EE8552 - