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For many years electric utilities and large industrial plants have reduced electrical current demands by the use of capacitors to increase the power factor on large electrical loads. Volt is the electrical unit of measurement used to express the electrical potential or force which causes current ?ow.
In alternating current systems, both voltage and amperage rise and fall thru cycles such as illustrated schematically in Figures 1 and 2.
The values which we measure for voltage and amperage in a circuit are actually mean values occurring during the cycle.
If the voltage and amperage are in phase, as in Figure3, the power consumed (watts) is equal to the product of the volts times amps. Power factor is de?ned as the ratio of the power consumed doing work (watts) divided by the apparent power (volts-amperes). The following basic formulas govern the relationship of voltage, amperage, and power in electrical circuits. The mathematical relationship between actual power and apparent power is shown by means of a vector diagram, Figure 5. All inductive devices, such as motors, transformers, and solenoid coils require magnetizing current to create the magnetic ?eld necessary for the device to operate. Capacitors have a directly opposite effect to inductive magnetizing current and cause the current to lead the voltage rather than lag.
Regardless of the actual power consumed, the electric distribution system sees volt-amperes. Generator and transformer output is measured in volt-amperes, so the greater the reactive current, the less actual (or usable) power the generator can produce and the transformer can handle. The reasons for this are that power companies must be prepared to satisfy normal transmission line (I2R) losses caused by low power factor and also increase generating capacity to provide apparent power.
The vector power diagram provides a convenient means of mathematically calculating power factor correction.
The power factor by de?nition is actual power divided by apparent power, and is equal to .80.
To determine the reactive power, it is necessary to calculate a leg of the power factor triangle.
Figure 7 (see following page) shows the motor performance curves for a typical three-phase induction motor.
Note that even with no load, and no power consumption, the motor continues to draw magnetizing current. It is always possible to correct a motor to unity power factor, but total correction is normally not recommended. Convenient Tables of power factor correction factors have been calculated to avoid the necessity for a laborious calculation for each application. The original compressor power factor can easily be calculated from the compressor speci?cation sheet.
It is possible to calculate the size of capacitors that must be used for power factor correction. For convenience, the Kc factor has been calculated and is shown in Table 2 (following page) for various power factor corrections.
For single-phase loads, the capacitor must be connected across the line ahead of the motor. The following examples illustrate the calculations necessary to select capacitors for power factor correction. Determine the capacitance required to correct the power factor to 85% for a small single-phase motor operating with a 72% power factor, drawing 865 watts and 10.44 amperes on 115 volt, single-phase, 60 hertz power. Determine the capacitance required to correct the power factor to 90% for a three-phase motor operating with a 77% power factor, drawing 7440 watts and 14.7 amperes on 380 volt, three-phase, 50 hertz power.

