than 1 GHz.
Harmonics: Most power applications require a THD measurement or the acquisition of n number of harmonic orders. The proper method to determine adequate bandwidth is to multiple the fundamental frequency of the signal times the harmonic order. For example, if a PWM signal has a fundamental frequency of 60 Hz and 100 orders of harmonics are required, then the bandwidth should be > than 60 * 100 = 6 kHz.
Analog / Digital Channels
Input channel types are important to understand when selecting the best instrument. A combination of analog and digital channels is often required. For example, a motor test can require measurements of 3 phase AC channels, 1 channel for DC, inputs for a motor resolver, and digital channels for CANbus.
Analog Channels: Generally the more channels the better, but they come at a cost. Some architectures such as the power analyzer, are modular so more input channels mean more capital cost. Other instruments like an oscilloscope are generally offered in 4 and 8 channel models.
Mixed Signal: For applications requiring the acquisition of both analog and digital channels may prefer the use of a mixed signal oscilloscope to show time correlated waveforms.
Motor Input Channels: Some instruments measure more than just electrical parameters. The motor input channels enable measurements of rotational speed and direction, synchronous speed, slip, torque, mechanical power, electrical angle and motor efficiency from an analog or pulse output of torque sensors or pulse outputs of rotation sensors.
Is there one instrument that will satisfy all my measurements needs?
The answer depends entirely upon the needs of the application across its development stages. With oscilloscopes focusing on waveform analysis, power analyzers on accuracy, and hybrids extending that to time domain measurements or flexible data acquisition, adopting a single instrument may call for a compromise on capabilities. That could be easier in some industries or applications than others. If more than one instrument is required, an integrated software experience to simplify measurement collection correlation and storage is a very important consideration.
Conclusion
On today&#;s market is a variety of instruments that can potentially meet power measurement requirements. Depending on the circumstances, one may need the waveform analysis of an oscilloscope, the high accuracy of a power analyzer, or a hybrid combination of the two with flexible data acquisition added into the mix.
Next Steps
For more information on Yokogawa Power Analyzers and related software, click here.
For more information on Yokogawa Power Scopes and related software, click here.
For more information on Yokogawa Oscilloscopes and related software, click here.
For more information on Yokogawa ScopeCorders and related software, click here.
For more information on Yokogawa data acquisition products and related software, click here.
Review our library of relevant content to further your education.
Still not sure which instrument(s) is best for your measurement needs, speak with a precision maker by submitting an inquiry here.
You would be wise to understand the standards and the requirements needed for the various applications you have for power quality analysis in order to choose the right instrument for the task.
In review, the requirements and standards for power quality analysis have advanced significantly particularly in the area of accuracy and repeatability of measurement. New features such as rapid voltage change monitoring and line carrier signal monitoring have added additional ability to analyze power quality issues along with the traditional voltage, current, harmonic, surge, sag and transient monitoring.
IEC -4-30 defines measurement of the frequencies for line carrier monitoring that are typically below 3kHz.The plot below depicts a line signal capture in the Hertz range. The green area represents the carrier signal and the red line represents the acceptable parameter tolerances. Deviations above the red line indicate areas of potential concern.
Line Carrier Signaling or Main Signaling Voltage (MSV) is a function that tracks non-harmonic bursts of remote-control signals that ride on the fundamental frequency. They are typically used in remote control of industrial equipment and on automation applications and eliminates the need for running additional wiring.
Class A Power Analyzers can typically withstand some shockwaves up to 12 kV which are sampled every 500 ns. The standard only requires up to 6kV.
Shockwave Usually caused by lightning or inductive loads turning on and off. Shockwaves are huge instantaneous electrical voltage surges. They also propagate in the digital network.
These features are available to make it a lot easier to analyze these conditions along with the more common monitoring of surges and sags and transients. Below is listed a transient capture with the ability to zoom in to the actual offending transient at the push of a button.
Lets take a look at a few of the new parameters outlined in the standard.
Once you have made the decision on the accuracy class you need to comply with the application, you should look at the most frequently required parameters to be measured and does the instrument you are selecting have the ability to measure these parameters.
If you are looking to purchase a power quality analyzer today, you should strongly consider either a Class A or Class S device depending on your accuracy needs. There are also cost considerations you need to review in the process. If you think of power quality analyzers as good (Class B), better (Class S) and best (Class A), the costs increase accordingly with pricing ranging from the $ to $12,000 range.
The latest edition of IEC -4-30 Edition 3 published in no longer lists Class B in the standard as a performance class.
The Class B performance class was included in the second edition of IEC -4-30. It facilitated making many older power analyzers still viable at the time the standard was issued and allows their measurements to still be useful. Class B compliant power analyzers offer accuracies that are typically used for power surveys, trouble shooting and other applications where a high level of accuracy is not needed. The inaccuracies associated with Class B Power Quality Analyzers for the typical measurements of voltage, ficker, current, harmonics and frequency are defined and listed by individual manufacturers.
Class S also defines the measurement time accuracy to be better than +/- 0.3 sec/day for parameters such as voltage, current and harmonics. It has less stringent defined measurement methods and tolerances for Line Voltage, Flicker and Harmonic measurement methods and uncertainties in accordance with IEC -4-7 Class I. The accuracy and performance requirements for Class S are not as rigorous as Class A.
Power Analyzers that meet Class A performance requirements of the IEC -4-30 standard, will repeatedly achieve the same measurement responses when connected to the same signals. Class A devices are required to meet the highest performance and accuracy requirements in the standard.
Class A identifies the measurement time accuracy to be better than +/- 0.3 sec/day for voltage, current, harmonics and other measurements. With specific procedures and understanding for uncertainties. This time accuracy requires that the instrument incorporates a Global Positioning System (GPS) which is a network of satellites and receiving devices used to determine the location of something on Earth.
One of the first, and most important choices, you need to make is the accuracy required for your ongoing applications for power quality monitoring. To do this you will need to review and understand the IEC -4-30 Standards classification on time accuracy. There are three classes of accuracy to consider. These are Class A, Class S and an older standard, Class B. A brief review of each of these accuracy conditions spelled out in the standard is presented here.
Lets first look at the international standard IEC -4-30. It describes the measurement requirements, and the way power quality analyzers should make measurements. The standard defines the specific measurement procedures and methods for measuring power quality parameters such as Line Voltage, Voltage Fluctuations, referred to as Flicker, Voltage Unbalance as well as Harmonics associated with Voltage and Current.
TODAY THERE ARE MANY CHOICES AVAILABLE when selecting an instrument for power quality analysis. The features and functions vary tremendously across offerings by the manufacturers of these products.
This application note is republished courtesy of Amprobe.
Regardless of popular belief, Power Quality is NOT difficult to understand. The problem is finding reliable sources of such information, provided in an easy-to-understand way with real-world examples. Many of the available sources are either too technical or are not complete. Easy-to-use devices and properly formatted information are crucial ingredients for a successful application. Lets learn more
What do I Need to Know About Power Quality?
Power quality can be generally described as the concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of that equipment. (Source: Heydt, G.T. . Electric power quality. West Lafayette, Indiana: Stars in a Circle Publications.) Well, this says a lot but without giving us any specifics. What we know for sure is that electrical equipment is powered either by AC or DC power sources. In the DC circuit, current flows in one direction. On the other hand, the AC power sources pump current back and forth.
In the DC circuit, the voltage is equal to 120V. But the AC signal, on the right drawing, changes from 0 to 169V, then it drops to 0 and then it falls to negative 169V. And so on. So it is changing with time. But in reality when you are measuring AC voltage at the outlet in the residential building you expect to see 120V. How can it be if the value of the AC voltage is constantly changing with time?
RMS Value of the Signal
To overcome this challenge, the RMS (Root Mean Square) formula is used to calculate the so-called effective value of AC voltage. Dont panic. Amprobe meters are going to make this calculation for you. In the example above, a sinusoidal signal with a voltage peak of 169V will produce an RMS value of 120V. So yes if the receptacle is powered by 120V RMS sine wave, its signal peak voltage is 169V! Keep in mind that the RMS is NOT the Average.
By definition, the RMS Value is equivalent to a DC voltage that would provide the same amount of heat generation in a resistor as the AC voltage would if applied to that same resistor.
And we would be done by now, but manufacturers of the test equipment took a little shortcut in their designs in the past. Here is what happened The easiest and the least expensive way to design an AC meter is to have a circuit, measuring the average or peak value. Now, we know that the average value is not the same as RMS. But the RMS for the pure sine wave, remember ONLY if you have pure undistorted sine wave, can be easily calculated by multiplying the Average reading by 1.11.
