MAHALAKSHMI ENGINEERING COLLEGE

TIRUCHIRAPALLI-621213.

QUESTION BANK

SEMESTER : V DEPARTMENT: EEE

SUBJECT NAME: POWER ELECTRONICS SUBJECT CODE: EE2301

UNIT V – AC to AC CONVERTERS

PART A – 2 marks

1. What is meant by cycloconverter (AUC NOV10, MAY 12)

Cycloconverter converts input power at one frequency to output power t different

frequency with one stage conversion.

2. Write the o/p RMS voltage for single phase AC voltage controller with resistance

load. (AUC NOV10)

Vo=Vs[1/π(π-α+sin 2α/2)]1/2

3. What is a matrix converter. ( AUC MAY11)

4. Matrix converter is capable of direct conversion from AC to AC using bi

directional fully controlled switches.

5. List out the different types of cycloconverter. (AUC MAY 11)

Single phase cycloconverters

Three phase to three phase cycloconverters

Three phase to single phase cycloconverters

6. What are the applications of cycloconverter? (AUC NOV 11)

Speed control of high power ac drives

Induction heating

Static VAR compensation

For converting variable speed alternating voltage to constant frequency

output voltage for the use as a power supply in aircraft or shipboards.

7. What is integral cycle control? (AUC NOV 11, MAY12)

Integral cycle control is a method to remove portion of full cycles/ one cycle of ac

signal for controlling ac power across the loads. Thyristors are connected as

switches to connect the load circuit to the source for a few cycles of the same

voltage and disconnected for another few cycles.

8. Write the principle of operation of cycloconverter.(AUC NOV12)

A cycloconverter is a frequency changer that converts ac power at one frequency

to ac power at different frequency without any intermediate dc link.

Ac power at one frequencycycloconverterAc power at different frequency

9. Write any two important applications of AC voltage controllers. (AUC NOV12)

Domestic & industrial heating.

Transformer tap changing

Lighting control.

Starting of induction motor

10. What are the types of ac voltage controllers? (AUC MAY 13)

Single phase ac voltage controllers.

Three phase ac voltage controllers.

11. What does ac voltage controller mean?

It is device which converts fixed alternating voltage into a variable voltage

without change in frequency.

12. What are the applications of ac voltage controllers?

a. Domestic and industrial heating

b. Lighting control

c. Speed control of single phase and three phase ac motors

d. Transformer tap changing

13. What are the advantages of ac voltage controllers?

a. High efficiency

b. Flexibility in control

c. Less maintenance

14. What are the disadvantages of ac voltage controllers?

The main draw back is the introduction of harmonics in the supply current and the

load voltage waveforms particularly at low output voltages.

15. What are the two methods of control in ac voltage controllers?

a. ON-OFF control

b. Phase control

16. What is the difference between ON-OFF control and phase control?

ON-OFF control: In this method, the thyristors are employed as

switches to connect the load circuit to the source for a few cycles of the load voltage

and

disconnect it for another few cycles. Phase control: In this method, thyristor switches

connect the load to the ac source for a portion of each half cycle o f input voltage.

17. What is the advantage of ON-OFF control?

Due to zero-voltage and zero current switching of thyristors, the harmonics

generated by the switching action are reduced.

18. What is the disadvantage of ON-OFF control?

This type of control is applicable in systems that have high mechanical inertia and

high thermal time constant.

19. What is the duty cycle in ON-OFF control method?

Duty cycle K = n/ (n + m), where n = no. of ON cycles, m = no. of OFF cycles.

20. What is meant by unidirectional or half-wave ac voltage controller?

Here the power flow is controlled only during the positive half-cycle of the input

voltage.

21. What are the disadvantages of unidirectional or half-wave ac voltage controller?

a. Due to the presence of diode on the circuit, the control range is limited and the

effective RMS output voltage can be varied between 70.7% and 100%.

b. The input current and output voltage are asymmetrical and contain a dc

component. If there is an input transformer, saturation problem will occur

c. It is only used for low power resistive load.

