CONTROLLED RECTIFIERS

POWER ELECTRONICS(IV SEM)

ByMrs. ANSHU BHALLA Sr. Lecturer in Electronics

Govt.Poly.LOHARU.

CONTROLLED RECTIFIERS(Phase control rectifiers)

•The basic function is to convert A.C input voltage to a controllable d.c output voltage .•They consists of SCRs. •The basic principle of phase controlled rectifiers is to control the point in the time at which the SCRs are allowed to conduct during each A.C cycle(i.e. firing angle) and thus it is feasible to have controlled mean d.c output voltage.

RECTIFIER/ CONTROL RECTIFIERS

RECTIFIER• Rectifiers provide fixed

output d.c for a fix magnitude of I/P A.C voltage. They can not by themselves control the output d.c voltage.

• Rectifiers made up of diodes

CONTROL RECTIFIERS• The basic function of

control rectifiers also known as phase control rectifiers is to convert A.C input voltage to a controllable d.c output voltage.

• They consists of SCRs.

VS

Why a phase control rectifieris called so?

•In phase control, power across the load is controlled bytriggering the thyristors at a fixed phase angle ’α’ whichis known as firing angle. And the duration during whichthe SCR conducts is known as conduction angle (β). Inthis case,β = л- α Lower the firing angle ‘α’, higher is theconduction period of SCR, so the output voltage acrossthe load and hence power delivered to the load will bemore. So, by controlling firing angle, d.c voltageavailable to the load can be changed.

Why a controlled rectifieris called so?

Applications of controlled rectifiers

• 1.Electroplating and other electrochemical processes

• 2. Battery chargers.• 3.Constant voltage supply.• 4. Speed controllers such as used in

paper mills, steel rolling mills etc.• 5. High voltage d.c transmissions.

Single phase half wave controlled rectifier

On state forward voltage drop (VT) across the SCR is assumed to be zero.

Let Vs=Vm sin wt be the input voltage to the circuit.

-In positive half of the input a.c cycle the SCR is forward biased and starts conducting when a firing pulse is given to its gate terminal at firing angle ’α’.

-Prior to firing ,SCR is off and the output voltage is zero as there is no link between input and output.

WORKING

Single phase half wave controlled rectifier

Single phase half wave controlled rectifier

-At α ,SCR turns ON and acts like a closed switch, thus whatever is the input voltage becomes available as output. -Load current (iL) flows whose magnitude is dependent on output voltage Vo and load resister RL i.e. iL =Vo/RLat any instant of time. ---All the waveforms are shown in next figure

-At wt= л, the input voltage becomes zero, so the current through the SCR also becomes zero and SCR turns off. Immediately, after this negative half cycle begins helping the SCR to turn off and SCR is reversebiased.-SCR remains off till the next gate pulse at (2л+ α) and output remains zero as shown in the figure. -At wt= (2л+ α), SCR is again fired to get the output voltage and current. Voltage across SCR i.e. Vscr = (Vi-Vo) is also shown in the figure.

Single phase half wave controlled rectifier

VSVO

ig

VS VO

ig

From the output voltage waveform time period of output (Vo) is given

T= (2л+α)-αor

T=2лAs the input voltage, Vi=Vm sin wt,the average output voltage will be given by

Vav = Vm/2л (1+ cos α)

Vav = 1/T Vm sin wt. d (wt)

Where, T is the time period of the output W/F.

Vav = 1/2л Vm sin wt . d (wt)

= - Vm/2лIcos wtIлα=-Vm/2л (cos л – cos α)

(as cos л=-1)

Vav = Vm/2л (1+ cos α)…….(1)

Л∫α

Л∫α

AVERAGE OUTPUT VOLTAGE

Thus average d.c voltage acrossthe resistive load is given by

This equation shows that by decreasing the firing angle (α) output voltage can be increased and vice versa. This is known as phase control.

PHASE CONTROL

Vav = Vm/2л (1+ cos α)

Single phase half wave controlled rectifier (with

inductive load):

•In the +VE half cycle SCR is fired at α & we get the output voltage as shown. But the load current in this case rises slowly and not

instantaneously & the inductive load stores energy.•During -VE half Cycle the stored energy in the load forces the current to flow in the same direction through SCR and the a.c. supply. So, SCR cannot commutate (turn off) and remains in on-state. Hence, negative output voltage is obtained through on SCR.

WORKING

Power can be controlled by changing the firing angle α. The effect of inductive load (as shown in the figure ) is that conduction angle of SCR is increased and is given by β>л-α(for inductive load) Also, the output voltage becomes -ve during the period SCR is forced to conduct. Hence, the average O/P voltage with inductive load is less as compared to average output voltage with resistive load.

