07-08-2012, 04:28 PM
Digital Power Factor Controller
Digital Power Factor Controller.docx (Size: 1.01 MB / Downloads: 28)
Introduction:
Power factor is the ratio between the KW and the KVA drawn by an electrical load where the KW is the actual load power and the KVA is the apparent load power. It is a measure of how effectively the current is being converted into useful work output and more particularly is a good indicator of the effect of the load current on the efficiency of the supply system.
All current will cause losses in the supply and distribution system. A load with a power factor of 1.0 result in the most efficient loading of the supply and a load with a power factor of 0.5 will result in much higher losses in the supply system.
A poor power factor can be the result of either a significant phase difference between the voltage and current at the load terminals, or it can be due to a high harmonic content or distorted/discontinuous current waveform.
Poor load current phase angle is generally the result of an inductive load such as an induction motor, power transformer, lighting ballasts, welder or induction furnace.
A distorted current waveform can be the result of a rectifier, variable speed drive, switched mode power supply, discharge lighting or other electronic load.
A poor power factor due to an inductive load can be improved by the addition of power factor correction, but, a poor power factor due to a distorted current waveform requires an change in equipment design or expensive harmonic filters to gain an appreciable improvement. Many inverters are quoted as having a power factor of better than 0.95 when in reality, the true power factor is between 0.5 and 0.75. The figure of 0.95 is based on the Cosine of the angle between the voltage and current but does not take into account that the current waveform is discontinuous and therefore contributes to increased losses on the supply.
Abstract:
This project discusses the need for power factor correction and provided a suitable solution that could be used for small-scale industries and establishments.
When voltage and current are in phase with each other in an AC circuit, the electrical energy drawn from the source is fully converted into another form of energy in the load and the power factor (cosine of the angle between voltage and current waveforms) is unity. As the power factor drops, the system becomes less efficient.
A drop from from unity to 0.9 in the power factor results in 15 per cent more current requirement for the same load. A power factor of 0.7 requires approximately 43 per cent more current; and a power factor of 0.5 requires approximately 100 per cent (twice as much ) more current to feed the same load. In small-scale industrial units and establishments, most of the load is in the form of electrical motors and airconditioning units. These loads are inductive in nature, wherein the current lags the applied voltage and their power factor is termed as leading power factor. In contrast, in capacitive loads, the current leads the voltage and their power factor is termed as leading power factor The objective therefore should be to neutralize the lagging power factor of inductive loads by using switching capacitors across the load, which have a leading power factor.
Power Factor Correction:
Capacitive Power Factor correction is applied to circuits which include inductive components as a means of reducing the inductive component of the current and thereby reduce the losses in the supply. There should be no effect on the operation of the motor itself.
An inductive load draws current from the supply that is made up of resistive components and inductive components. The resistive components are:
1) Load current.
2) Loss current.
and the inductive components are:
3) Leakage reactance.
4) Magnetizing current.
The current due to the leakage reactance is dependant on the total current drawn by the motor, but the magnetizing current is independent of the load on the motor. The magnetizing current will typically be between 20% and 60% of the rated full load current of the motor. The magnetizing current is the current that establishes the flux in the iron and is very necessary if the motor is going to operate. The magnetizing current does not actually contribute to the actual work output of the motor. It is the catalyst that allows the motor to work properly. The magnetizing current and the leakage reactance can be considered passenger components of current that will not affect the power drawn by the motor, but will contribute to the power dissipated in the supply and distribution system. Take for example a motor with a current draw of 100 Amps and a power factor of 0.75 The resistive component of the current is 75 Amps and this is what the KWh meter measures. The higher current will result in an increase in the distribution losses of (100 x 100) /(75 x 75) = 1.777 or a 78% increase in the supply losses.
In the interest of reducing the losses in the distribution system, power factor correction is added to neutralize a portion of the magnetizing current of the motor. Typically, the corrected power factor will be 0.92 - 0.95 Some power retailers offer incentives for operating with a power factor of better than 0.9, while others penalize consumers with a poor power factor. There are many ways that this is metered, but the net result is that in order to reduce wasted energy in the distribution system, the consumer will be encouraged to apply power factor correction.
Power factor correction is achieved by the addition of capacitors in parallel with the connected motor circuits and can be applied at the starter, or applied at the switchboard or distribution panel. The resulting capacitive current is leading current and is used to cancel the lagging inductive current flowing from the supply.