22. What is meant by bidirectional or half-wav e ac vo ltage controller?

Here the power flow is controlled during both cycles of the input voltage.

23. What type of gating signal is used in single phase ac voltage controller with RL

load?

High frequen cy carrier gating signal is used for single ph ase ac voltage controller

with RL load.

24. What are the disadvantages of continuous gating signal?

a. More heating of the SCR gate.

b. Increases the size of pulse transformer.

25. What is meant by sequence control of ac voltage regulators?

It means that the stages of voltage controllers in parallel triggered in a proper

sequence one after the other so as to obtain a variable output with low harmonic

content.

26. What are the advantages of sequence control of ac voltage regulators?

a. System power factor is improved.

b. Harmonics are reduced in the source current and the load voltage.

27. What is meant by cyclo-converter?

It conv erts input power at one frequency to output power at another frequency

with one-stage conv ersion. Cycloconverter is also known as frequency changer.

28. What are the two types of cyclo-converters?

a. Step-up cyclo-conv erters

b. Step-down cyclo-converters

29. What is meant by step-up cyclo-converters?

In these converters, the output frequency is less than the supply frequency.

30. What is meant by step-down cyclo-converters?

In these converters, the output frequency is more than the supply frequency.

31. What are the applications of cyclo-converter?

a. Induction heating

b. Speed control of high power ac drives

c. Static VAR generation

d. Power supply in aircraft or ship boards

32. What is meant by positive converter group in a cycloconverter?

The part of the cycloconverter circuit that permits the flow of current during

positive half cycle of output current is called positive converter group.

33.What is meant by negative converter group in a cycloconverter?

The part of the cycloconverter circuit that permits the flow of current during negative half

cycle of output current is called negative converter group.

PART B(8 & 16 marks)

1. Describe the three phase to three phase cycloconverter with relevant circuit

diagram using 18 thyristors. (AUC NOV10)

Three-phase to Three-phase Cyclo-converter

The circuit of a three-phase to three-phase cyclo-converter is shown in Fig. 31.1.

Two three- phase half-wave (three-pulse) converters connected back to back for

each phase, with three thyristors for each bridge, are needed here. The total

number of thyristors used is 18, thus reducing the cost of power components,

and also of control circuits needed to generate the firing pulses for the thyristors,

as described later. This may be compared to the case with 6 (six) three- phase

full-wave (6-pulse) bridge converters, having six thyristors for each converter,

with total devices used being 36. Though this will reduce the harmonic content in

both output voltage and current waveforms, but is more costly. This may be

used, where the total cost may be justified, along with the merit stated. This has

also been discussed in the last section of the previous lesson (#30). The ripple

frequency is 150 Hz, three times the input frequency of 50 Hz. In Fig. 31.1, the

circulating current mode of operation is used, in which both (positive and

negative) converters in each phase, conduct at the same time. Inter-group

reactor in each phase as shown, is needed here. But, if non-circulating current

mode of operation is used, where only one converter (positive or negative) in

each phase, conducts at a time, the reactors are not needed.

the circuit, and the operation, in brief, of the three-phase to three-phase cyclo-

converter, is described. Six three-phase half-wave converters are used in this

case, with two converters, connected back to back, per phase. A total of 18

thyristors are needed as power switching devices, having three thyristors for

each converter. Lastly, the analysis of the output waveform for the cyclo-

converter is presented. The procedure for obtaining the expression for the output

voltage (rms) per phase for cyclo-converter is described

2. Show that the fundamental RMS value of per phase output voltage of low

frequency for a m pulse cycloconverter is given by π πm n E Epn or sin.

(AUC NOV10)

3. Explain the working of multisatage sequential control of AC voltage controller.

(AUC MAY 11)

Sequence control of AC voltage regualtors:

Sequence contro, of ac voltage regualttors are used for reduction of harmonics

and the improvement of system power factor in the input current & the output

voltage. Sequence control of ac regulators means the use of two or more stages

of voltage controllers in parallel for the regulation of output voltage. The

sequence control of ac voltage controllers can be used as voltage controllers in

supply systems & for the speed control of induction motors. These types of

controllers are known as synchronous tap changers or transformer tap changers.

