Power Electronics Rectifiers (Chapter 3) by Bakshi

 

Phase Controlled Rectifiers

______ (AC/DC Converters) Objectives • Principle of controlled rectification. • Single phase and 3 phase converters. • Half wave and full wave converters. semiconverter  • Bridge converters  — --------► semicoi *

ful fulll brid bridge ge convert converter  er 

• Resistive, inductive and motor (RLE) loads on converters. • Continuous and discontinuous output current operation and its effects. • Inver Invertin ting g operation (power flow from load to sou source) rce) in case o f full con convert verters ers.. • Effects of feedback diode and freewheeling operation. • Harmonic analysis of converters.

3.1

Introduction

3.1.1 Principle of AC/DC Conversion (Controlled Rectifier) • Controlled Contro lled rectifie rectifiers rs are basically AC to DC Controlled converters. The power rectifier 

transferred to the load is

controlled by controlling triggering angle of the devices. Fig. 3.1.1 shows this operation.

Load

a Control circuit

Fig. 3.1.1 Principle of operation of a controlled   rectifier 

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•

The trigg triggering ering angle ' a ’ of the device devicess is control controlled led by the control circuit.

•

The input to the controlled rectifie rectifierr is normally AC mains. The outp output ut of the controlled rectifier is adjustable DC voltage. Hence the power transferred across the load is regulated.

 A p p l i c at i o n s :

The controlled rectifiers are used in battery chargers, DC drives, DC power supplies etc. The controlled rectifiers can be single phase or three phase depending upon the load power requirement. 3.1.2 Concept of Commutation  A n s w er fol follow lowii n g quest questii on after r ead eadii ng thi th i s t opi c  1 . What do  you mean   by commut commutati ation on o f SCR ? Give types of   commutations. Explain natural commutation in details. Marks (6), (6) , May -20 07 I \  J

Mos ostt lik likely ely and masked masked In previous previous niversity niversi ty E xam

: Commutation is the collective operation, which turns of the conducting SCR. Definition

Commutation requires external conditions to be imposed in such a way that either current through SCR is reduced below holding current or voltage across it is reversed. There are two types of commutation techniques.

Fig. 3.1.2 •

Forced com m utation : It requires external components to store energy and it is

used to apply reverse voltage across the SCR or reduce anode current below holding current of the SCR to turn it off.

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Phase Phase Contro lled Rectifiers Rectifiers (AC/DC (AC/DC Conv erters)

Cu rrent com mu tation tation : The SCR is turned off by reducing its anode current

below holding current. •

Voltage com m utation : The SCR is turned off by applying large reverse voltage

across it. •

Principle of line comm utation

The natural commutation does not need any external components. It uses supply (mains) voltage for turning off the SCR. Hence it is also called as line commutation. •

Explanation

Mains AC

Fig. 3.1.3 A half wave rectifier uses natural c om m ut at i on t o t urn of f S CR

Fig. 3.1.3 shows the circuit using natural commutation. It is basically half wave rectifier. The mains AC supply is applied to the input. The SCR is triggered in the positive half cycle at a. Since the SCR is forward biased, it starts conducting and load current i0  starts flowing. The waveforms of currents and voltages are shown in Fig. 3.1.4. Since the load is resistive,

 j:::::  j: :::: :::: :::: :::: :::::::: I::: I:::::: ::::::: :::: ;,: ;,:::: ::: ::: :::;:: ;:: ::: :::;:: ;::::: ::: ::: :::::: ::: : :::::: ::::::::::: ::::: :

.........I...... ................. *.........   • ......... .

31113S HIS sstS sstS :!tH :3S !•t:S !•t:S!? !?:!: :!:

.

.

....

.

.

