what kind of meters you use to test power lines

If a product uses ability, then ability consumption and ability quality measurements must be a function of product design and test. These measurements are essential to optimize product design, comply with standards, and provide nameplate information to customers. Only how do you measure electricity?

This article will hash out best practices to measure out electrical power, starting with power measurement basics and proceeding to the types of instruments and associated components typically used to make measurements. The article will conclude with existent-world examples, which apply the information imparted earlier in the article to solve applied measurement problems.

Although most of u.s.a. accept been exposed to basic power measurement equations, a primer is helpful to summarize this information and to show how information technology applies to product design and test.

Power Measurement Basics

How is electrical current measured?

DC power measurement is relatively elementary as the equation is simply watts = volts x amps. For Air-conditioning electrical power measurement, the power cistron (PF) introduces complication as watts = volts x amps x PF. This measurement of Air-conditioning power is referred to equally active power, true ability, or real power. In Ac systems, multiplying volts 10 amps = volt-amps, likewise called apparent power.

Power consumption is measured by calculating information technology over time, using at least 1 complete cycle. Using digitizing techniques, the instantaneous voltage is multiplied by the instantaneous current then accumulated and integrated over a specific time period to provide a measurement of electrical current. This method provides a truthful power measurement and true RMS measurements for any waveform, sine or distorted, including harmonic content upwardly to the bandwidth of the instrument.

Single-stage and 3-phase Power Measurement of Electrical Ability

The Blondel Transformation states that total power is measured with one less wattmeter than the number of wires in the system. Therefore, a single-phase, ii-wire system will require one wattmeter, a unmarried-phase, three-wire system will require two wattmeters (Figure 1), a 3-phase, three-wire system will require two wattmeters, and a iii-phase, four-wire organization will crave three watt meters.

What Device Measures Electric current?

Figure one. The 2-wattmeter method can measure out power through direct connections to a 3P3W organisation. Pt = P1 + P2

In this context, a wattmeter is a device used to measure out current via one current and one voltage input. Many Ability Analyzers and DSOs have multiple current/voltage input pairs capable of measuring watts, in effect acting as multiple wattmeters inside a unmarried instrument. Thus, information technology's possible to mensurate iii-stage 4-wire power with one correctly specified Ability Analyzer.

With a unmarried-phase, two-wire system (Figure 2), the voltage and current detected past the wattmeter are equal to the full power dissipated by the load. The voltage is measured between the two wires, and the current is measured in the wire supplying ability to the load, often called the hot wire. Voltage can typically exist measured directly by a Power Analyzer upwards to 1000 V RMS. Higher voltages volition require the use of a VT (Voltage Transformer) on an Air-conditioning organisation to step downwardly the voltage to a level that can be measured by the instrument. Currents can typically exist measured directly by a Ability Analyzer up to fifty A, depending on the instrument. Higher currents will crave the use of a CT (Current Transformer) on an AC system. There are different types of CTs. Some are placed directly in-line. Others have a window which the electric current-carrying cablevision passes through. The third kind is a clench-on type. For DC currents, a shunt is typically used. The shunt is placed in line and a low level millivolt indicate is measured by the instrument.

Effigy two. A unmarried-phase, ii-wire arrangement uses a current transformer and voltage transformer.

With a single-stage, three-wire arrangement (Figure 3), the total power is the algebraic sum of the ii wattmeter readings. Each wattmeter is continued from ane of the hot wires to the neutral, and current is measured in each hot wire. Total Power is calculated as Pt = P1 + P2.

Figure 3. Ii wattmeters connect to a single-phase, three-wire system (1P3W).

With a three-phase, four-wire system (Figure four), the 3 wattmeters each measure voltage from a hot wire to the neutral, and each wattmeter measures current in 1 of three hot wires. The total power for the 3 phases is the algebraic sum of the three wattmeter measurements, as each meter is in essence measuring a single phase of the iii-phase system. Pt = P1 + P2 + P3

Figure four. This iii-phase, 4-wires organisation uses three wattmeters.

With a 3-phase, three-wires system (Figure v), the two wattmeters mensurate phase current in any 2 of 3 wires. Each wattmeter measures a line-to-line voltage between two of the three power supply lines. In this configuration, the full ability, watts is accurately measured by the algebraic sum of the 2 wattmeter values. Pt = P1 + P2. This holds true if the organisation is balanced or unbalanced.

If the load is unbalanced, meaning that the phase currents are different, total power will be correct, but the full VA and ability cistron could exist in error. Ability analyzers, though, may accept a special 3V3A wiring configuration to ensure authentic measurements on 3-phase, three-wire systems with a counterbalanced or unbalanced load. This method uses three watt meters to monitor all three phases. One wattmeter measures voltage between the R and T phases, a 2nd wattmeter measures voltage between the S and T phases and a 3rd wattmeter measures voltage between the R and S phases. The phase currents are measured past each wattmeter. The two-wattmeter method is yet used to calculate total power. Pt = P1 + P2. The total VA, still, is calculated every bit (√3/3)(VA1 + VA2 + VA3). All iii voltages and currents are used to make accurate measurement and calculations on the unbalanced load.

Figure 5. A three-phase, three-wire system uses the three-wattmeter method to achieve accurate measurements on an unbalanced load.

Power Factor Measurement

Power factor must oft be measured, and this value should exist kept as shut equally possible to unity (1.0).