Single phase transformer connection changes:

Thyristors are used as static switches for on load changing of transformer

connections.

Static connections changers have the advantage of very fast switching

action 7 the change over can be controlled to cope with the load condition

& is smooth.

The turns ratio of the input transformer are such that if the primary

instantaneous voltage is

Secondary instantaneous voltages are,

When thyristors T3 & T4 are alternately fired with delay angle of α=0, the

load voltage is Vo=V1.

If full output voltage is required, thyristors T1 & T2 are alternately fired with

delay angle of α=0 and full vltage I Vo=V1+V2.

The gating pulse of thyristors can be controlled to vary the load voltage.

The RMS value of load voltage Vo can be varied within three possible

ranges.

0 < Vo < V1, 0 < Vo < (V1+V2), V1 < Vo < (V1+V2)

Case1: 0 < Vo < V1

To vary the RMS voltage within this range, T1 & T2 are turned off. T3 & T4 can

be operated as a single phase ac voltage regulator. The RMS load voltage is

given by, Vo=V1[1/π(π-α+(sin2α/2))]1/2 and the firing angle range is 0<α<π.

Case2: 0 < Vo < (V1+V2)

T3 & T4 are turned off. T1 & T2 operate as a single phase ac voltage regulator,

the load voltage is Vo=(V1+V2)[1/π(π-α+(sin2α/2))]1/2 .

Case3: V1 < Vo < (V1+V2)

T3 is turned on at ωt=0 and the secondary voltage V1 appears across the load. If

T1 is turned on at ωt=α, T3 is reverse biased due to secondary voltage V2 & T3

is turned off. The voltage across the load is (V1+V2). At ωt=π, T1 is self

commutated & T4 is turned on. The secondary voltage V1 appears across the

load until T1 is fired ωt=π+α, t4 is turned off due to reverse voltage V2 and the

load voltage is (V1+V2). At ωt=2π, T2 is self commutated, T3 is turned on again

the cycle is repeated. This type of controller is also czlled as synchronous tap

changer.

The RMS load voltage can be found from,

Vo=

Multistage sequence control:

The number of stages used in 2 stage control can be increased for harmonic and

power factor improvement.

The transformer has n secondary windings. Each secondary is rated for Vs/n,

where Vs is the source voltage. The voltage of node p with respect to K is Vs.the

load voltage at terminal Q is (n-1)V3/n and so on. If voltage controlfrom Vsk=(n-

3)Vs/n to Vrk=(n-2)Vs/n is required, then SCR pair $ is triggerer at α=0 and firing

angle of SCR pair3 is controlled from α= 0 to 180 and all other SCRs are kept off.

Similarly for controlling the voltage from Vqk=(n-1)Vs/n to Vpk=Vs, SCR pair 2 is

triggered at α=0, whereas for SCR pair 1, firing angle is varied from 0 to 180

keeping the remaining (n-2) SCR pairs off. Thus the load voltage can be varied

from Vs/n to Vs by an appropriate control of triggering the adjacent SCR pairs.

The presence of harmonics in the output voltage depends upon the voltage

variation. If this voltage variation is a small fraction of the total output voltage, the

harmonic content in the output voltage is also small.

4. Explain the principle of single phase to single phase step down cycloconverter

with power circuit & waveforms. (AUC MAY 11)

Single phase to single phase step down cycloconverter:

In a single phase cycloconverter whose input & output voltage are single phase

ac, the input ac voltage of supply frequency 50 Hz is converted into lower

frequency ac output. The two configurations of this type are,

Centre tapped (midpoint) transformer configuration

Bridge type cycloconverter

Centre tapped transformer configuration

In the power circuit there are four thyristors P1, N1, P2, N2. Ot of four thyristors

P1 & P2 are responsible for generating the positive group. The other two

thyristors N1 & N2 are responsible for producing negative halves forming the

negative group. This configuration generates 1/3 of the input frequency.