VRMS = 1.11 x VAVG (true for pure sinewaves only)
Therefore standard meters, also called Average Sensing Meters, will simply measure the average reading and then multiply it by an appropriate coefficient to display RMS value of the measurement. Easy? So what is the problem?
Distorted Sine Waves
The problem is that the above-mentioned coefficients apply only to pure sine waves. However, a majority of the loads today are non-linear, so pure sine waves are rare.
By definition, the Non-Linear Loads are devices that draw non-sinusoidal currents when a sinusoidal voltage is applied. Frequently these are devices that convert AC to DC.
Instead of analyzing the above definition, I am going to list below some of the typical examples of non-linear loads:
Depending on the distortion of the sine wave, the relation between the average value and the RMS value varies. Do you remember when we said that typical average sensing meters measure average value and then multiply this value by 1.11? This calculation holds ONLY for undistorted sine waves! It DOES NOT apply to distorted sine waves.
If we use a standard average sensing meter on a distorted sine wave, the measuring error can reach over 30% (usually these meters tend to measure lower). So instead of displaying 120V at the receptacle, you may read as low as 90V! This is a huge measuring error!
True RMS Reading (TRMS)
A True RMS (TRMS) meter uses a complex RMS converter to read the true RMS (heating) value for any type of AC waveform. In todays sophisticated world, you can no longer count on pure sine waves. When the exact waveform is unknown, you should always use a TRMS instrument to measure an accurate value.
Sine Wave Distortion Harmonics Distortion
Besides deceiving an average sensing meter, distortion of the voltage or current sine waves may cause other problems, such as:
In order to troubleshoot a system, we somehow need to connect a cause (the type of distortion) with the symptom (the resulting problem). For example, certain distortions cause transformers to overheat, others may reduce a power factor. So we should try to link the shape of the distorted sine wave, to the specific symptoms. But there is an infinite number of different shapes of distorted waves, so how can we make a reference to each of them?
To solve this puzzle we are going to use some help from the music industry! In the 19th century, German scientist Hermann Helmholtz, was racking his brain to answer a theoretically very easy question: why do two different instruments playing the same note sound different?
Why do flutes and violins sound different? If the sound is a vibration of air molecules, and by playing the same note these instruments vibrate air molecules with the exact same frequency, why do they sound different? He discovered that musical instruments produce an entire array of sounds (many frequencies). But all of these sounds (frequencies) are multiples of the fundamental frequency. So if we use 60Hz as fundamental frequency, instruments will generate second harmonic (60Hz 2 = 120 Hz), third harmonic (60Hz 3 = 180Hz) and so on
The combination of individual harmonics for each instrument varies, and that is a reason instruments playing the same note sound different.
Also in the 19th century, French mathematician, Jean Baptiste Joseph Fourier proved, in essence, any waveform could be decomposed or separated into sinusoids (sine waves) of different frequencies (harmonics). All these sine waves would sum up to the original waveform. The Fourier Transform identifies or distinguishes the different frequency sinusoids and their respective amplitudes.
In other words, no matter how distorted a sine wave is, it can be broken down into a fundamental frequency and individual harmonics. If you perform harmonic analysis with a power quality instrument, the distorted sine wave will be broken down into individual harmonics. Individual harmonics can then be linked to specific malfunctions in an electrical system. The harmonic analysis is performed separately for voltage and current, and for each phase of the three-phase system.
As a result of harmonic analysis, the instrument should measure and display the following information:
THD This is Total Harmonic Distortion, a cumulative number, which represents the percentage of all harmonics in the signal. The more harmonics, the more distortion and more trouble you can expect.
DC This is the DC content of the signal. Generally the more DC content, the more problems can be expected.
Individual Harmonics Knowing the percentage and/or amplitude of each harmonic, you can troubleshoot the system and prevent potentially damaging situations.
How to Diagnose a Harmonic Distortion Problem
When maintaining or troubleshooting electrical systems, you should always perform harmonic analysis. The IEEE Standard 519- IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems has suggested limits on the number of harmonics that a customer or facility should produce. Below are some of the highlights:
Harmonics Distortion Example
Lets move to some real-world examples. In this three-phase four-wire system (WYE), each phase conducts a current of 100A. The system is balanced, so the neutral current should be zero, (three phase currents should cancel each other). You measure neutral and it reads 120A. Is this possible?