SH i !H9 5SI 5SI!! §8: HIE Bi:! HHi IS

:::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::: :::::::::::::::::::::: ::::::::::::::::::: ::::::::::::::::::: ::::::::::::::: :::::: I:::::::::::::::::;::::::::::::::::::::::::::::::::::::::::::::::::::::;:;::::-.;:

Fig. 3.1.4 Waveforms of half wave controlled rectifier to illustrate natural c om m ut at i on Copyrighted mate mate

 

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Phase Contro lled Rectifiers Rectifiers (AC/DC (AC/DC Conv erters)

Hence the shape of the output current is same as output voltage. Observe that the output current is basically SCR current. At 'rf the supply voltage is zero. Hence current through SCR becomes zero. Therefore the SCR turns off. The supply voltage is then negative. This voltage appears across the SCRs and it does not conduct. Thus natural commutation takes place without any external components. Here note that natural commutation takes place only when the supply voltage is AC. Thus the controlled rectifiers use natural commutation. 3. 3.1. 1.3 3

Forced Com m utatio n

3.1.3.1 Principle of Forced Commutation Forced commutation is used when the supply is D.C. A commutation circuit is connected across the SCR as shown in Fig. 3.1.5. The commutation circuit is normally LC circuit. The LC circuit stores energy when the SCR is ON. This energy is used to turn-off the SCR. The LC circuit imposes reverse bias across the SCR due to stored energy. Hence forward current of SCR is dropped below holding current and the SCR tums-off.

I

LC circuit

Fig. 3.1.5 Principle of forced   commutation

There are different types of forced commutation circuits depending upon the way they are connected.

3.1.3.2 Classification of Forced Commutation Forced commutation circuits can be classified depending upon whether voltage or current is used for commutation. Similarly the classification can be made based on whether the load resonates or commutation components are separate. Some times additional SCR is used for commutation main SCR. Such techniques are called auxiliary commutation methods. Based on these classifications following are some of the main commutation techniques : 1. Self commutation by resonating load aand nd LC circuit 2. Auxiliary v voltage oltage commutation (impulse commutation) 3. Auxiliary current commutation (resonant pulse commutation) 4. Complementary commutation 5.

External pulse commutation.

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Phase Contro lled Rectifiers Rectifiers (AC/DC (AC/DC Converters)

3.1.3.3 Comparison of Natural Commutation and Forced Commutation Table 3.1.1 shows the comparison between natural and forced commutation techniques. Sr. No.

Natural commutation

Forced commutation

1.

No external commutation components are

External commutation components are

2.

required. Requires AC voltage at the input.

required. Works on DC voltages at the input.

3.

Used in cont contro rolllle ed rec recti tifi fie ers rs,, AC volta oltag ge controllers etc.

Used in choppers, inverters etc.

4.

No power power loss loss takes takes place place during during com commu mutatio tation n

Power Power loss loss takes takes place place in com commu mutatin tating g components.

5.

SCR tur turns ns off off due due to neg egat ativ ive e sup suppl plyy volt volta age.

SCR can be be tur turne nedd-of offf du due e to to volt voltag age e an and current both.

6.

Cost of the commutation circuit is nil.

Cost of the commutation circuit is significant.

Table 3.1.1 Natural and forced commutation

3 .2 .2

Single Phase Half Wave Con verter and Effect of Free ew wh ee ell i n g   Diode

3.2.1 Single Phase Half Wave Controlled Rectifier with Resistive Load  A n s w er fo foll llow owii n g quest i on after r ead eadii ng thi th i s topi c  1. Explain the th e ope operatio ration n o f 1half wav wavee con conver verter ter wit withh the h elp o f  ci circ rcui uitt diagram and waveforms.

Most likely and

Important Question

The principle of phase controlled operation can be explained with the help of half wave controlled rectifier shown in Fig. 3.2.1. The secondary of the transformer is connected to resistive load through thyristor or SCR T y   The primary of the transformer is connected to the mains supply. In the positive cycle of the supply, Tj is forward biased. T{   is triggered at an angle a. This is also called as triggering or firing delay angle. Tj conducts and secondary (i.e. supply) voltage is applied to the load. Current i0  starts flowing through the load. The output current and voltage waveforms are shown in Fie. 3.2.2.

Fig. 3.2.1 Half wave controlled rectifier with   R-load.

