CONTROLLED RECTIFIERS LINE COMMUTATED AC TO DC CONVERTERS

CONTROLLED RECTIFIERS (LINE COMMUTATED AC TO DC CONVERTERS)

INTRODUCTION Type of input: Fixed voltage, fixed frequency ac power supply. Type of output: Variable dc output voltage Type of commutation: Natural / AC line commutation 2

WHAT IS LINE COMMUTATION? The turning –off, or commutation, of a thyristor by supply voltage itself is called natural, or line commutation. INDUSTRIAL APPLICATION OF CONTROLLED D C POWER Steel-rolling mills, paper mills, printing presses and textile mills employing dc motor drives Traction system working on dc Electromechanical and electrometallurgical processes Magnet power supplies Portable hand tool drives High-voltage dc transmission 3

DIFFERENT TYPES OF LINE COMMUTATED CONVERTERS AC to DC Converters (Phase controlled rectifiers) AC to AC converters (AC voltage controllers) AC to AC converters (Cyclo converters) at low output frequency. 4

DIFFERENCES BETWEEN DIODE RECTIFIERS & PHASE CONTROLLED RECTIFIERS

DIODE RECTIFIER VS SCR RECTIFIER The diode rectifiers are referred to as uncontrolled rectifiers. The diode rectifiers give a fixed dc output voltage. Each diode conducts for one half cycle. Diode conduction angle = 1800 or radians. We can not control the dc output voltage or the average dc load current in a diode rectifier circuit. The SCR rectifiers are referred to as controlled rectifiers. The SCR rectifiers give variable dc output voltage. Each SCR may conduct from 00 to 1800 for one half cycle. Controlling of the dc output voltage or the average dc load current is possible in SCR rectifier circuit. 6

PHASE CONTROLLED RECTIFIER Need no commutation circuit Simple Less expensive and Widely used in industries where controlled dc power is required. 7

CLASSIFICATION OF PHASE CONTROLLED RECTIFIERS Single Phase Controlled Rectifiers. � Half wave controlled rectifiers. � Full wave controlled rectifiers. Semi converter (half controlled bridge converter). Full converter (fully controlled bridge converter). Three Phase Controlled Rectifiers. � Half wave controlled rectifiers. � Full wave controlled rectifiers. Semi converter (half controlled bridge converter). Full converter (fully controlled bridge converter). 8

ASSUMPTIONS In the study of thyristor systems SCR’s and Diode are assumed ideal switches i. e � No voltage drop across them � No reverse current exists under reverse voltage conditions � Holding current is zero 9

PRINCIPLE OF PHASE CONTROLLED RECTIFIER OPERATION

SINGLE PHASE HALF-WAVE THYRISTOR CONVERTER WITH A RESISTIVE LOAD 11

WORKING…. 12

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