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Controlled Rectifiers

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1 Controlled Rectifiers
Lecture 9 Controlled Rectifiers

2 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

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

4 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.

5 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

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


8 Controlled, Half-wave R-L load



11 Controlled full-wave rectifiers
Resistive load:  

12 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.

13 Full wave Controlled Rectifier with RL load

14 Discontinuous and Continuous Operations

15 Discontinuous Mode

16 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  .


18 Continuous Mode

19 continuous current



22 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

23 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

24 Controlled Single-phase converter operating as an inverter

25 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

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