In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same corporeality of useful power transferred. The college currents increase the energy lost in the distribution system and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electric utilities will usually charge a higher cost to industrial or commercial customers exhibiting a low power factor.

Effigy 6 shows current lagging voltage by 44.77°, which gave a power factor of 0.70995. The credible power, S1, was 120.223 VA. The true power, or existent power, P1, notwithstanding, was simply 85.352 W.

Effigy 6. A ability analyzer screen shows a stage difference between voltage and current.

If power-consuming devices have practiced power factors, so the unabridged power arrangement will too, and vice versa. When power factors drop, ability-cistron-correction devices must often exist used, at considerable expense. These devices are typically capacitors considering the majority of most power-consumption loads are anterior.

The electric current lags the voltage in an inductor; this is known as a lagging power factor. The current leads the voltage in a capacitor; this is known as a leading power factor. An AC motor is an example of an inductive load, and a compact fluorescent lamp is an case of a capacitive load.

To determine full power factor on a 3-phase 4-wire system, three wattmeters are required. Each meter measures watts, and measurements are also made of the volts and amps. The ability gene is then calculated by dividing the full watts from each meter by the total volt-amps.

With a three-stage, three-wire system, power factor should be measured using the 3 wattmeter method instead of the two wattmeter method if the load is unbalanced, that is, if the phase currents are unlike. Because the ii wattmeter method merely makes two amp measurements, any differences in the amp reading on the third phase will crusade inaccuracies.

Household Apparatus Power Measurement

A typical application for ability measurement is standby power for household appliances that are based on Energy Star or IEC62301 standards. Both standards ascertain the required ability accurateness, resolution and other ability measurement parameters, such as harmonics. Within the IEC62301 standard, at that place are an additional 25 standards that define the specific test parameters for diverse appliances. For case, IEC60436 defines the methods for measuring the performance of electric dishwashers.

Standby way is defined equally the lowest power consumption mode that cannot be turned off by the user and that may persist for an indefinite time when an awarding is connected to the principal electricity supply and used in accordance with the manufacturer'southward instructions. Standby ability is the average power in standby manner when measured in accordance with the standard.

There are three main methods to measure out ability consumption for standby ability or other similar applications. If the power value is stable, then the instantaneous instrument reading at any point in time can exist used. If the power value isn't stable, then accept either an average of the instrument readings over time or measure total free energy consumption. Watt-hours tin can be measured over a specified period of fourth dimension and and so divided by that time.

Measuring total-energy consumption and dividing past time yields the most authentic values with both steady and fluctuating power and is the method commonly employed when using our Power Analyzers. Simply measuring total free energy consumption requires a more sophisticated instrument because power must be continuously measured and totalized.

Tools for Ability Measurement

Power is typically measured with a digital ability analyzer or a DSO (digital storage oscilloscope) with power-assay firmware. Near modernistic ability analyzers are entirely electronic and apply digitizers to convert analog signals to digital forms. Higher terminate analyzers utilize digital signal processing techniques to make the calculations required to decide values.

Power-analysis DSOs apply special firmware to make truthful power measurements. They are, however, somewhat limited because they rely on sample data from digitized moving ridge forms. Their current and voltage probes make them well suited for board and component level piece of work where absolute accuracy isn't a must and power frequency is relatively high.

Power analyzers can typically measure up to 50 A RMS direct at voltage levels upwardly to 1000 V RMS, so about products under test can be directly continued. On the other hand, a DSO will require the employ of voltage and electric current probes to make the power measurements.

CTs are rated by the ratio of input to output current, for example 20:5. Other important CT parameters are accuracy and phase shift and frequency range for AC power measurement. VTs are used to step downwardly the actual voltage to a level that can be accommodated by the power measurement instrument. For example, if the product under test is rated at 480 VAC and the instrument is limited to 120 VAC, and then a 4-to-1 VT is required.

A DSO typically doesn't provide the accurateness of a Power Analyzer and can't directly accept loftier current and voltage inputs, but it tin measure power at much college frequencies up to 500 MHz with the appropriate probes. It also provides other advantages over Ability Analyzers in certain applications including dedicated probes for ease of connection, probe phase compensation and upwards to 8 multiple channel inputs.

A typical application for a DSO would exist any type of lath-level measurement, such every bit when designing excursion boards for a switching power supply. Parameters that are typically measured and analyzed with a DSO or a ability analyzer include, only aren't limited to, switching power loss, device ability consumption, switching racket level, harmonics, output power, and stability of output.

When using a DSO, the required equipment will include differential voltage probes and a current probe (Figure 7). The Current Probe is connected around 1 of the main current carrying wires equally shown in the picture. Often the voltages of components are not referenced to a ground level. Therefore a Differential Voltage Probe is required to isolate the DSO ground from the component ground. In addition to a Power Analyzer or a DSO and CTs and PTs as needed, other ancillary components for making power measurements are probes, clamps and wires. Once all needed instruments and components are on paw, the next step is to ascertain exactly what instruments are needed, and how these instruments should be connected to the load.

Figure seven. Use voltage probes and a current probe with an oscilloscope to measure voltage and current.

Power Analyzers are generally the instrument of selection for household apparatus ability measurements and other power measurements with relatively high voltage levels, low frequencies and high accuracy requirements. Even so, for board level measurements, a DSO is usually employed.

Using the information presented in a higher place, the choice and connection of the correct instruments and tools tin be made for various ability measurement applications. Data gathered from these instruments tin can and so be used to optimize designs, comply with standards and provide nameplate information.

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