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Phase Phase Contr olled Rectifiers Rectifiers (AC/DC (AC/DC Converters)

Since the load is resistive, output current is given as,

Hence the shape of output current waveform is same as output voltage waveform. At n  supply voltage drops to zero. Hence current i0  flowing through 7^ becomes zero and it turns off. In the negative half cycle of the supply Tj is reverse biased and it does not conduct. There is only one pulsve of V0  during one cycle of the supply. Hence ripple frequency of the output voltage is,  fripp  fri pple le =  50 Hz ‘‘-ee - supply frequency

 

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Mathematical analysis

The average value of output voltage is given as,

1 T 

Vo(av) = f ! vc

0

 M

dot

The period of one pulse of v 0 (cot)  can be considered as T = 2 n .   And v0 (cot) =Vm sin  cot  from a to jr. For rest of the period v0  (cof) = 0. Hence above equation can be written as, V o( av )

1 71

7T—

f Vm sin cot du> du>tt 

2 n J 

... (3.2.1) The power transferred to the load will be, V„U)

o( av )

R

Thus the output average voltage and power delivered by the controlled rectifier can be controlled by phase control (i.e. a). The phase control in converters means to control the delay (or triggering) angle a. 3.2.2 Half Wave Controlled Rectifier with RL Load Now let us study the operation of single phase half wave controlled rectifier for inductive (RL) load. Normally motors are inductive load. L is the armature or field coil inductance and R is the resistance of these coils. Fig. 3.2.3 shows the circuit diagram of half wave controlled rectifier with RL load.

. . . .

Fig. 3.2.3 3.2.3 Half w ave co nt ro lled rec tif ier with RL load

The SCR will be forward biased in the positive half cycle of the supply. Hence SCR is applied with the firing pulses in the po posi siti tive ve ha half lf cycle. The wave wavefforms orms are 00 x u iU shown in Fig. 3.2.4. Fig. 3.2.4(a) shows the supply voltage and Fig. 3.2.4(b) shows the

firing pulses to the SCR.

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Phase Contr olled Rectifiers (AC/DC (AC/DC Converters)

Fig. 3.2.4 Waveforms of half wave controlled rectifier for RL load

When the SCR is triggered, the supply voltage appears across load. We normally neglect small voltage drop in SCR. Hence v0 =vs  when SCR is conducting. This is shown in Fig. 3.2.4(c). Observe that output voltage is same as supply voltage after a. Because of the RL load, output current starts increasing slowly from zero. The shape of i0  depends upon values of R and L. At n  , the supply voltage becomes zero and i0  is maximum. Due to negative supply voltage after n ,   SCR tries to turn-off. But energy stored in the load inductance induc tance ge generat nerates es the voltag voltagee L - ~ . This indu induced ced volt voltage age for forwar ward d biases the SCR and maintains it in conduction. This is shown in Fig. 3.2.5. The basic property of inductance is that it opposes change iin n current. At n ,   the current i0  is maximum. As SCR tries to turn-off due to negative supply voltage, the output current i0  tries to go to zero. Such change in i0  is opposed by load inductance. Hence the energy stored in an inductance tries to maintain i0.  To maintain the flow of i0,  inductance generates the voltage

with

polarity as shown in Fig. 3. 3.2.5. 2.5. This voltage is higher than negative supply voltage. voltage. Hence Tj is forward biased and it remains in conduction. The output current and supply current

 

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Phase Phase Contro lled Rectifiers Rectifiers (AC/DC (AC/DC Converters)

flow in the same loop. Hence i0 =is  all the time. The waveform of i0  is shown in Fig. 3.2.4(d) and is  is shown in Fig. 3.2.4 (e). After 7 1 , i0  (i.e. is )  flows against the supply. Hence energy is consumed in the supply. i0  flows due to load inductance energy. In other words, the inductance energy is partially fed to the mains and to the load it self. Therefore energy stored in inductance goes on reducing. Hence i0  also goes on reducing Fig. 3.2.5 SCR conducts due to in as shown in Fig. 3.2.4 (d). At P the energy stored ductance voltage after n in the inductance is finished. Hence i0  goes to zero. Therefore T. tums tums-off. -off. In Fig Fig.. 3.2.4(c) observe that v0  is negative from n  to to p. Because Tj conducts from n  to p . Hence whenever Tj conducts v0 =vs. The SCR is triggered again at 2 71 + a. Hence outpu outputt v voltage oltage remain remainss zero from p to 271+eex. Outpu 271+ Outputt curren currentt as well as ssupply upply current are also zero from p to 2? i+ a. At 2n + a r Tj is triggered again and the cycle repeats. Here i0  goes to zero at p. Hence this is called discontinuous conduction.

