A power meter is used to measure electrical power, the product of voltage and current. In order to record both parameters, those devices are equipped with analogue and digital measurement techniques. Often they varies in the count of possible recordable channels.
More specialized power meters, which have extremely high accuracy and a large number of additional analysis functions (e.g. FFT), are also called power analyzers. These are used to determine the degree of efficiency in the development of electrical drive components such as electric motors or power inverters.
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Accuracy specifications of power meters are highly complex and usually comprise several pages of the product documentation. The accuracy depends on the frequency, the power factor and the modulation of the ranges.
It is common to state the accuracy of the power at a value of 50Hz. An accuracy of <0.05% is one of the values that good power analyzers achieve there. However, this is only one of many parameters that describe the accuracy of a device. There is also no standardization. Which information is given depends on the manufacturer. These indicate their own boundary conditions. This is why the reputation of a brand is particularly important, as customers have to be able to rely on the manufacturer's information. A high level of accuracy is important because individual changes to the test item to be examined are often only marginally noticeable, but can have an enormous effect on the overall efficiency in case they sum up. In addition, technical modifications to the product are only made step by step, so the influence on the overall result can be precisely documented and controlled. A clean and comprehensible documentation is essential for later processing, because future developers will be able to build on these experiences and use them in their own work.
The most important characteristic of a power analyzer is the acquisition of current and voltage. The speed at which the data is acquired and processed depends on the dynamics of the application. For this it is particularly important that power analyzers record this extremely accurately and reproducibly even at low power factors. The system must be able to perceive the smallest changes to the device under test (DUT). An increase in efficiency of only 0.5% can, for example, lead to enormous competitive advantages in the development of electric motors. The system should also be error-resistant. If the operation is too complex, incorrect settings may occur which falsify the result of the measurement. The operator effect and user experience is often underestimated. As a consequence incorrect measurements may occur.
Concluding, a power analyzer should have the following characteristics:
After longterm use, aging and switching cycles (but also thermal cycles) can lead to drifting of the components. It is the task of quality assurance to check that the required levels of accuracy are maintained. If drifting occurred, this may mean a change in the measured parameters. Mostly this happens in the lower ppm range, but even these small deviations add up quickly and can lead to incorrect results. In order to ensure the most accurate measurement possible, it is therefore essential that devices have to be calibrated within fixed intervals. As a rule, it is not necessary to readjust the devices.
The intervals for calibration depend on the device and field of application. A period of 2 years is possible with a very stable measurement equipment. Others should be recalibrated after a year. Calibration intervals should therefore always be adapted to the respective application.
When calibrating the measuring devices, a distinction is made between DAkkS and ISO calibration. ISO calibrations are carried out in accordance with DIN EN ISO 9001, whereas DAkkS calibration is carried out by a accrediated laboratory which is monitored by an accreditation body. This accreditation guarantees that the calibration is carried out according to the strictest rules and a validated procedure. The DAkkS calibration certificates created are recognized worldwide. Another advantage of the DAkkS calibration certificate is the formula it contains for determining the measurement uncertainty. With their help, the user of the measuring device can correctly evaluate the readings.
When measuring power, a distinction is made between electrical and mechanical power.
are considered to be electrical power values.
An input for torque and speed is required for the calculation of mechanical power. The actual sensing of these values takes place within a torque measuring shaft.
Current and voltage are measured directly. After the analog quantification, the signals are digitized. These values are then multiplied and averaged in real time. This is how the real power is calculated. The digital availability of the measured values also enables to derive other variables. Furthermore these data also offer the option of filtering.
The basic measuring method does not depend on AC or DC. With AC, the entire signal period must be taken into account. With a sine, for example, the power also looks like a sine. However, it is essential to average the entire period, otherwise there will be beats (flickering measured values). That makes an exact reading impossible. With DC signals the power can be read off at any time.
The measured data is transmitted to the PC via Ethernet (LAN), USB or other common communication interfaces. In most cases the device is portable. Some manufacturer offer specialized analyzer for stationary use only, e.g. for EOL verification.
Reactive power: Reactive power does not affect the energy balance of the system. But it still has to be made available. It is a pendulum power that does not affect the energy balance due to the recurring energy flow. It is not entirely necessary to keep the reactive power low, but this is desirable. The greater the reactive power, the stronger the electrical components have to be designed. Resulting in more volume, more weight and more costs.
Real power: Unlike reactive power, real power actually has an effect on the energy balance. It is the electrical power that "actually" matters. It is an effective gain or loss in energy.
Apparent power: Apparent power describes the sum of active and reactive power. The indication of the apparent power does not allow any information about whether there is actually a lot or little "real" power, as it does not indicate the proportion of reactive power. For example, a high apparent power with high reactive power can have a lower real power than a low apparent power with low reactive power.
Measuring / measuring technology: Measurement technology describes the pure process of observing and acquiring the corresponding values. There is no impact on the system.
Control techniques: When controlling, the system is actively intervened (e.g. by switching on a voltage for a certain period of time). However, there is no feedback that provides information about the amount of influence. Control technology without feedback sensors is always used when the behavior of the system is known very precisely and no significant interference is to be expected.
Feedback control techniques Also feedback control techniques actively intervenes in the system. However, here various measured variables are fed back to the regulation in order to be quantify the effects of the intervention. A response can then be made accordingly and the process can be precisely regulated. Feedback control technology is a combination of control and measurement technology.
In the segment of power meters, a distinction must be made according to the application use. For end user applications, "simple" power meters, also called wattmeters, are more interesting. These have a limited range of functions and are also less precise than highly professional devices in terms of their accuracy. The areas of application for wattmeters are particularly in the home for tracking down "energy guzzlers". Watt meters are quite easy to use and are therefore particularly suitable for experienced home users, electricians or energy consultants.
Power analyzers differ from wattmeters in that they are much more accurate, work with a larger range of functions, but are also a lot more complicated. They are used in professional environments for R&D activities and EOL testing. The technicians and engineers who use these devices on test benches and in laboratories rely on the high-precision results of the power analyzers.
Therefore, the measurement data from watt meters and power analyzers cannot be compared. Particularly in professional areas of application, absolute synchronicity and the best possible accuracy are required, which cannot be guaranteed by "simple" wattmeters.