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CHAPTER l

INTRODUCTION

Power converters are used in power supplies, power conditioners, and motor drives.

As a consequence they are currently in very wide use and recent improvements in power

electronic devices, topologies, and controls are extending their range of application. Inmany of these applications, ac utility power is converted to de by means of uncontrolled

diode rectifiers that are driven by the ac line frequency. These rectifiers can be made

controllable by replacing diodes with ilicon !ontrolled "ectifiers #!"$ to provide a

controlled de output when required. Though simple in design and high in efficiency, these

conventional rectifiers produce problems in that they do not draw sinusoidal current from

the utility source and in%ect current harmonics into the supply. This results in poor power

factor seen by the utility and loads drawing poor power factor can disrupt other equipment

connected to the same source. This is why utility companies today have power factor

requirements. Power factor determines how much real power is drawn from the utility line.

Power factor is defined as the ratio of average power to apparent power. &ohan in ' 1 (

defines power factor as

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defines power factor as

ideal case, the power factor is unity which means that only real power is supplied by the

utility to a load that is, or appears as, resistive. Power factors less than unity indicate thatreactive power is present with the real power. /hen reactive power is flowing in a circuit,

it is not usable but still results in circuit power losses. 0sers are charged by utility compa

nies for reactive power and equipment must be designed to handle it based on the powerfactor rating. quipment that has poor power factor will be oversi2ed #higher fabrication

cost$ and be expensive to operate. To illustrate this with an example, consider a -3 A, 1-3

4ac circuit. The maximum power that can be drawn from this circuit is -533 / providedthe power factor is unity. )ecreasing the power factor will decrease the maximum powerthat can be drawn from that circuit without tripping circuit protection. If one had a loadoperating at 3.6 pf, then the maximum power that could be drawn without tripping circuit

protection would be 17-3 /. *or this reason alone it is advantageous to draw high powerfactor. 0tility requirements however also have cost penalties for not complying with

power factor requirements and more requirements are being considered to specifically

address excessive harmonic pollution of the utility supply.

/hen designing power factor correction # pfc $ circuits, it is essential to recogni2e

that both harmonic distortion and displacement contribute to power factor. If both of these

t id d i f d i f i it l th f t * l

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conduction mode to perform the input current waveshaping and de output voltage

regulation. A three:phase version of this design is illustrated in *ig. 1. The boost converter

*ig. 1. #a$ Active three:phase power factor correction circuit.

#b$ Actual source implemented experimentally.

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