3.1. Benchtop vs. Portable Powermeters
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Picture 1: The CW240
portable type powermeter |
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Picture 2: The WT3000
benchtop type powermeter |
Powermeters can be generally classified into portable and benchtop (rack mount) types. The former take advantage of miniaturization and lightweight design for portability, are equipped with clamp-on probes, and enable on-site measurements of live lines (see Picture 1). Particularly with the promotion of the ISO14000 international standards in recent years relating to energy-saving measures and preservation of the environment, this type of powermeter can be used to take measurements enabling easier power management at factories and offices, as well as for power line quality control.
On the other hand, numerous types of benchtop models are available, ranging from 1-channel input models for single-phase measurements and 2- or 3-channel models for three-phase measurements, on up to 6-channel models that can measure two three-phase systems simultaneously. These types generally allow direct current input which affords higher measurement precision than the portables, perform measurement together with other types of measuring instruments, and are frequently rack-mounted into systems for product evaluation and testing (see Picture 2).
· Powermeter Architecture
Figure 6 illustrates the structure of a typical digital powermeter. The unit consists of a voltage input section, current input section, DSP, CPU, display, and interface.
Input voltage is normalized in the voltage input circuit by a voltage divider and OP amp, then sent to the A/D converter. In the current input circuit, a flow divider forms a closed circuit, and the voltages on either side of the flow divider are amplified and normalized in the OP amp before being routed to the A/D converter. With this system, the current input circuit will not open even if the current range is switched, enabling safe range switching even when electrified as well as remote control via communications. The output from the A/D converters in the voltage and current circuits (the digital data) is insulated with an isolator and sent to the DSP. In the DSP, sampled digital values that have been converted to the second power or instantaneous power are summedadded for measuring the data update rate, and those summed values are divided by the number of samples to determine the measured values of voltage, current, and active power.
There are other methods in addition to the resistive potential voltage input method shown in figure 6, such as the VT (voltage transformer) method. One must select a measuring instrument that has the input format best suited to the circuit under test. For current input, there are the shunt input and CT (current transformer) methods. And especially with portable types, there is the clamp-on probe current input method. With the VT, CT, and clamp-on probe methods, the powermeter itself has no isolators because the input section is insulated from the primary circuit.
Figure 6: Powermeter architecture
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