12/30/2010
Electrical Engineering Department Kota Bharu Polytechnic
Single-Phase Controlled Rectifiers Wan Zain Bin Wan Dollah
SINGLE PHASE AC TO DC CONVERTER 1. HALFWAVE CONTROLLED RECTIFIER 2. FULLWAVE HALFCONTROLLED RECTIFIER 3. FULLWAVE FULLY CONTROLLED RECTIFIER
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The Half-wave Controlled 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
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The simplest controlled rectifier uses a single device, such as a thyristor, to produce variable voltage d.c. from fixed voltage a.c. mains. The circuit arrangement is shown below
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• The thyristor is turned on in the positive half-cycle, some time after supply voltage zero, by the application of a gate pulse with delay angle α. In the negative halfcycle, the thyristor is reverse biased and cannot switch on. The larger the delay angle, the smaller is the average load voltage.
Voltage waveforms for two delay angles are shown below
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Controlled, Half-wave R load • A gate signal is applied at t = , where is the delay/firing angle. Average"DC" outputvoltage ,
Vo Vave VDC
1 Vm sin(t) dt Average power absorbed by resistor, 2
V ms [1 cos] 2
I o , rms
Vo , rms R
Vs 2R
P I 2 rms R
V 2 rms R
where, Vo , rms
1 [Vm sin(t )]2 d (t ) 2 0 Vm sin(2 ) 1 2 2
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. Briefly describe what happen if the circuit is replaced by diode to produce the same average output.
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Example (Cont) • Solution In such that to achieved 40V average voltage, the delay angle must be
V Vo ms [1 cos ] 2 120 2 40 [1 cos ] 2 61.2o 1.07 rad
Vo , rms
Vm sin( 2 ) 1 2 2
120 2 1 .07 sin 2 (1 .07 ) 1 2 2 75 .6V
P
V 2 rms 75.6 2 57.1W R 100
• If an uncontrolled diode is used, the average voltage would be
Vo
Vms
2 (120)
54V
• 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
Half - Wave Controlled Rectifier Circuit with an RL Load
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Figure 1 : Half-wave controlled Rectifier with RL Load
Let vs(t) be Vm sin (ωt). At ωt = 0, the current through the circuit is zero. As ωt becomes > 0, vs becomes positive. If a diode instead of an SCR has been used, the diode would start conduction at ωt = 0. With an SCR, the conduction does not start till the SCR is triggered. Let the SCR be triggered when ωt = α. Then α is called the firing angle and the SCR continues to conduct.
When ωt = π, the source becomes zero, but at this instant, the current through the circuit is not zero and there is some energy stored in the inductor. When vs becomes negative, the current through the circuit would not become zero suddenly because of the inductor. The inductor acts as a source and keeps the SCR forward-biased till the energy stored in the inductor becomes zero. Let the current through the circuit become zero at ωt = β and the value of β > π. For β < ωt < 2π, the current through the circuit is zero
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With an Inductive (RL) Load
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With Inductive Load and Freewheeling Diode
Average output current, Idc 14
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FULL-WAVE CONTROLLED CENTER-TAP RECTIFIERS
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With Resistive Load
Average output voltage, Vdc
Average output current,
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With an Inductive (RL) Load
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Control Characteristics for center-tap rectifier
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With Freewheeling Diode
Average output voltage, Vdc
Average output current, 19
Example 6.4 Explain with the help of waveforms the operation of a full-wave center-tap rectifier with RL load for the following firing angles: (a) 0° (b) 45° (c) 90° (d) 135° (e) 180° 20
Assume highly Inductive Load
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Voltage and current waveforms for α=0° •
During positive-half cycle of source voltage, SCR1 is forward biased and SCR2 is reverse biased. During negative half-cycle, SCR2 is forward biased and SCR1 is reverse biased. In either case voltage across the load is Vs.
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Output is similar to uncontrolled rectifier.
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Each SCR conducts for 180° and supplies current to the load for this period
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Voltage and current waveforms for α=45°
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Average DC output voltage decreases.
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If SCR1 is triggered at 45°, SCR2 will conduct upto that point, even though the source voltage is zero, due to highly inductive nature of load.
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When SCR1 is turned on, SCR2 is turned off.
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Current to the load is supplied by SCR1 and SCR2, each conducting for 180° 22
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Voltage and current waveforms for α=90°
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Average DC voltage is zero, so there is no transfer of power from AC source to DC load.
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Each SCR remains in conduction for 180°
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As firing angle is increased from 0 to 90°, the power supplied to the DC load decreases, becoming zero at α=90°
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Voltage and current waveforms for α=135° •
Average DC voltage is negative.
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Load current still flows in each SCR for 180° in its original direction.
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Load voltage has changed polarity.
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Power now flows from DC load to AC source .
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Circuit acts like an inverter.
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Voltage and current waveforms for α=180°
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Average output DC voltage is at its maximum negative value.
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SCRs remain in conduction for 180°
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Example 6.5 Show direction of power flow and operating mode (rectifying or inversion) of center-tap rectifier circuit with following firing angles: A) α > 0° B) α < 90° C) α > 90° D) α < 180° 26
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Solution • For firing angle in the range 0° < α < 90° 1. 2. 3. 4.
Average output voltage is positive. Converter operates in the rectifying mode. Power to the load is positive Power flow is from AC source to the DC load.
• For firing angle in the range 90° < α < 180° 1. 2. 3. 4.
Average output voltage is negative Converter operates in inversion mode Power to the load is negative Power flow is from DC load to AC source
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Assignment # 2 • In example 6.4, draw waveforms for voltage across thyristor 2. i.e., VSCR2
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FULL-WAVE CONTROLLED BRIDGE RECTIFIER
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With Resistive Load
Average output voltage, Vdc
Average output current,
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With an Inductive (RL) Load
Average output voltage, Vdc
Average output current,
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For L >>> R
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Control characteristics for bridge rectifier
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With RL load and freewheeling diode
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HALF-CONTROLLED OR SEMICONTROLLED BRIDGE RECTIFIERS
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• In fully-controlled rectifier, only rectification can be obtained by connecting a freewheeling diode across the output terminals of the rectifier. • Another method of obtaining rectification in bridge rectifiers is replacing half of the SCRs with diodes. These circuits are called semicontrolled bridge rectifiers.
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Full-wave semicontrolled bridge rectifier circuit
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Semicontrolled bridge rectifier with FWD
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Dual Converter
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Exercise Problems 6.1, 6.5, 6.6, 6.9, 6.12, 6.21, 6.22, 6.24 Also give analysis of waveforms in each case of all above exercise problems
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