It is possible to make a direct calculation of the relationship between a correction in kilovars and the capacitors necessary to create the kilovar correction. In the three-phase example above, the kilovar factor KK, from Table 1, for a correction from 77% to 90% would be .345.
A large bank of capacitors installed on the line side for correction of a group of compressors carries the danger of over-correction should some of the compressors be cycled off.
The simplest and most effective installation is to connect the correction capacitors to the load side of the contactor, so that the capacitor correction and the motor are switched simultaneously in and out of the circuit. For example, in Figure 9 three 15KVAR capacitors are connected in wye, the result would be a total KVAR of 15.
The most critical application of capacitors for power factor correction is on motors using two contactors (part winding start), multi speed, or perhaps dual voltage.
A part winding start connection as shown below is basically a transformer and when capacitors are connected to each set of windings, a circuit is completed and circulating currents begin to ?ow when power is applied. A special contactor arrangement should be considered to disconnect the capacitors from the de-energized winding during start or bring on the required KVAR after the compressor is operating. Remember: power factor correction does not decrease the power consumed by the motor, and on small compressors, power factor correction may not be economically justi?able. In electrical plants the loads draw from the network electric power (active) as power supply source (e.g. To get this, it is often necessary that the load exchanges with the network (with net null consumption) the reactive energy, mainly of inductive type. This energy, even if not immediately converted into other forms, contributes to increase the total power flowing through in the electrical network, from the generators, all along the conductors, to the users. The power factor correction obtained by using capacitor banks to generate locally the reactive energy necessary for the transfer of electrical useful power, allows a better and more rational technical-economical management of the plants. Moreover, the present spreading of direct current users, such as electronic circuits and electric drives, involve the generation of current harmonics which are injected into the network, with the consequent pollution and distortion of the waveforms on other connected loads.
Therefore, the use of harmonic filters, both of passive as well as of active type, contributes to improve the overall powerquality of the network, carrying out also power factor correction at the network frequency, when such filters are properly sized.
The reactive component IQ, in quadrature to the voltage, is used to generate the flow necessary for the conversion of powers through the electric or magnetic field and it is index of the transfer of energy between supply and load. In the most common case, in the presence of ohmicinductive type loads, the total current I lags with respect to the active component IR. Electrical Corner StoreCheck out our new #Electrical Corner Store with selected Electrician's Books, Testing and Safety equipment and other electrician's stuff!
Science at GlanceOur mission is to be the leading provider of scientific information in the field of power and engineering in general.
DIYTrade accepts no responsibility whatsoever in respect of such content.To report fraudulent or illegal content, please click here. With the growing emphasis on the need to conserve electrical energy, there is increasing interest in power factor correction for three-phase motors, even on small installations. Fortunately, the practical application of electricity involves exact scienti?c relationships that follow precise physical laws, so the application engineer needs be concerned only with basic formulas and relationships. In direct current circuits, since there is no reversal of voltage or current, the power factor in effect is always unity. This magnetizing current, or reactive current as it is termed, does not produce usable power, but the effect of the magnetic ?eld is to cause the current drawn from the power line to lag the voltage.
As a result, capacitors installed in circuits with low power factors tend to cancel the effects of the reactive current and increase the power factor.
The presence of reactive current means the power supply lines must carry more current than that actually consumed by the load, and this additional current causes greater line losses, more voltage drop, and imposes a greater load on generators, transformers and distribution lines. The combined effects of low power factor greatly increase the power company’s cost for capital equipment, so power companies frequently charge penalties for low power factors.

As energy production costs rise and energy conservation becomes more important, it is probable that electrical speci?cations will increasingly call for power factor correction. As you will recall, the square of the hypotenuse (side opposite the right angle) is equal to the sum of the squares of the other two sides.
The only scales shown are for power factor and motor torque, but the remainder of the curves are shown for reference. Since this reactive magnetizing current is relatively constant, the power factor declines rapidly as the motor loading is reduced.
The in?uence of other reactive forces on the power line, such as changing motor or transformer load, is unpredictable, and if the power factor is over-corrected, it can cause high currents, high magnetic side pull forces on the motor rotor, high voltage, and transient motor over-torque much greater than full load motor torque. Table 1 (following pages) gives multipliers to be used to determine the capacitor kilovars required.
For any given application, the kilovars required for power factor correction are determined by the operating condition selected as a basis for correction and the amount of correction desired. While the actual electrical components used to obtain the power factor corrections are capacitors, manufacturers sell kits consisting of pre-wired capacitors in assemblies in terms of kilovars. For three-phase loads, the value determined is for a single capacitor, three being required, connected line to line. No complicated engineering studies are needed, and the compressor is fully protected against over-correction.
The decision will depend on the overall economics, availability of units and total KVAR required. Some Copeland brand compressors may use part winding start connections and extreme caution must be used when applying power factor correction. There is a time delay of approximately 1 second between the contactors and the chances that voltages are in phase when the second contactor closes are unlikely. To smooth such negative effect, the power factor correction of the electrical plants is carried out. Without this, there could be no net transfer of power, for example, thanks to the magnetic coupling in the core of a transformer or in the air gap of a motor. Every day, the generation and usage of energy produces more pollution than any other single industry.
In alternating current circuits with lagging current (caused by inductive loads), the actual power available for work is the product of the volts times amperes times the power factor.
If the current is in phase with the voltage, then the apparent power is equal to the actual power, and the power factor is 1.0. The term reactive power is used to describe the product of the reactive current and the operating voltage, and is measured by line BC. Whether over correction will cause motor damage is uncertain, but there is evidence that motor life can be shortened by voltage spikes caused by over-correction. The multiplier (or KK) to be used is found by locating the original power factor in the left hand column, and then reading the required value at the intersection of the original power factor row, and the desired corrected power factor. Power factor correction should be made only if absolutely necessary on two contactor or part-winding start compressor applications, and care must be taken to avoid overcompensation under all operating conditions. The greater the reactive current, in proportion to the useful current, the greater the reactive power and the lower the power factor. Capacitors should be oil ?lled, and the capacitor rated voltage should be in excess of maximum line voltage. In other words, When Power Factor increases, Current Decreases, and when Power Factor decreases, Current Increases.

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