SINGLE-PHASE CONTROLLED THYRISTOR BRIDGE RECTIFIER
Rectifier converts an input AC voltage into the output DC voltage. Controlled version allows
to change averaged (mean) value of output voltage applying a time delay for turn-on of
thyristors in respect to the AC voltage wave.
Such a single phase system comprises a 4 thyristors – controllable on turn-on unidirectional
conductivity semiconductor switches- connected in scheme of 4 pole bridge with two
interconnected a cathodes of the thyristors as positive polarity output DC pole and two
anodes as negative polarity output pole of the rectified DC voltage ud (instantaneous values)
with measured by voltmeter averaged (mean) value of voltage Ud (Fig.1).
Input AC sinus shape voltage with instantaneous values uin and magnitude Um is applied to
the two other poles of the bridge.
Fig.1
When AC voltage uin is in its half-wave with upper clamp a positive (as on Fig.1) a both
thyristors V1 and V2 can be turned on with some angular delay Į in respect to initial point of
this half-wave (accepted as positive one). In this case input current i1 neglecting of input line
inductance L1 is same as current id of DC load.
When uin is in its half wave with upper clamp negative (in brackets on Fig.2) with angular
delay Į in respect to the initial point of this half-wave (accepted as negative one) can be
turned on both thyristors V3,V4 but if V1 and V2 conducts in this turn-on instant current then
they will be turned off for to them a reverse voltage is applied. Current i1 in this case is
opposite directed to the load current id.
In both positions – when conducting pair is V1,V2 and when - V3,V4 – output current of the
bridge – load current can pass only in one direction appointed by conductivity direction of
thyristors, i.e., id • 0.
Shape of current id depends on structure of load. On Fig.1 DC load is presented as serial
connection of resistor R, inductance Ld and EMF E. If the load comprises only a resistor R
then shape of current id is repeating one of the AC voltage uin : id = uin/R when conducting
V1,V2 and id= - uin/R when conducting V3,V4 (Fig.2). Averaged value Id of current id is
measured by ammeter of galvanoscope mode.
Fig.2. Diagrams of output rectified voltage and current at different kinds of load – active (a),
active-inductive (b), with endless inductance (c), with EMF E and R,L in series (d)
If it is applied load with L and R in series (b case) then changes of output current can be
calculated from equation
did
id R U m sin Zt
dt
where in conducting circuit id •0 and AC voltage uin is applied in range when Ȧt>Į and while
exists id >0.
Ld
At same value of Į in latter case averaged value of voltage Ud is smaller than one for the R
load but mean value of the load’s current
Ud
Id
R
i.e., can be calculate using same equation because an averaged by full cycle influence of L on
mean value of voltage is equal to zero. If id falls down to zero value before turning-on the next
pair of thyristors then such an id is accepted as discontinuous one.
If Ld is increased then deviations of output current is smaller, i.e., wave of current is smoothed
and it is closer to the DC one. At certain large value of inductance Ld current id is always
more than zero and such a current is accepted as continuous one.
In the last case averaged output voltage
U d U d 0 cosD
where Ud0 stands for output voltage of bridge realized using only diodes instead of thyristors.
This voltage is 0.9 Uin where Uin stands for RMS voltage of the AC network ( U m
2U in ).
As it is seen from equation at continuous case of current it is possible operate only with
0”Į” 0.5S . When Į=0.5ʌ then Ud=0. Really at Į=0.5ʌ some current Id exists but not
continuous shape.
Introducing in content of load an EMF E and if E is directed opposite to the direction of the id
then processes are similar to described only equation will be
di
Ld d id R U m sin Zt E
dt
and it is possible to start conducting of current if in turn on instant id >0 or an instantaneous
meaning 0f voltage U m sin Zt ! E . If in turn-on instant of the thyristors pair id>0 then exists
continuous case, voltage Ud= U d 0 cosD , load’s current
Ud E
Id
.
R
If at turn-on instant id is zero then rising of current id starts only at instant when U m sin Zt ! E
(see Fig 2,d). Current is discontinuous and curve of ud comprises an intervals when id=0 and
when ud=E. Nevertheless
Ud E
Id
R
only Ud must be calculated taking into account an intervals with ud=E.
Some influence on the processes has presented in the AC circuit a line inductance L1. This
inductance excludes possibility of instantaneous change of direction of AC line current i1 at
turn-on instant of thyristors pair. At changing of polarity of AC voltage uin current i1 is
pressed to change its direction but it takes certain time delay angle Ȗ named as commutation
angle. If i1 is positive directed and with conducting for example V1,V2 turn-on of thyristors
V3,V4 takes place then a commutation starts and when i1 is smaller than id then difference in
balance of instantaneous values of currents will be hold up with further stay in conducting
position all thyristors V1,V2,V3,V4, i.e., in commutation interval all four thyristors will be
conducting current and bridge clamps will be shortened. Changing of i1 then describes
equation
§ di1 ·
¨ L1 ¸ U m sin Zt
© dt ¹J
with initial value of i1=Id when V1,V2 are turned on i1=-Id when V3,V4 are turned on.
In shape of output voltage ud curve there would be short intervals with ud=0 (Fig.3) and
therefore at continuous case (only then commutation phenomenon occurs)
2 I ZL
U d U d 0 cosD d 1
S
where Z
2Sf - angular frequency of input AC voltage.
Fig.3. Diagrams of rectified voltage and current of the thyristors at commutation phenomenon
Situation is different when direction of EMF E is coherent with direction of the current id.
Then EMF E is as source of energy and this energy can be transmitted to the AC network.
Such a case exists when Į>0.5ʌ and Į< ʌ. Then polarity of mean value of voltage ud is
negative one:
U d ( E I d R) 0
and if
E U d 0 cosD
at S t D t 0.5S ,
i.e., E is more negative than in this case a negative EMF of rectifier U d 0 cosD . Case of
inverting exists, i.e., energy of DC source is transmitted to the AC network – DC to AC
conversion takes place in controlled rectifier. Processes are explained with diagrams on Fig.4.
In shape of instantaneous value diagrams of ud there must be certain intervals with positive
polarity which is necessary for forced turn-off action of previous conducting pair of thyristors.
These intervals must be longer as 20 electrical. If commutation is considered then maximum
length of delay angle is
D max S 20 J
where commutation angle depends on value of inductance L1 and value of current Id. If both
are larger then Ȗ is bigger too. It means that Įmax must be shortened at rising of Id.
Fig.4. Diagrams of voltages and currents at inverter operation regime