Download For Negative half cycle

PHASE CONTROLLED
RECTIFIERS
Phase Controlled Rectifier
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3.1 Single phase half wave controlled rectifier with
resistivel oad
Circuit Diagram & Waveforms :
OPERATION :
For Positive half cycle :
In a positive half cycle of supply voltage, i.e. when the anode
of SCR is positive with respect to cathode and SCR is fired, power
is delivered to the load.
For Negative half cycle :
In the négative half cycle the SCR is reverse bised and blocks
the line voltage, and no curent can flow th rough the load.
Phase Controlled Rectifier
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3.2 Single phase full wave Controller rectifier with
Resistiveload
Circit Diagram& Waveforms :
OPERATION :
Single phase half wave rectifires are used in practice, but
the voltage and current relations developed for the circuits are quite
useful in understanding the operation of more practical full wave or
bridge circuit.
Two phase half wave or single phase full wave circuits
with transformer midpoint connections are generally used for
rectifires of low ratings.
Phase Controlled Rectifier
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OPERATION :
For positive half cycle :
 In the positive half cycle of the supply ,i.e. when the terminal
‘a’ of the transformer is positive with respect to the terminal
‘b’,if the SCR1 is fired at an angle α,current would flow from
terminal ‘a’ to SCR1,the node resistance RL and back to the
centre tap of the transformer.
 This current continues to flow up to the angle Π when the line
voltage reverses its polarity and SCR1 is turned off.
For negative half cycle :
 In the negative half cycle, SCR2 is fired at angle Π+α.current
flows through SCR2 the load resistance and back to centre
tap of the transformer.
 This current continues till 2Π when SCR2 is turned off. Thus
across the load there are two pulses of current in the same
direction.
 when SCR1 is turned on at the angle α,the voltage across it is
negligibly small, and about 1.0v would be maintain across it
up to the angle Π till the line voltage reverse and it is turned
off.
Phase Controlled Rectifier
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3.3 Fullwavecontrolledrectifier with RL load
(without freewheeling diode)
Circuit Diagram :
OPERATION :
It consist of single phase fullwave or two phase halfwave
controlled rectifier with RL load.
In the halfwave rectifier that the presence of inductance in
the load does not allow the load current to be zero at the point of
supply voltage reversal.
Phase Controlled Rectifier
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Waveforms :
For positive half cycle :
 At the positive half cycle if SCR1 is fired at an angle α,the
load current starts to flow from α and continues beyond П.
 The load current may be in the form of discontinuous
pulses or continuous depending on the firing angle α.
For negative half cycle :
 In the negative half cycle of the supply voltage, the
conduction is taken over by SCR2, so it is turn ON.
 The reverse voltage that turns OFF the conducting SCR1 is
line voltage itself, this type of commutation is known as
line commutation.
 Due to line commutation SCR1 turn OFF. The important
difference between RL load and purely resistive load is
that, in the RL load, some portion of load voltage is
negative (from П to β).
Phase Controlled Rectifier
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3.4 Effect of freewheeling diode
 A flyback diode is a diode used to eliminate flyback,
which is the sudden voltage spike seen across
an inductive load when its supply voltage is suddenly
reduced or removed.
 When power to inductive loads such as coils and inductors
is turned off, there is a sharp voltage spike. The direction
of this voltage is opposite to the applied voltage in
accordance with Lenz’s Law.
 When a current flows through the coil of a relay, the coil
gets electromagnetically charged. The energy is stored in
the magnetic field around the coil. When the power supply
to the coil is interrupted and the current in the coil tends to
decrease, the magnetic field discharges causing a surge in
voltage.
 The voltage, thus induced, can jump across the contacts of
relays connected to the coils. The sparks and arcing
produced can affect the life of the contacts. The voltage
spikes can also damage electronic components like
transistors which may be driving the relay coils
.
 Freewheel diodes are connected in reverse bias vis-à-vis
the supply voltage. Hence, when the voltage spike appears
in the opposite direction, they are short-circuited through
the diode. The voltage spike is thus short-circuited across
the coil. This protects the connected circuits.

Phase Controlled Rectifier
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 The inductor has property to store energy .In ac current,
during positive high cycle the inductor stores energy with
opposite polarity. At negative half cycle the inductor
deenergies ,so there must be reverse current which will
make SCR on .This may cause reverse damage to circuit
.To avoid energy dissipation freewheeling diode is used.
 To cut off the negative portion of the instantaneous output voltage
& smooth the output current ripple free, a freewheeling diode is
used.
Phase Controlled Rectifier
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3.5 SINGLE PHASE FULL CNTROLLED
RECTIFIRE WITH RL LODE &
FREEWILING DIODE
Circuit Diagram & Waveforms:
Phase Controlled Rectifier
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OPERATION:
CASE 1 –
 During positive half cycle of secondary winding is negative
with respect to B .In this case SCR1 is in forward blocking stage
and SCR 2 becomes off and current flows through SCR1
 A-SCR1-RL –Mand some charge that is electromagnetic energy
is red due to RL load.