Depending upon the polaritis of points B&C of the transformer, SCRs are gated.

Natural or line commutation is used for turning off the SCRs. This circuit

configuration can be analyzed for purely resistive load.

During the first +ve half cycle 0 to π, when point B is +ve and point C is –ve.

SCR P1 being in conducting mode is gated. The current flows through +ve point

B, SCR P1, load and –ve point O.

In the –ve half cycle π to 2π, point C is +ve & point Bis –ve, P1 is automatically

turned off by line commutation and P2 is triggered simultaneously. Current flow is

from point C, SCR P2, load and – ve point O. direction of current flow remains

same as in +ve half cycle.

Next moment again point B becomes +ve and point C becomes –ve. Thus P2 is

automatically line commutated. P1 is gated simultaneously. The current path

becomes as in previous case when P1 was conducting. Thus the direction of

current flow through the load remains in all the three half cycle, the three +ve half

cycles are obtained across the load to produce one combined +ve half cycle as

output.

Similarly in next half cycle of the ac input , when point C is again +ve & point B is

–ve, P1 is switched off. Now instead of P2, N1 is forward biased and is gated.

The path for current flow will be from point C, load, N1 and back to –ve point B.

thus the direction of current flow through the load is reversed.

In the next +ve half cycle, point B is +ve & point C is negative. N1 is automatically

turned off. N2 whicch is in conducting mode is simultaneously turned on. The

current flow path becomes +ve point O, load, N2 to the –ve point C. the directiojj

of current flow through the load remains the same.

For the next half cycle of the ac input when the point C is +ve and point B is –ve.

N2 is automatically switched off and N1 turned on. The current flow through the

load again remains in the same direction.

The circuit produces one half cycle at the output by combining three negative

halves of the input so three cycles of the input is conbined o produce one cycle at

the output. This clearly indicates that the input frequency 50 Hz is reduced to

1/3(16*2/3)Hz. The input & output waveforms are shown

Bridge Type cycloconverter:

The circuit shows step down bridge type cycloconverter. Here two single phase

fully controlled bridges are connected in opposite directions. Bridge 1- +ve group

converter supplies load current in the +ve half of the output cycle and bridge 2

-ve group converter supplies load current in the negative half of the output cycle.

The two bridges should not conduct togethernas this will produce a short circuit

at the output. The input & output voltage waveforms are shown.

During the +ve half cycle 0 to π SCR P1 & P2 are forward biased and is triggered

at ωt=α. Then P1 & P2 are on state & the output is positive. The current flows

from B-P1-R-P2-C. at ωt=π, P1 & P2 are turned off.

During –ve half of the cycle π to 2π SCR P3 & P4 are forward biased and is

triggered at ωt=π+α. Then P3 & P4 are in on state. Again the output voltage &

current is positive. Current flow is through C-P3-R-P4-B. at ωt=2π , SCR p3 & P4

are turned off due to natural commutation.

Now bridge 2 can be operated and the output is negative. During +ve half cycle

2π to 3π, SCR N1 & N2 are forward biased. It is triggered at ωt=2π+2. Then it

comes to on state. The current flows through B-N1-R-N2-C. the output voltage &

current is negative. At ωt=3π SCR N1 & N2 are turned off due to natural

commutation.

During negative half cycle 3π to 4π, SCR N3 & N4 are forward biased. It is

triggered at ωt=3π+α. Then it comes to on state. The current flows through C-

N3-R-N4-B. now negative voltage & current is got as the output. At ωt=4π, SCR

N3 & N4 are turned off due to natural commutation.

Now bridge 1 can be operated to get the positive output voltage. This cycle is

repeated. The below figure shows the output waveform for fo=fs/3 and fs/4

5. Discuss the working of 2 stage sequence control of ac voltage controller (AUC

NOV11).

