MAHALAKSHMI ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

QUESTION BANK

SEMESTER : V DEPARTMENT: EEE

SUBJECT NAME: POWER ELECTRONICS SUBJECT CODE: EE2301

UNIT 2 – PHASE CONTROLLED RECTIFIERS

PART A(2 Marks)

1. What is displacement factor for two pulse converter?(AUC MAY 13)

It is defined as cosine of displacement angle. Df=cosα

2. What is circuit turn off time of single phase converter? (AUC MAY 13)

Circuit turn off time for single phase full converter is defined as time duration from

the start of SCR reverse biasing of outgoing SCR to the forward biasing of incoming

SCR

3. Why is the power factor of semi converter better than full converter? (AUC NOV 12)

For supplying given load, the semi converter receives less reactive power due to

freewheeling action when compared with full converter. This freewheeling action is

not present in full converter.

4. What is inversion mode of rectifiers? (AUC NOV 12, MAY 12 )

For firing angle, α>90o the voltage at the load terminal is negative and as a result

the power flows from the load to the source i.e., from dc side to ac side and the

converter behaves as inverter. Such type of inverter is called line commutated

inverter.

5. Define Total Harmonic Distortion (THD). (AUC MAY 12)

The harmonic factor of the input current is defined as,

Hf=((IS2-IS1

2)/IS12)1/2

6. Give any two difference between Semi converter & Full converter. (AUC NOV 11)

S.NO Semi Converter Full Converter

1 It is a mixture of diodes &thyristors It uses only thyristors

2 There is a limited control over the There is wider control over the level of

level of dc output voltage dc output voltage

3 One quadrant converter, one

polarity of dc output voltage

Two quadrant converter, voltage

polarity can reverse

PART B (16, 8 marks)

1. Explain single phase bridge rectifier with neat circuit and waveform.(AUC MAY13,MAY 12,

NOV11)

Single phase full wave Bridge converters:

A single phase full converter using four SCRs is shown in the figure. The load is assumed

to be RLE type, where E is the load circuit EMF. Voltage E may be due to a battery in the

load circuit or may be generated EMF of a DC motor. Thyristors T1 & T2 is

simultaneously triggered and π radians later, pair T3 & T4 is gated together. When a is

positive w.r.t b, supply voltage waveform is vab. When b is positive w.r.t., a, supply

voltage waveform is dotted vba. Vab= - vba. The current direction and voltage polarities

are treated as positive.

Load current or O/P current io is assumed continuous over the working range, this means

that the load is continuously connected to the ac voltage source through the thyristors.

CONVERTER OPERATION

Between ωt=0 & ωt= α, T1, T2 are forward biased through already conducting SCRs

T3,T4 and block the forward voltage. For continuous current, thyristors T3, T4 conduct

after ωt=0 even though these reverse biased. When forward biased SCRs T1, T2 are

triggered at ωt=α, they get turned on. As a result the supply voltage Vmsinα immediately

appears across T3, T4 as a reverse bias, these are turned off due to natural or line

commutation.

At the same time, load current io flowing through T3, T4 is transferred to T1,T2 at ωt=α.

Note that even when T1,T2 are gated at ωt=α, these SCRs will turn on only when

Vmsinα>E. T1,T2 conduct from ωt=α to ωt=π+α. At ωt=π+α, forward biased SCRs T3,T4

are triggered. The supply voltage turns off T1, T2 by natural or line commutation and the

load current is transferred from T1, T2 to T3,T4.

INVERTER OPERATION

Voltage across T1,T2 is shown as vT1 & vT2 and that across T3, T4 as vT3 & vT4.

Maximum reverse voltage across T1, T2, T3, T4 is Vm and the instant of triggering with

firing angle α, each SCR is subjected to a reverse voltage of Vmsinα. Source current is is

treated as positive in the arrow direction. Under this assumption, source current is

positive when T1, T2 are conducting and negative when T3, T4 are conducting.

During α to π, both vs andisare positive, power flows form ac source to load. During

interval π to (π+α), vs is negative but is is positive, the load therefore returns some of its

energy to the supply system. But the net power flow is from ac source tto dc load because

(π-α)>α.

The terminal voltage Vo is given by,

Vo =(1/π) = (2Vm/π) cosα.

The above equation shows that if α>90o, Vo is negative. This is illustrated in above

figure, where α>90o.

In this figure , the average terminal voltage Vo is negative. If the load circuit EMF is

reversed, this source E will feed power back to ac supply. This operation of full converter

is known as inverter operation of the converter. The full converter with firing angle

delay greater than 90o is called a line commutated inverter. Such an operation is used

in regenerative braking mode of a dc motor in which case then Eis the counter EMF of dc

motor.

During 0 to α, ac source vs is positive, but ac source current is negative. Power flows

from dc source to ac source. From α to π, both vs& is are positive, power flows from ac

source to dc source. But the net power flow is from dc source to ac source, because

(π-α)<α in the figure.

In converter operation, the average value of O/P voltage Vo must be greater than the load

circuit EMF, E. during inverter operation, load circuit EMF when inverted to ac must be

more than ac supply voltage or dc source voltage E must be more than the inverter

voltage Vo, only then power would flow from dc source to ac supply system. Both in

converter and inverter modes, SCRs must be forward biased and current through the m

must flow in the same direction as these are unidirectional devices.

The variation of voltage across thyristors T1, T2, T3 or T4 reveals that the circuit turn off

time for both converter & inverter operation is tc=(π-α)/ω.

