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The threatened limitations of conventional electrical power sources have focused a great deal of attention on power, its application, monitoring and correction. The electric utility's primary goal is to meet the power demand of its customers at all times and under all conditions. But as the electrical demand grows in size and complexity, modifications and additions to existing electric power networks have become increasingly expensive. The measuring and monitoring of electric power have become even more critical because of downtime associated with equipment breakdown and material failures.
Typical Voltage Configurations
Single-Phase Systems
Single-phase residential loads are almost universally supplied through 120/240V, 3-wire, single-phase services. In this system the two "hot" or current carrying conductors are 180 degrees out-of-phase with respect to the neutral.
Three-Phase, 3-Wire Systems
In this type of system, commonly known as the "DELTA" configuration, the voltage between each pair of line wires is the actual transformer voltage. This system is frequently used for power loads in commercial and industrial buildings. In such cases, service to the premises is made at 208V, three-phase. Feeders carry the power to panel supplying branch circuits for motor loads. Lighting loads are usually handled by a separate single-phase service. The 480V distribution is often used in industrial buildings with substantial motor loads.
Three-Phase, 4-Wire Systems
Known as the "WYE" connection, this is the system most commonly used in commercial and industrial buildings. In office or other commercial buildings, the 480V three-phase, 4-wire feeders are carried to each floor, where 480V three-phase is tapped to a power panel or motors. General area fluorescent lighting that uses 277V ballasts is connected between each leg and neutral; 208/120V three-phase, 4-wire circuits are derived from step-down transformers for local lighting and receptacle outlets.
Typical voltage:
phase-to-phase = 208/480V
phase-to-neutral = 120/277V
Balanced vs. Unbalanced Loads
A balanced load is an AC power system using more than two wires, where the current flow is equal in each of the current carrying conductors. Many systems today represent an unbalanced condition due to uneven loading on a particular phase. This often occurs when electrical expansion is affected with little regard to even distribution of loads between phases or several nonlinear loads on the same system.
RMS vs. Average Sensing
The term RMS (root-mean-square) is used in relation to alternating current waveforms and simply means "equivalent" or "effective," referring to the amount of work done by the equivalent value of direct current (DC). RMS measurements provide a more accurate representation of actual current or voltage values. This is very important for nonlinear (distorted) waveforms. With expanding markets of computers, uninterruptible power supplies, and variable speed motor drives, resulting nonlinear waveforms are drastically different. Measuring nonsinusoidal voltage and current waveforms requires a True RMS meter. Conventional meters usually measure the average value of amplitudes of a waveform. Some meters are calibrated to read the equivalent RMS value (.707 x peak); this type calibration is a true representation only when the waveform is a pure sine wave (i.e., no distortion). When distortion occurs, the relationship between average readings and True RMS values changes drastically. Only a meter which measures True RMS values gives accurate readings for a nonsinusoidal waveform. RMS measuring circuits sample the input signal at a high rate of speed. The meter's internal circuitry digitizes and squares each sample, adds it to the previous samples squared, and takes the square root of the total. This is the True RMS value.
Power Factor
Power factor is the ratio of ACTUAL POWER used in a circuit to the APPARENT POWER delivered by a utility. Actual power is expressed in watts (W) or kilowatts (kW); apparent power in voltamperes (VA) or (kVA). Apparent power is calculated simply by multiplying the current by the voltage.
Power Factor = Actual Power = kW/Apparent Power kVA
Certain loads (e.g., inductive type motors) create a phase shift or delay between the current and voltage waveforms. An inductive type load causes the current to lag the voltage by some angle, known as the phase angle. On purely resistive loads, there is no phase difference between the two waveforms; therefore the power factor on such a load will be 0 degrees, or unity.
The following examples of a soldering iron and a single-phase motor illustrate how power factor is consumed in different types of loads. In a soldering iron, the apparent power supplied by the utility is directly converted into heat, or actual power. In this case, the actual power is equal to the apparent power, so that the power factor is equal to "1" or 100 percent (unity).
In the case of a single-phase motor, the actual power is the sum of several components:
- the work performed by the system; such as lifting with a crane, moving air with a fan, or moving material, as with a conveyer.
- heat developed by the power lost in the motor winding resistance
- heat developed in the iron through eddy currents and hysteresis losses
- frictional losses in the motor bearings
- air friction losses in turning the motor rotor.
Reactive Compensation Power
Reactive compensation power refers to the capacitive values required to correct low power factor to as close to unity (1.0) as possible. Most industrial loads are inductive, so the load current lags the line voltage by some degree. In order to bring the value closer to unity, something must be added to the load to draw a leading current. This is done by connecting a capacitor in parallel with the load. Since a capacitor will not dissipate any real power, the charge for real power will be the same.
Many meters today have the ability to accurately work with these non linear signals and display accurate results. A typical clamp-on meter can measure power, power factor as well as volts, amps, watts and VARs.
John Olobri is Director of Sales & Marketing for AEMC Instruments, a leading manufacturer of professional electrical test and measurement instruments for the industrial, commercial and utility marketplace. For more information, visit www.aemc.com
author: By John Olobri, AEMC Instruments