Well, when we performed harmonic analysis on this system, we noticed that the 3rd harmonic (60Hz 3 = 180Hz) on each leg contributed 40 Amps RMS. After research, we learned that in the 3 phase, 4 wire system, the 3rd harmonics add up on the neutral wire instead of canceling each other because they are in phase (see graphs below).
In our example, the currents are going to add up to 120A (40A + 40A + 40A). Always rely on a Power Quality or TRMS meter to measure neutral current, since it will read the combination of harmonics and system imbalance.
The odd multiples of the third harmonic (called TRIPLENS) are added together in the neutral and can cause overheating even with balanced loads.
Solution
There are many companies producing specialized equipment to solve harmonic distortion problems. Contact them to find the best solution for the problem you are experiencing. One of the solutions for the problem we indicated above would be a filtering system, to mitigate a third harmonic.
All Faces of Power
Electric power makes the bulbs light, motors turn and your computer run. Electric power does many jobs. So what is it? What is the difference between Apparent, Reactive and Active Powers? What is a Power Factor? What are Watts, kilowatts, VARS, VA? Let us take a closer look
Active or working power is power, which performs useful work. This is the type of power we want to see. This power is measured in Watts [W].
Reactive Power is simply power we are losing to produce the electromagnetic field in motor windings or to charge capacitors. It is wasted power, so we want to reduce it as much as possible! Reactive power is measured in Volt-Amps-Reactive [VAR].
Apparent Power is a total (vector sum) of Active and Reactive powers. In other words, this is the total power that needs to be provided to cover useful, working power as well as wasted power. It is measured in VoltAmps [VA].
Peak Demand If a power plant is not able to supply enough energy using their regular resources; they need to produce or deliver additional energy on demand. The cost of such energy is higher. The cost is calculated based on the maximum value of average power measured in 15 or 30 minutes intervals with a rate of 1 second, throughout 30 days time. The Demand is measured in Watts [W] or Volt-Amps [VA] depending on the energy provider.
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Power Factor is a measure of how efficient a system is.
Power factor is measured from 0 to 1.
The lowest efficiency is 0 (0%) meaning we lose all energy, and the highest is 1 (100% efficient system), meaning we use all produced energy.
A power factor of 0.5 describes a system that is 50% efficient.
A power factor of 0.95 describes a system that is 95% efficient. And so on. The bottom line is: the higher the power factor, the better!
Displacement Power Factor
Lets see how we can make electrical systems more efficient and save on energy bills. To do so we need to learn how different types of loads affect our Power Factors.
The Resistive Loads
The resistive loads do not create a phase-shift. The voltage and current in above drawing are in-phase, meaning that they cross the X (time) axis in the same places. Such a system is very efficient, with a Displacement Power Factor of 1 (100% efficient) since there is no Reactive power present.
The inductive loads cause current to lag the voltage. Motors are good examples of such loads. Motors create Reactive power, which affects the power factor in a negative way. The more motors you connect to the system the more energy you are losing. Due to this very reason, the Displacement Power Factor can be as low as 0.6 (or even lower), meaning that only 60% of supplied energy is used and almost 40% is wasted! That also means that somebody is paying for this 40% wasted energy. Wouldnt it be nice to save this money?
Here is the good news: Capacitive loads cause the current to lead the voltage.
Compare the above drawings. Did you notice that capacitance (capacitor) shifts current in an opposite direction to inductance (motor coil)?
Displacement Power Factor (DPF) Correction
This means that the inductive lagging DPF produced by motor windings can be balanced with leading capacitance. So if we add capacitors to these systems, we will correct (shift) the DPF and bring it closer to 1 (100% effective system). This will reduce the electric bill. This technique is known as Power Factor correction.
TRUE Power Factor
True Power Factor consists of DPF and harmonic distortion. The harmonic distortion component is more difficult to correct since it requires removing harmonics from the system. This process usually requires specialized equipment like harmonic filters and K-rated transformers.
Energy
Energy is power used over time. Energy, like power, can be active, reactive or apparent.
Active Energy is active power consumed over time. It is measured in Watt-hours [Wh]
Reactive Energy is reactive power consumed over time. It is measured in Volt-Amp-Reactive hours [VARh].
Apparent Energy is apparent power consumed over time. It is measured in Volt-Amp hours [VAh].