»>■► Example 3.2.1 : Derive an expression for average value of output voltage for 1  = 10 A'

« = 60° or |

i) RMS supply current

ln-a ) v 71 71 =  10 * " 3 1 71 8.165 A ii) Output voltage

 V„  V„o(av) bridge (Full) conv converter erter with resistive load. In the positive half cycle of the supply SCRs Tj and T2  are triggered at firing.angle a. Hence current starts flowing through the load. The equivalent circuit for this operation is shown in Fig. 3.4.2. It is clear from Fig. 3.4.2 that, when T{   and conducts, V0 = Vs  (i.e. sup supply ply voltage) V a nd nd ,

'» = i f

... (3.4.1)

V  = T

- ( 3 A2 A2 )

Fig. 3.4.3 shows the waveforms of this circuit. Observe that load voltage is same as supply voltage from a to n.   Since the load is resistive, waveforms of V0  and i0  are same. The supply current i$  and i0  are in the same direction hence i$ =i0. T]  and T-, turn off when supply voltage becomes zero at n.  In the negative half cycle T3 and T4 are triggered at 7c+a.

Fig. 3.4.4 shows the equivalent circuit when T3 and T4 conduct. In the adjacent figure observe that supply current is  and load current i0  flow through the same loop. But directions of i$  and i0  are opposite hence h  =

-*o

The supply current waveform is also shown in Fig. 3.4.3. T3 and T4 turn off when supply voltage becomes zero at 2 k   . At 2 k  + a,   Tj and T2 are triggered again and the cycle repeats.

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Phase Con tro lled Rectifiers (AC/DC (AC/DC Con verters )

 

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Phase Contr oll ed Rectifi ers (AC/DC (AC/DC Converters)

Solution : This is a fully controlled bridge with resistive load of 100 Q in series with the battery of 50 V. Hence output voltage of the converter appears across resistance of 100 Q  and battery of 50 V. Hence let us first calculate average value of output voltage. The given data is,

a = 30° Vs

= 220   V

/.

Vm  =

220V2

The average output voltage for resistive load is given by equation 3.4.3 as,

Vo(av)  =

~ < 1+ C ° S « )

=

- ( 1 + cos cos 30°) 30°) 71

= 184.8 V This voltage is applied to the load. Fig. 3.4.6 shows the equivalent circuit.

By applying KVL to above circuit, Vo(av) =  '«,(, '«,(,»)* »)* + 50 184.8 = 'fu ll c on ve rter hav ing res istive load

 

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The SCR pai pairr T6 -T j condu conduct ct fr from om during thi thiss per period. iod. At

Phase Phase Controlled Rectifiers Rectifiers (AC/DC AC/DC Converters)

+ txj to ^

+ a j . Line Line vol voltag tagee VRY   is applied

+ a j SCR T2  is triggered (Fig. 3.6.2 (c)). Here note that Te

tums-off, since T2 is triggered. Hence Tj -T2  starts conducting and it is marked as interval-II. In this interval supply line voltage VR VRB B  is appli applied ed acro across ss the loa load. d. At

+ aj ,

T3 is triggered. Hence Tj tum s-of s-offf and T2  -T3 starts conducting. Therefore line voltage VYB  is applied across the load. It is marked as interval-III. Load and supply currents

Fig. 3.6.4 Output current and supply current waveforms for a   = 30°

•

Since the load is resistive, the shape of outp output ut current curren t waveform will be similar to that of output voltage. Its amplitude will be i0  =

•

When Whenever ever Tj cond conducts, ucts, R-phas R-phasee curren currentt will be positive and whenev whenever er T4 conducts, R-phase current will be negative.