Same as Q3 in part B

6. Discuss the working of 3 phase to single phase cyclo converter with neat voltage

& current waveforms.(AUC NOV11, MAY12, NOV12)

Three-phase to Single-phase Cyclo-converter

Circulating Current Mode of Operation

In all the cases described earlier both for single-phase to single-phase and for

single-phase to three-phase cyclo-converters, circulating current-free or non-

circulating current mode of operation was described, wherein only one of two

bridge converters conducts at a time, but not both, in which case the converters

would be short- circuited. The positive (P) converter conducts, when the current

is in the positive half of the cycle, whereas the negative one conducts with the

current flowing in the negative half. But, in this case, i.e. circulating current mode

of operation of the cyclo-converter, both the converters would conduct at a time,

with an inter-group reactor (IGR) between the positive and negative groups as

shown in Fig. 30.4. It may be noted that, though the output voltages of two

converters in the same phase have the same average value, but their output

voltage waveforms as a function of time are, however, different, and as a result,

there is a net potential difference (voltage) across two converters. Due to this

voltage, the reactor is inserted to limit the circulating current. As described, the

main converter − positive/negative, as the case may be, acts in the rectifier

mode, and the other one acts in the inverter mode, with the average value along

with the sign, of the output voltage being same. Thus, the sum of their firing delay

angle must be 180° (π ) . In other words, if α p and α n are the firing angles for

positive and negative group of converters, respectively, then these firing angles

must be controlled so as to satisfy the condition ( αp+αn)= 180o (π )

The continuous current of each group in the circulating current mode imposes a

higher loading on each group compared to the non-circulating current mode of

operation. In practice, this mode, i.e. circulating current one, would only be used,

when the load current is low, so that continuous load current with a better

waveform can be maintained. At the higher levels of load current, the groups

would be blocked to prevent circulating current. Control circuits would be used to

sense the level of the load current, allowing firing pulses to each group at low

current level, but blocking firing pulses to one or the other group at the higher

current levels. The reactor would be designed to saturate at higher current levels,

when the cyclo-converter is operating in the non-circulating current mode, thus

permitting a smaller one.

Cyclo-converter, using two three-phase half-wave converters

7. Describe the principle of operation of PWM type and multistage sequential ac

voltage controller.(AUC MAY 12)

Multistage is explained in Q3

PWM type control:

The input power factor of the phase controlled AC voltage controller is low in

large firing angle. Therefore lower order harmonics are introduced in the supply

& load system. The input power factor can be improved by using PWM type of

control.

In the circuit switch S1 is turned on & off several times during positive half cycles

of the input voltage. Similarly switch S2 is turned on & off several times during

negative half cycles of the input voltage. Switch S1’ and S2’ are providing the

freewheeling paths for the load current whereas S1 & S2 are in off state. The

figure shows the gating signals. The diodes are used to prevent reverse voltages

from appearing across the switches.

8. A resistive load of 5 ohms is fed through a single phase full wave AC voltage

controller from 230V, 50 Hz source. If the firing angle of thyristor is 120 degree

find the output RMS voltage, input power factor and average current of thyristor.

(AUC NOV12)

9. Explain about multi stage sequence control of voltage controllers. ( AUC MAY

13)

Same as Q3

10. Explain the principle of integral cycle control(AUC MAY 13)

Principle of on / off control:

The principle of on – off control can be explained with a single phase full wave

controller. The thyristor switch connects the ac supply to load for a time tn. The

switch is turned off by a gate pulse for time to. The on time tn usually consists of

an integral number of cycles. The thyristors are turned on at the Zero voltage

crossings of ac input voltage. The gate pulses for thyristors T1 & T2 and

waveforms are shown.

Due to zero voltage and zero current switching of the thyristors, the harmonics

generated by the switching actions are reduced.

For a sinusoidal input voltage Vs= Vm sinωt=21/2 Vs sinωt. If the input voltage is

connected to load for n cycles and is disconnected for m cycles, the RMS output

voltage can be found from

Where K=n/(m+n) and is called the duty cycle.

11. A single phase voltage controller has input voltage OF 2330V, 50 Hz and a load

of R=15 ohms. For 6 cycles ON and 4 cycles OFF determine the RMS output

voltage, input power factor and average and RMS thyristor currents. (AUC

MAY13)