Mystery of Prefixes: Prefixes can be used with any physical value: Volts, Amps, Watts, VAR, etc In test and measurement they are used to simplify readings. For example, instead of saying 0.A , I can say 2µA [micro Amps]. Instead of displaying 4,000,000,000Ω, we can display 4GΩ.
The table below explains the most often used prefixes. In order to convert units, simply multiply reading by ratio (see examples in the table):
You probably have heard about kWatts [kilowatts] or kWh [kilowatt hours]. 1kW is W [watts] and 1kWh is Wh [watt hours].
A Amps or Amperage
AC Abbreviation for Alternating Current
COM Reference voltage input. On a three-phase four-wire system it is connected to a neutral wire.
dB Decibels Used in communication to indicate Volume. It is not a linear relationship, i.e. as dB increases the apparent volume increases more quickly.
dBm Measure of absolute power. Used in communication work. Zero dBm equals one milliwatt.
DC Abbreviation for Direct Current
DPF Displacement power factor
Hz Hertz or frequency Formerly called cycles, as in 60 cycles. Frequency is expressed as the number of cycles per second, usually 60 Hz in the U.S., 50 Hz in Europe.
I Electrical current measured in Amps. I1, I2, I3 or (IA, IB, IC) respectively indicate current in phases 1, 2, and 3. IN indicated current in a neutral wire.
kW 1,000 wafts the common measure of Power.
kWh kilowatt Hour 1,000 watt hours. The measure of energy used.
P Active or Working Power
PF Power Factor
Q Reactive Power
S Apparent Power
TPF True Power Factor
V Volts or Voltage, V1, V2, V3 (or VA, VB, VC) respectively indicate phases 1, 2, and 3.
Some Other Electrical Terms:
Auto Ranging Instrument automatically selects the best range for measurement being made.
Capacitance A capacitor is an electronic component that temporarily stores energy and resists a sudden change in circuit voltage. Its value (capacitance) is usually stated in microfarads.
Crest Peak or topmost part of a waveform from the zero reference.
Crest Factor Ratio of the peak to TRMS value of a waveform.
Continuity An electrical test to determine if a continuous path for the flow of current is established.
Counts The maximum number (counts) a digital instrument can display (a display is counts).
Diode Test Instrument can test the forward and reverse conduction conditions of a diode.
Duty Cycle The ratio of Time On vs. Time Off of a machine or waveform.
Harmonic Distortion In the early s, a new efficient power supply called a switch mode power supply began showing up in electronic products. This supply converts the incoming sine wave to a distorted waveform. If a device converts AC to DC or vice versa (via a switch mode power supply) as part of its routine operation, it is a harmonic generating device. Such devices include copiers, computers, UPS systems and many more common everyday devices.
Harmonics Voltages or currents that are at a multiple of the fundamental frequency. In the US, the fundamental frequency is 60 Hz.; therefore, harmonics can occur at 120 Hz, 180 Hz, 240 Hz and so on. Harmonics are usually specified by their numeric order. For example, the third harmonic is 180 Hz (60×3).
Hold The ability of a digital instrument to retain (Hold) its Last Reading after connections to the circuit are removed.
Inductance An inductor is an electrical component that resists a sudden change in electric current. Its value (inductance) is typically stated in millihenries.
Input Impedance The loading (impedance), in ohms, an instrument presents to the circuit being tested. (This is typically 10 Meg-ohms for digital multimeters).
Peak The absolute maximum (crest) excursion of a measured waveform from the zero reference.
Polarity The ability of a digital instrument to display polarity to determine the direction of current flow.
Power Factor Ratio of Actual Power to the Apparent Power.
Range A measurement span (i.e.: 0-200 is range of measurement).
RS-232 An EIA-defined standard permitting serial communications between computer terminals, modems, instruments, etc.
Sag (Valley) A sudden DECREASE in the amplitude of the nominal electrical value. This disturbance lasts for 1/2 cycle or longer.
Spike (Noise) An abrupt transient of less than one millisecond which comprises parts of a waveform and may considerably exceed its average amplitude.
Surge (Swell) A sudden INCREASE in the amplitude of the nominal electrical value. This disturbance lasts for 1/2 cycle or longer.
True RMS Indicates an instruments ability to measure a waveform other than sinusoidal. A true RMS instrument will display the correct voltage or amperage despite a distorted waveform.
Vars (Q) Volt-amp reactive (wasted power).
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