1 phase controlled rectifier

7/27/2019 1 Phase Controlled Rectifier

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Single-Phase ControlledRectifiers

JM610

Electrical Engineering Department

Kota Bharu Polytechnic

Mohd Azlan bin Ashari

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SINGLE PHASE

CONTROLLED RECTIFIER

1. HALFWAVE CONTROLLED

RECTIFIER

2. FULLWAVE HALFCONTROLLED

RECTIFIER

3. FULLWAVE FULLY CONTROLLEDRECTIFIER

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Normal rectifiers are considered as uncontrolledrectifiers.

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

The Half-wave Controlled Rectifier

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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 positivehalf-cycle, some time after supply voltage

zero, by the application of a gate pulse

with delay angle a. In the negative half-

cycle, the thyristor is reverse biased and

cannot switch on. The larger the delay

angle, the smaller is the average load

voltage.

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Voltage waveforms

for two delay angles are shown below

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Controlled, Half-wave R load

]cos1[

2

)sin(2

1

,""

a

a

s

mDCaveo

V

tdtVVVV

voltageoutputDCAverage

a

a

2

)2sin(1

2

)()]sin([2

1,

resistor,byabsorbedpowerAverage

0

2

,

22

m

mrmso

rms

V

tdtVVwhere

R

VRIP rms

A gate signal isapplied at t = a,

where a is the

delay/firing angle.

R

V

R

VI srms

o

rmso 2

,

,

m

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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.

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Example (Cont) Solution

rad

VV

o

s

o

07.12.61

]cos1[2

212040

]cos1[2

a

a

a

In such that to achieved 40Vaverage 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 mustbe 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

V

VV m

rmso

6.75

2

)07.1(2sin07.11

2

2120

2

)2sin(1

2,

a

a

WR

V

P

rms

1.57100

6.75 22

63.0

100

6.75)120(

1.57

pf

VV

V so 54)120(2

m

m

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Half - Wave Controlled

Rectifier Circuit with an RLLoad

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Figure 1 : Half-wave controlled

Rectifier with RL Load

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Let vs(t) be Vm sin (t). At t =

0, the current through the

circuit is zero. As t becomes >0, vs becomes positive. If a

diode instead of an SCR has

been used, the diode would

start conduction at t = 0. With

an SCR, the conduction doesnot start till the SCR is

triggered. Let the SCR be

triggered when t = a. Then a

is called the firing angle and

the SCR continues to conduct.

When t = , 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 andkeeps the SCR forward-biased till

the energy stored in the inductor

becomes zero. Let the current

through the circuit become zero

at t = b and the value ofb > .Forb < t < 2, the current

through the circuit is zero

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With an Inductive (RL) Load

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With I nductive Load and

Freewheeling Diode

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FULL-WAVE HALF-CONTROLLED BRIDGE

RECTIFIER WITH RESISTIVE LOAD

The half-controlled is the easiest to implement since the two thyristors

can be arranged to have a common cathode.

The firing circuit can have a common train of pulses and only the

forward-biased device will switch on at the arrival of a pulse on the two

gates.

Figure 1 : Circuit Diagram16


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In the positive half-cycle, T1 is turned on at delay anglea, and current flows to the load through the path T1, load

and D1. The supply voltage then passes through zeroand reverses; since the load is resistive, T1 and D1

would turn-off.

Figure 2 : The flow of load current during +ve half cycle

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At delay angle + a, T2 is fired, and load current flowsthrough T2, load and D2. Once again, the supply voltagepasses through zero, and T2 and D2 would turn-off

Figure 3 : The flowing of load current during ve half cycle

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Figure 4 : The input and output waveforms

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Average load voltage (V av)

The average load voltage is found by calculating the area under the

voltage curve and then dividing by the length of the base. For any delay

angle a, the average load voltage is given by

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In the circuit in Fig. 1, the load is replaced by a large inductance.The assumption is that the load inductance is high enough to causecontinuous steady load current.