Single phase half wave controlled rectifier (with

inductive load)

VSVO

VS

Single phase half wave controlled Rectifier (with inductive load and free-wheeling diode):

Working•Free wheeling diode is connected across the inductive load to prevent output from going -VE.

•When output is positive, FWD becomes reverse biased has no effect on output. •After wt=л, the load current continues to flow through the load due to stored energy but now it gets a less resistive path through the FWD.

Working•As the diode is connected in shunt with the load, the output voltage (Vo) remains zero till

the energy is fly wheeled or exhausted through diode. As the current is not forced through SCR, it gets commutated at wt= л

Conduction Angle β = л-αRoles of commutating diode

(i)To prevent the negative load voltage.(ii) To allow the SCR to regain its blocking state at the voltage zero by transferring the load current away from the thyristor.

Single phase half wave controlled Rectifier (with inductive load and free-wheeling diode):

SINGLE PHASE FULL WAVE CONTROLLED

RECTIFIER(WITH RESISTIVE LOAD)

TYPES:

• Bridge converter or B-2 connection

• Mid-point converter or M-2 connection.

SINGLE PHASE FULL WAVE CONTROLLED RECTIFIER(WITH RESISTIVE LOAD)Single phase full wave rectifier can be of two types:1. Bridge converter or B-2 connection2. Mid-point converter or M-2 connection.

•Bridge connection is generally preferred asthey do not need transformer. If one end ofthe load is to be grounded mid-pointconfiguration is used. As centre-tappedsecondary is used, the rating of thetransformer has to be double the load ratingin the mid-point configuration. The circuitdiagrams are shown in the next figure.

B-2/M-2 connection.

L (+ve) SCR1 A — load — B SCR2 L (-ve)

Working•During +ve half cycle S1 and S2

are fired at firing angle α & current flows in the direction.

The line current (iL) is equal to the load current (id). At wt = л, the applied a.c voltage becomes zero and SCR1 and SCR2 get commutated. So

the output voltage becomes zero.

•In the negative half-cycle atwt=л +α, SCRs 3 and 4 are fired.Now the current flows in thedirection.

N (+ve) SCR3 A — load — B SCR4 L (-ve)

Working

•The direction of load current (iL) remains the same i.e. from A to B in the load but polarity of the line current (iL) has changed, though magnitudes of both are same. The negative half of the input thus, becomes available as positive across the load.

The voltage waveform across SCR1

The average load current

MATHEMATICALLY ANALYSIS

•Assumptions: On state voltage drops across SCRs to be zero.

•Assumed input voltage to be V=Vmsinwt •The voltage appearing across the load will begiven by,

Vav =1/T ∫ Vm sin wt. d (wt.)= л

Vav =1/л Vm sin wt.d(wt)

=-Vm/лIcos wtIлα=-Vm/л (cos л - cos α)

Vav = Vdc = Vm/л (1+cos α)

α

∫л

MATHEMATICALLY ANALYSIS

:

1.On-state voltage drop of SCRs to be zero.2.Source inductance assumed to be zero.3.Load inductance ( ) assumed very

very high. d L → ∞

Assumption :

Single Phase Full Wave Controlled Bridge Rectifier

with inductive load :

*

vo

io

a

b

p+a

a

b

p+a

Discontinuous current mode:

*

vo

io

a

p+a

a

p+a

Continuous current mode:

L(+ve) SCR1 A Load B SCR2 N(-ve)

→→→

→→→

In +ve cycle of a.c. i/p at firing angle ’α ’ the SCRs 1 and 2 are fired, positive voltage at input becomes available at the output and the current flows in the direction.

Operation:

The magnitude of line current id is same as that of load current.

- As -ve half cycle starts,the load current continues to flow in thesame direction with the same

magnitude (load current assumed constant) due to the stored energy in the inductor.

As a result SCR1 and SCR2 remain in onstate. So the negative voltage appears at the output terminals. Then, at (л + α), SCR3 and SCR4 are fired, as a result reverse voltage is applied across SCRs 1 and 2 and they get commutated.

N (+ve) SCR3 A Load B SCR4 L (-ve)

Same so far as the load current is concerned,While Opposite so far as the line current isconcerned. load current is unidirectional butthe line current is reverses its polarityperiodically.