 So we use freewiling diode across the RL load .so that the diode
uses the electromagnetic energy &freewheeling diode is in
forward biased condition.
CASE2 During negative half cycle of input AC signal terminal A of
secondary winding is positive with respect to B.
 In this case SCR1 is reverse blocking state i.e off &SCR2 is in
the forward blocking stage and current flows through SCR2BSCR2-RL-M.
Phase Controlled Rectifier
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3.6
SINGLE PHASE
HALF CONTROLLED
BRIDGE RECTIFIER WITH RESISTIVE
LOAD:-
For symmetrical operation:CIRCUIT DETAIL:The fig. shows circuit diagram of single phase half
controlled bridge rectifier with resistive load. In this circuit, a
single gate pulse can be used for triggering either of SCR. The
SCR which is in forward blocking state is only triggered at
instant of firing.
Circuit Diagram:
Phase Controlled Rectifier
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CASE 1:
 During positive half cycle L is positive with respect to N..
 Then SCR T1 is in forward blocking state and diode D2 is
ON.
 When gate pulse is applied at SCR T1 then current flow
through path L-T1-RL-D2-N.
CASE 2:
 During negative half cycle, N is positive with respect to L.
 Then SCR T2 is in forward blocking state and diode D1 is
conducting.
 When gate pulse is applied to SCR T2 then current starts
to flow through path N-T2-RL-D1-L.
Waveforms:
Phase Controlled Rectifier
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3.7 3 phase half controlled bridge converter (basic
construction): The 3 phase half controlled bridge converter is as shown in the fig.
(1)in this there are all 6 devices,3 SCR’s and the remaining 3
diodes.
 The load is connected between the common cathode point of the
thyristors and the common anode of the diodes.
 The bridge circuit in fig. shows that the neutral point N of the input
3 phase ac supply is not connected anywhere in the ckt.
Circuit Diagram:
 The half wave converter can be redrawn as shown in fig(2). The
SCR’s S1,S2& S3 form one half wave control rectifier and the
diodes D1,D2 &D3 form another rectifier ckt.
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 In this way half wave converter is equivalent to addition of two
half wave ckts.
 The load is fed via the upper 3 phase half wave rectifier & the
return path for the current is provided via the lower half wave
rectifier to one of the 3 supply lines.
 No neutral is required.
 At a time two devices conduct simultaneously one from each half
wave rectifier. One SCR & one diode will conduct simultaneously.
 The load vtg therefore will be a line vtg. & not the phase vtg.
Which line vtg. Appears across the load is decided by which pair
of devices conduct at the given instant of time.
Phase Controlled Rectifier
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3.7.4 3hase half converter with purely resistive load
 With a purely resistive load (R) the load current will be in phase
with the load vtg.
 The instantaneous load current will be equal to the instantaneous
load vtg divided by the resistance R.
 Naturally the shape of the load current waveform will be same as
that of the load vtg.
 Depending on the value of firing angle “α”, this ckt can operate in
two different modes of operation i.e., continuous & discontinuous.
Circuit Diagram:
Phase Controlled Rectifier
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Important Points: 6 PULSE OUTPUT: the load vtg waveform contains 6 pulses per
cycle of the input vtg. As shown in fig(4). The ripple frequency of
the output vtg waveform is therefore 6 times the supply frequency
i.e,300Hz.
 CONDUCTION ANGLE: each device SCR or diode conduct for
2π⁄3 radians or 120⁰.
 NO. FREEWHEELING: as load is resistive, it cannot store energy.
Therfore freewheeling action doesn’t take place in any of two
modes of operation with resistive load.
Phase Controlled Rectifier
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3.8 Operation of 3 phase fully control rectifier with
purely resistive load: The voltage & current waveforms through purely resistive
load are as shown in fig.
 The load voltage waveform is same as that with the load
inductive load. The load current however will not be ripple
free.
 Instead the instantaneous load current is equal to the
instantaneous load voltage divided by the load resistance R.
 Therefore the shape of the load current waveform is same as
that of the load voltage and it is in phase with the load
voltage.
 The sequence in which the diodes are turned on & off also
remains same as that with the inductive load. The outgoing
diodes are turned off due to line communication.
 Each diode conducts for a duration of 120°or 2 π/3 radians
and the current through each diode during its conduction
period is equal to the instantaneous load current during that
period.
Circuit Diagram:
Phase Controlled Rectifier
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Result:
 the three phase bridge rectifier & fullwave circuit have the
advantages of higher average output voltage ,lower rippel
factor,higher rippel frequency,better rectification efficiency &
better transformer utilization factor.
 the condution angel for diode is 120°or 2π⁄3 radians for half
wave & bridge but 60° for full wave circuit.
 The peak inverse voltage also remains same i.evm(line) or
√3vm(ph) or √6vs(rms) for all the three configuration.
Phase Controlled Rectifier
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