.

HALF-CONTROLLED BRIDGE WITH

HIGHLY INDUCTIVE LOAD

Inductive

load

Figure 5 : Half-controlled bridge rectifier with inductive load

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Operation:

In the positive half-cycle, T1 is turned on at delay angle a, andcurrent flows to the load through the path T1, load and D1. Thesupply voltage passes through zero and reverses; if this was aresistive load T1 would turn-off. However, due to the inductivestored energy, the load voltage reverses in order to keep the loadcurrent flowing, D2 is forward-biased and conducts, and clamps thebottom of the load to virtually zero voltage. Energy stored in the loadinductance keeps load current flowing through the path of D2, T1and the load until T2 is fired at t = ( a + )

The flow of load current

during ( a < t < )


during ( < t < a ) 22


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At delay angle + a, T2 is fired, T1 is reverse-biased and turns off,and load current flows through T2, load and D2. Once again, thesupply voltage passes through zero, and load inductive energyforward biases D1 to keep load current flowing. T1 is then fired, T2turns off and the cycle is repeated.

.

The flow of load current during ( a) < t < 2

The flow of load current during 2 < t < ( 2 a)

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Input supply

Load voltage

Load current

Input current

Current flow due

to inductive load

Figure 6 : Waveforms 24


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Average load voltage

A full-wave half-controlled bridge has a supply voltage of 220V at 50Hz. The

firing angle delay a = 90o . Determine the values of average and rms

currents load power and power factor for a resistive load of R = 100,

Example :

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HALF-CONTROLLED BRIDGE WITH FLYWHEEL

DIODE AND INDUCTIVE LOAD

Although the half-controlled bridge has a fly-wheel diode action built in, it

uses one of the thyristors in the fly-wheeling path. If a third diode isused, connected directly across the inductive load, then when the load

voltage attempts to reverse, this diode is forward-biased and the

inductive stored energy circulates the load current in the closed path of

the load and third diode

Figure 7 : circuit diagram

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The advantage of this method is that at mains voltage zero the conducting

thyristor turns off instead of hanging on for fly-wheel diode action, and thisreduces the thyristor duty cycle.

The circuit arrangement shown in figure 7 and resulting waveforms are shown

in Figure 7b. It is clear from observation of the waveforms that values of

average and rms voltage and current are unaffected by the addition of the third

diode.

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Input supply

Load voltage

Load current

Input current

Current flow due

to inductive load

Figure 7b : Waveforms 28


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FULL-WAVE FULLY CONTROLLED BRIDGE

WITH RESISTIVE LOAD

During the positive half-cycle, T1 and T3 are turned on simultaneously at the delay

angle ofa, and current flows to the load through the path T1, load and T3. The

supply voltage then passes through zero and reverses; since the load is resistive,

T1 and T3 would turn-off.

At delay angle of ( + a), T2 and T4 are fired simultaneously, and load current

flows through T2, load and T4. Once again, the supply voltage passes through

zero, T2 and T3 would turn off.

The waveforms are shown in figure 8. 29


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Figure 8Average load voltage

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FULL-WAVE FULLY CONTROLLED BRIDGE

WITH INDUCTIVE LOAD

In the positive half-cycle, T1 and T3 are turned on at delay angle a, and current

flows to the load through the path T1, load and T3. When the supply voltage

passes through zero and reverses. the stored energy in the load is regenerating

back to the supply; T1 and T3 are maintained in conduction state. Energy stored

in the load inductance keeps load current flowing through the path of T1, load

and T3 until (T2 and T4) are fired at delay angle of ( a).

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When T2 and T4 are fired at delay angle of ( a), current flows to the load

through the path T2, load and T4 until T1 and T3 are fired at the next cycle.


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supply voltage

Output voltage

Output current

supply current

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Average Load Voltage :

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1 phase controlled rectifier

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