•As –ve half of the input appears across the output terminals A and B as positive through SCRs 3 and 4 and the output current flows in the path Which is in the direction

MATHEMATICALLY ANALYSIS

•Assume input Vi =Vm sin wt

Vav = Vdc = 1/T Vm sin wt . (d.wt)

Here, T = (л+ α) - α= л

So, Vav = Vdc = 1/л sin wt d(dt)

= -Vm/л lcos wtlα л+α

-Vm/л (cos (л+ α) – cos α)

-Vm/л (-cos α –cos α)

Vav = Vdc = 2Vm/л cos α

(л+ α)∫α

(л+ α)∫α

When α =0, Vdc = 2Vm/л cos 0 = 2Vm/ л

When α = л/2, Vdc = 2Vm/ л cos л/2=0

When α = л, Vdc = 2Vm/л cos л =-2Vm/л

This shows the variation of Vdc with α

Load Power = Output voltage X load current= Vdc.IL

.

Vav = Vdc = 2Vm/л cos α

Here,Load current ILis unidirectional and always positive.

For α < л/2, Vdc is +ve i.e.Load Power (Vdc.IL ) is also +ve. This means that power flows is from a.c. side to d.c. side. This is known as conversion operation or rectification mode.But, for α > л/2, Vdc is -ve,and IL is always +ve. So the load power comes out to be -ve. This means that load will supply power back to a.c. This is known as inversion I.e. converter is said to be operating in Inverting mode

Why a fully controlled converter is called so?

This converter can convert a.c. to d.c. and d.c. to a.c. Power flow is possible in both directions and because of this feature, this circuit with inductive load is known as Full Wave Fully Controlled Converter. The conversion of A.C. to D.C. and D.C. to A.C. is also known as Two—quadrant operation of converter circuit.

Why a fully controlled converter is called so?

The waveform of load current

The waveform of line current

Single Phase Full wave controlled rectifier with free wheeling diode and inductive load: OR Single phase full wave half controlled rectifier

WorkingIn the +ve half SCR 1 and 2 are fired and positive output voltage is obtained. The FWD remains R.B and has no effect on the output.

But when -ve half of the a.c. cycle starts, stored energy in the load forward biased FWD and gets fly wheeled through the FWD. Thus, output becomes zero. Also the SCR 1 and 2 get commutated due to -ve potential of the a.c. input. Though the output voltage becomes zero, the load current continuous to flow through the load and FWD.

Next at wt = л + α, SCRs 3 and 4 are fired So, the -ve alternation of a.c. input appears as +ve across the load and FWD becomes R.B & the current follows the path:

Working

N (+ve) SCR3 A Load B SCR4 L (-ve)

In this ckt, the line current becomes zero during the period FWD is conducting. Similarly, in the next +ve half FWD is used to fly wheel the stored energy and SCR1 &SCR2 are fired symmetrically firing angle to get the positive output.

The waveform of bridge output voltage

The waveform of load current

The waveform of current through the free-wheeling diode

The waveform of mains current

Vav = Vdc = 1/T α∫ Vmsin wt. d (wt)

Here, T = л – 0 = л

So, Vav = Vdc = 1/ л Vm sin wt d (wt)

-Vm/ л Icos wtIлα-Vm/ л (cos л – cos α)

Vav = Vdc = Vm/ л (1+ cos α)

л

Л∫ α

Input voltage is Vi = Vm Sin wt.The average output voltage Vav will

be given by

AVERAGE OUTPUT VOLTAGE

When α = 0, Vdc = Vm/л (1+cos 0) = 2Vm/ лWhen α = л/2, Vdc= Vm/л(1+cos л/2) = Vm/л

When α = л, Vdc = Vm/ л (1+cos л) =0

The average output voltage Vav will be given by

Vav = Vdc = Vm/ л (1+ cos α)

Thus control has become half ,Vdc is always +ve & also the load current is always +ve. So, the load power which is equal to vdc* IL is also +ve

for this circuit.

Why a half controlled converter is called so?

In this circuit power flow can be from a.c. to d.c. side only. Power cannot flow from d.c. to a.c.. Thus, in this converter, inverter mode is not possible, therefore, it can be said that control has become half .So, this circuit is known as single phase full wave half controlled converter

Single phase full wave half

controlled converters

(using SCRs and diodes)

•These types of half controlled converters are cheaper as two SCRs are replaced by two diodes and are used for applications where converter is not required to work as an inverter.

•Half controlled converters can be fabricated in two connections: Symmetrical Configuration &Asymmetrical Configuration

Symmetrical Configuration:

-In Positive half SCR1 is fired &

SCR1 and diode D3 conduct.

-In negative half SCR2 is fired and SCR2 & D4 conduct.

- After positive half fly wheeling takes place through SCR1 and D4. After negative half flywheeling takes place through SCR2 and D3.

- Free wheeling diode is not required as inherent flywheeling action is there.

Symmetrical Configuration

(ii) Asymmetrical Configuration

•During [positive half SCR1 and D3 conduct and then energy is flywheeled through diodes D4 and D3..