The following points are important about 3full converter operation.

i) Only two SCRs conduct in any interval. ii) Each SCR conduct for 120°. iii) Each SCR pair conduct for one interval of 60°. iv) SCRs are triggered in following sequence : ......... T’l - h - h   -T4 -T5 -T6  -T, -T2 ............

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Phase Contr olled Rectifiers (AC/DC (AC/DC Conv erters)

= 4 iv vh = 4 i - ^ v Here firing angle is 45°. Hence the conduction will be continuous for resistive as well as inductive load. Therefore the average DC output is given by equation 3.6.2 i.e., VV)

3>/3 VL = -T ^ co sa

Putting values in above equation , 400

3 V 3 x - J 2  ■

V*»)

= --------- ^

V3 cos 45°

= 382 volts

3.6 3. 6.2 Operation Opera tion wit with h Highly Inductive Induc tive Load Load  An  A n sw er fo ll lloo w in g q u e s t io n a ft e r re ad in g th is t o p ic

i

1. Draw the circ circuit uit diagram o f 3 ^ full conv converte erterr wi with th a highly highly inductivee load. Explain iitt's workin inductiv workingg an andd draw th thee lload oad voltag voltagee, K load current and current through SCR waveforms for rectification  I y.

^ revious

Let us consider con sider the o operation peration of 3 full conve converter rter with highly inductiv inductivee load. The output current will be continuous and ripplefree. In the waveforms of Fig. 3.6.2, observe that voltage waveform is continuous till a =60°. But with inductive load, voltage waveform is continuous continu ous for any value o off a. Fig. 3.6.5 show s the wave waveforms forms o off 3 4>full co converte nverterr for highly inductive load. Fig. 3.6.5 (c) shows the output voltage waveform for a =60°. Observe that this waveform is same as that of resistive load shown in Fig. 3.6.2 (e). Fig. 3.6.5 (d) shows the continuous and ripplefree output current. Fig. 3.6.5 (e) shows supply phase current waveforms iR , iY   and iB.  Observe that the R-phase current is positive whenever Tj conducts and it is negative whenever T4 conducts. All the three current waveforms are of the same nature (quasi square wave) having 120° phase shift with respect to each other. Fig. 3.6.5 (f) shows the outp Fig. output ut voltage waveform for a = 90°. 90°. The waveform goes negative for same period, because of inductive load. The load inductance generates a large voltage to maintain the load current in the same direction. Hence SCRs continue to conduct and load voltage becomes negative occasionally. Note that there is no freewheeling in full converter.

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Phase Phase Cont rol led Rectifiers (AC/DC (AC/DC Converters )

Fig. 3.6 3.6.5 .5 Wa veform s o f 3 3< 90°.

•

The wavefo waveforms rms of inverting operation are shown in F Fig. ig. 3.6.6 (b) (b) for a =120°. The inversion takes place and average output voltage is negative. Exam ple 3.6 3.6.3 .3 : Derive an expression for average output voltage of   3 fu ll converter  

having highly inductive load. Solution : In the Fig. 3. 3.6.2 6.2 observe that ou output tput volt voltage age wave waveform form is con continuous tinuous for complete range of a. Hence single expression can be derived. In Fig. 3.6.2 (d), observe that one ripple period of output voltage can be, 71

K  

2 + a

6 + “ =3

\

71

During this period line voltage VRY   is applied acro across ss the load. From Fig. 3.6.3 (b), VRY   will be, RY 

V3 Vm sin

Here Vm  is the peak value of supply phase voltage.

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Phase Phase Contro lled Rectifiers (AC/ (AC/DC Converters)

The average output voltage is given as, T 

V.o(av) ■j ^73

2

I

n 

VRY  VR Y  

(“0

6+a

71

2 +“ rfcof

71

3V3 V„

3V3 V.,

COS  (Of +

fl

r a

cos a

(3.6.4)

This equation holds for complete range of a. Example 3.6.4 :  A 3 fu ll converter operated fro from m   3 = a i +a 2 = 1 8 0 ° ■cir

2V l ® rl c0 c0 ssw w ,, - c 0 ss““ i j i  =S

□ □ □

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