Controlled Rectifiers Lecture 9 Controlled Rectifiers

The Controlled Half-wave Rectifier Normal rectifiers are considered as uncontrolled rectifiers. Once the source and load parameters are established, the dc level of the output and power transferred to the load are fixed quantities. A way to control the output is to use SCR instead of diode. Two condition must be met before SCR can conduct: The SCR must be forward biased (VSCR>0) Current must be applied to the gate of SCR

Controlled, Half-wave R load A gate signal is applied at t = , where  is the delay/firing angle.

Example Design a circuit to produce an average voltage of 40V across 100 load resistor from a 120Vrms 60 Hz ac source. Determine the power absorbed by the resistor and the power factor. Briefly describe what happen if the circuit is replaced by diode to produce the same average output.

Example (Cont) Solution In such that to achieved 40V average voltage, the delay angle must be If an uncontrolled diode is used, the average voltage would be That means, some reducing average resistor to the design must be made. A series resistor or inductor could be added to an uncontrolled rectifier, while controlled rectifier has advantage of not altering the load or introducing the losses

Controlled, Half-wave R-L load The analysis of the circuit is very much similar to that of uncontrolled rectifier.

Controlled, Half-wave R-L load

Controlled full-wave rectifiers Resistive　load： 　

The rms current in source is the same as the rms current in the load. 　The　power　delivered　to　the　load 　The　rms　current　in　source　is　the　same　as　the　rms　current　in　 the　load.

Full wave Controlled Rectifier with RL load

Discontinuous and Continuous Operations

Discontinuous Mode

For discontinuous current 　Analysis　of　the　controlled　full-wave　rectifier　operating　in　the　discontinuous　current　mode　is　identical　to　that　of　the　controlled　half-wave　rectifier, except　that　the　period　for　the　output　current　is　 .

Continuous Mode

continuous　current

R-L　Source　load　：　Fig.4-14 　The　SCRS　may　be　turned　on　at　any　time　that　they　are　forward　biased, which　is　at　an　angle

average load current is 　　For　continuous　current　case, the　average　bridge　output　voltage　is average　load　current　is　 　The　ac　voltage　terms　are　unchanged　from　the　controlled　rectifier　with　an　R-L　load. 　The　ac　current　terms　are　determined　from　circuit. 　Power　absorbed　by　the　dc　voltage　is Power　absorbed　by　resistor　in　the　load　is

Controlled Single-phase converter operating as an inverter

Average DC output voltage Assuming AC side inductance is zero Note that output voltage can go negative for alpha > 90 degrees. This means negative power flow or inversion Copyright © 2003 by John Wiley & Sons, Inc. Chapter 6 Thyristor Converters