2. A single phase bridge converter is utilized to produce regulated DC output voltage.

The input voltage is 230V and the load current is 8A for firing angle of 30 degree.

(i) calculate the dc output voltage.

(ii) calculate the dc output voltage and currents if a freewheeling diode is used at the

output for the same firing angle.

Vdc=(2Vm/π)cosα = 179.3V

Vdc=(Vm/π)(1+cosα)=193.20V

Idc=Vdc/R=6.72A

3. A single phase two pulse bridge converter feeds power to RLE load with R=6 ohms, E=60V,

AC source is 230V, 50 Hz. Find the average value of load current for a firing angle of 50 degree.

In case one of the 4 SCRs gets open circuited, find the new value of average load current

assuming the output current as continuous.

Vdc=(2Vm/π)cosα = 12.181V

Idc=(Vdc-E)/R=12.181A

When one SCR is open circuited,

Vdc=(Vm/π)cosα=66.54A

Idc=(Vdc-E)/R=1.09A

4. Explain the operation of three phase full converters(AUC NOV 11).

The three phase fully controlled bridge converter has been probably the most widely used power

electronic converter in the medium to high power applications. Three phase circuits are

preferable when large power is involved. The controlled rectifier can provide controllable out put

dc voltage in a single unit instead of a three phase autotransformer and a diode bridge rectifier.

The controlled rectifier is obtained by replacing the diodes of the uncontrolled rectifier with

thyristors. Control over the output dc voltage is obtained by controlling the conduction interval

of each thyristor. This method is known as phase control and converters are also called “phase

controlled converters”. Since thyristors can block voltage in both directions it is possible to

reverse the polarity of the output dc voltage and hence feed power back to the ac supply from the

dc side. Under such condition the converter is said to be operating in the “inverting mode”. The

thyristors in the converter circuit are commutated with the help of the supply voltage in the

rectifying mode of operation and are known as “Line commutated converter”. The same circuit

while operating in the inverter mode requires load side counter emf. for commutation and are

referred to as the “Load commutated inverter”. In phase controlled rectifiers though the output voltage can be varied continuously the load

harmonic voltage increases considerably as the average value goes down. Of course the magnitude of harmonic voltage is lower in three phase converter compared to the single phase

circuit. Since the frequency of the harmonic voltage is higher smaller load inductance leads to

continuous conduction. Input current wave shape become rectangular and contain 5th

and higher

order odd harmonics. The displacement angle of the input current increases with firing angle.

The frequency of the harmonic voltage and current can be increased by increasing the pulse number of the converter which can be achieved by series and parallel connection of basic 6 pulse

converters. The control circuit become considerably complicated and the use of coupling transformer and / or interphase reactors become mandatory. With the introduction of high power IGBTs the three phase bridge converter has all but been

replaced by dc link voltage source converters in the medium to moderately high power range.

However in very high power application (such as HV dc transmission system, cycloconverter

drives, load commutated inverter synchronous motor drives, static scherbius drives etc.) the basic

B phase bridge converter block is still used. In this lesson the operating principle and

characteristic of this very important converter topology will be discussed in source depth.

Operating principle of 3 phase fully controlled bridge converter A three phase fully controlled converter is obtained by replacing all the six diodes of an

uncontrolled converter by six thyristors as shown in Fig. 13.1 (a)

For any current to flow in the load at least one device from the top group (T1, T3, T5) and one

from the bottom group (T2, T4, T6) must conduct. It can be argued as in the case of an

uncontrolled converter only one device from these two groups will conduct. Then from symmetry consideration it can be argued that each thyristor conducts for 120° of the

input cycle. Now the thyristors are fired in the sequence T1→ T2→ T3→ T4→ T5→ T6→ T1

with 60° interval between each firing. Therefore thyristors on the same phase leg are fired at an interval of 180° and hence can not conduct simultaneously. This leaves only six possible conduction mode for the converter in the continuous conduction mode of operation. These are

T1T2, T2T3, T3T4, T4T5, T5T6, T6T1. Each conduction mode is of 60° duration and appears in the sequence mentioned. The conduction table of Fig. 13.1 (b) shows voltage across different devices and the dc output voltage for each conduction interval. The phasor diagram of the line voltages appear in Fig. 13.1 (c). Each of these line voltages can be associated with the firing of a

thyristor with the help of the conduction table-1. For example the thyristor T1 is fired at the end

of T5 T6 conduction interval. During this period the voltage across T1 was vac. Therefore T1 is

fired α angle after the positive going zero crossing of vac. Similar observation can be made about other thyristors. The phasor diagram of Fig. 13.1 (c) also confirms that all the thyristors are fired in the correct sequence with 60° interval between each firing. Fig. 13.2 shows the waveforms of different variables (shown in Fig. 13.1 (a)). To arrive at the

waveforms it is necessary to draw the conduction diagram which shows the interval of

conduction for each thyristor and can be drawn with the help of the phasor diagram of fig. 13.1 (c). If the converter firing angle is α each thyristor is fired “α” angle after the positive going zero crossing of the line voltage with which it’s firing is associated. Once the conduction diagram is drawn all other voltage waveforms can be drawn from the line voltage waveforms and from the conduction table of fig. 13.1 (b). Similarly line currents can be drawn from the output current and the conduction diagram. It is clear from the waveforms that output voltage and current waveforms are periodic over one sixth of the input cycle. Therefore this converter is also called the “six pulse” converter. The input current on the other hand contains only odds harmonics of

the input frequency other than the triplex (3rd

, 9th

etc.) harmonics. The next section will analyze

the operation of this converter in more details.