•During negative half D1 and SCR2 conduct & flywheeling is through diodes D4 and D3.

Asymmetrical Configuration

In Symmetrical Configuration, the cathodes of two SCRs are at the same potential so their gates can be connected & a single gate pulse can be used for triggering either SCR.

In Asymmetrical Configuration, separate-triggering circuits are to be used

Half Controlled Rectifier1.Contains mixture of

diodes and SCR.2.Mean d.c load voltage

can be controlled but reversal of load voltage is not possible.

Fully Controlled Rectifier1. All rectifying elements

are SCRs.2. By suitable control of

phase angle (firing angle) at which the SCRs are turned ON, it is possible to control mean d.cvoltage and to reverse d.c load voltage as well.

Half Controlled/ Fully Controlled Rectifier

Half Controlled/ Fully Controlled Rectifier

Half Controlled Rectifier

• It is also a unidirectional converter as power flow is only from a.c supply to d.c load.

Fully Controlled Rectifier

• It is a bidirectional converter, as it allows the power flow in either direction between the a.c supply and d.c load.

CONTROLLED RECTIFIERS�POWER ELECTRONICS�(IV SEM)CONTROLLED RECTIFIERS�(Phase control rectifiers)RECTIFIER/ CONTROL RECTIFIERSSlide Number 4Slide Number 5Slide Number 6Slide Number 7 Applications of controlled rectifiers Slide Number 9� ���� -In positive half of the input a.c cycle the SCR is forward biased and starts conducting when a firing pulse is given to its gate terminal at firing angle ’’.�� -Prior to firing ,SCR is off and the output voltage is zero as there is no link between input and output. Single phase half wave controlled rectifier�������-At wt= л, the input voltage becomes zero, so the current through the SCR also becomes zero and SCR turns off. Immediately, after this negative half cycle begins helping the SCR to turn off and SCR is reverse biased. �-SCR remains off till the next gate pulse at (2л+ ) and output remains zero as shown in the figure. �-At wt= (2л+ ), SCR is again fired to get the output voltage and current. Voltage across SCR i.e. Vscr = (Vi-Vo) is also shown in the figure. Slide Number 13Slide Number 14Slide Number 15����From the output voltage waveform time period of output (Vo) is given� T= (2л+)-� or� T=2л�As the input voltage, Vi=Vm sin wt,�the average output voltage will be given bySlide Number 17 � � � Thus average d.c voltage across� the resistive load is given by�����This equation shows that by decreasing the firing angle () output voltage can be increased and vice versa. �This is known as phase control. Slide Number 19Slide Number 20� Power can be controlled by changing the firing angle . The effect of inductive load (as shown in the figure ) is that conduction angle of SCR is increased and is given by β>л-(for inductive load) Also, the output voltage becomes -ve during the period SCR is forced to conduct. Hence, the average O/P voltage with inductive load is less as compared to average output voltage with resistive load. Slide Number 22Slide Number 23Slide Number 24Slide Number 25Slide Number 26Working�Single phase half wave controlled Rectifier (with inductive load and free- wheeling diode): �Slide Number 29SINGLE PHASE FULL WAVE CONTROLLED RECTIFIER(WITH RESISTIVE LOAD)�Single phase full wave rectifier can be of two types:�1. Bridge converter or B-2 connection�2. Mid-point converter or M-2 connection.Bridge connection is generally preferred as they do not need transformer. If one end of the load is to be grounded mid-point configuration is used. As centre-tapped secondary is used, the rating of the transformer has to be double the load rating in the mid-point configuration. The circuit diagrams are shown in the next figure.Slide Number 32L (+ve) SCR1 A — load — B SCR2 L (-ve)In the negative half-cycle at �wt=л +, SCRs 3 and 4 are fired. Now the current flows in the direction. Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39� � 1.On-state voltage drop of SCRs to be zero.� 2.Source inductance assumed to be zero.� 3.Load inductance ( ) assumed very � very high. Slide Number 41Slide Number 42L(+ve) SCR1 A Load B SCR2 N(-ve) - As -ve half cycle starts,the load � current continues to flow in the� same direction with the same magnitude (load current assumed constant) due to the stored energy in the inductor. Slide Number 45Slide Number 46Slide Number 47Slide Number 48Slide Number 49Slide Number 50Slide Number 51Slide Number 52Slide Number 53Slide Number 54Slide Number 55Slide Number 56Slide Number 57Slide Number 58Slide Number 59Slide Number 60Slide Number 61Slide Number 62Slide Number 63Slide Number 64Slide Number 65Slide Number 66Half Controlled/ Fully Controlled RectifierSlide Number 68