Power factor correction circuits

Power factor correction circuits are intended to increase the conduction angle of the rectifiers and to make the ac input current waveform sinusoidal and in phase with the voltage waveform. The input waveforms can be seen in Figure C-2. This means that all the power drawn from the power line is real power and not reactive. The net result is that the peak and RMS current drawn from the line is much lower than that drawn by the capacitive input Alter circuit traditionally used. 

Active power factor correction circuits can take the form of nontransformer isolated switching power supply topologies, such as buck, boost, and buck/boost. The buck topology in Figure C-3 produces an output dc voltage lower than found at its input, whenever the PFC stage is operating (F > Fom). In other... 

C.1 A Universal Input, 180W, Active Power Factor Correction Circuit... 

The resulting schematic of the power factor correction circuit is given in Figure C-9. 

Figure 3.11 Typical power factor correction circuit.

Figure 3.12 Current and voltage for MOSFETs operating in the power factor correction circuit using (a) Si diodes and snubber circuit and (b) SiC diodes only. (From [14]. 2002 Cree, Inc. Reprinted with permission.)...

Singh, R., and J. Richmond, SiC Power Schottky Diodes in Power Factor Correction Circuits, Cree Inc. Application Note, CPWR-ANOl, 2002. 

Within any particular installation, in instances where no financial penalties are incurred from the supply authority the power factor of individual circuits will influence both the losses within the installation and the system s voltage regulation. Therefore, power factor correction is... 

Capacitors as bulk units can be connected to the supply busbar via a fuse switch, molded-case circuit breaker of air circuit breaker. In this type of installation control is purely manual, and in cases of a reasonably constant load and where the amount of power factor correction is limited such a manually controlled system is perfectly adequate. The supply authority may, however, require to be informed that a capacitor bank is permanently connected to the supply. Capacitors are more generally connected either in banks controlled from a VAR sensitive relay or across individual loads (e.g. motors). 

Recently there has been growing interest in power factor correction circuitry. Power factor, which is defined as the ratio of the apparent required power to the actual true power, ultimately affects the circuit s efficiency, thus varying the cost of electricity. It seems that almost all AC-powered equipment now require some form of active power factor correction in order to operate efficiently. Active power factor correction utilizes electronics to force the input current to look like a... 

Sometimes, tuned filters are configured to provide power factor correction for a facility as well as harmonic current filtering. In such cases the filter would be designed to carry the resonant harmonic frequency current and also the normal frequency current at the fundamental frequency. In either case, a power system harmonic study is paramount to ensure that no ill effects would be produced by the application of the power factor correction/filter circuit. 

In an electrical system, if the power factor is 0.80, 80% of the apparent power is converted into useful work. Apparent power is what the transformer that serves a home or business has to carry in order for that home or business to function. Active power is the portion of the apparent power that performs useful work and supplies losses in the electrical equipment that are associated with doing the work. Higher power factor leads to more optimum use of electrical current in a facility. Can a power factor reach 100% In theory it can, but in practice it cannot without some form of power factor correction device. The reason why it can approach 100% power factor but not quite reach it is because all electrical circuits have inductance and capacitance, which introduce reactive power requirements. The reactive power is that... 

Polystyrene capacitors have exceptionally low tan S values (< 10 q, making them well suited for frequency-selective circuits in telecommunications equipment. Polymer capacitors are widely used for power-factor correction in fluorescent lighting units, and in start/run circuitry for medium-type electric motors used in washing machines, tumble-dryers and copying machines for example. They are also used in filter circuits to suppress radio frequencies transmitted along main leads. Such interference noise may originate from mechanical switches, furnace controllers and switch mode power supplies it not only spoils radio and television reception but can also cause serious faults in data-processing and computer equipment. 

Arrangements have been made for power factor correction at some time in the future if required. This is simply the provision of available space for the addition of capacitor banks to the operating circuits of the large motors and to the main substation bus. The "bus correcting" capacitor bank at the substation will in effect correct for the reactive power demand of the numerous small motors inuse throughout the site. 

With DC electric motors, power factor correction is sometimes used to improve the power factor of the drive [30]. This is done by incorporating capacitive components into the circuit. Capacitors produce leading reactive power whereas the phase-controlled rectifiers produce lagging reactive power. Thus, by adding appropriately sized capacitors, the power factor of the drive can be improved. 

Power factor correction The practice of placing or connecting one or more devices (usually capacitors) onto an AC power circuit in order to improve its operating power factor. 

Applications are primarily for use in DC circuits such as blocking, coupling, decoupling, bypass and DC line filtering where the radio frequency (RE) and audio components are small in comparison with the DC rating. They have also been used successfully in such AC power applications as power factor correction and AC line filtering. They may be operated at all temperatures from -40 °C to +85 °C with derating to 50% at 125 °C. They are intended for use in sealed encapsulated assemblies. 

When the main arc has been struck the current flowing in the circuit is limited by the choke. A capacitor connected across the supply provides power-factor correction. The circuit is shown in Fig. 3.55. 

The power-factor correction capacitor used in a domestic fluorescent luminaire would typically have a value of 8 iF at a working voltage of 400 V. In an electronic filter circuit a typical capacitor value might be 100pF at 63 V. 

Power Supplies and Controls. Induction heating furnace loads rarely can be connected directiy to the user s electric power distribution system. If the load is to operate at the supply frequency, a transformer is used to provide the proper load voltage as weU as isolation from the supply system. Adjustment of the load voltage can be achieved by means of a tapped transformer or by use of a solid-state switch. The low power factor of an induction load can be corrected by installing a capacitor bank in the primary or secondary circuit. 

The power factor cos 6 is always a positive fraction between 0 and 1 (as long as 161 < 90°). The smaller the power factor, the greater the current that must be supplied to the circuit for a given active (useful) power output requirement. The increase in current associated with low power factors causes greater line losses or requires an increase in the capacity of the transmission equipment (wire size, transformers, etc.). As a result, for industrial applications there is often a power factor charge in the rate structure for supplying electricity. The usual situation is for loads to be inductive, and the industrial consumer may add capacitance to their circuits to correct the lagging power factor. 

Power factor in an alternating current circuit is defined as the ratio of actual circuit power in watts (W) to the apparent power in voltage amperes (VA). The need for correction arises from fact that the majority of A.C. electrical loads take from the supply a lagging quadruple current (voltage amperes reactive, var) and thus operates at a lagging power factor due to the reactive (rather than capacitive) nature of their construction. 

A coil, or any similar device, is said to generate self-induction or, in brief, it creates a circuit with inductance. Electric motors, for example, are wired through numerous coils with high inductance. Thus, electric motors, if not corrected, can have very low power factors. 

In this section, we have described a demonstration of an all-SiC dc-ac inverter (using SiC power switches and SiC diodes) operated in excess of 400W and at case temperatures of approximately 150°C. Two factors related to the system s implementation limited the operational power levels of this circuit. First, load instability caused excessive current spikes and GTO failures and was corrected using a closed-... 

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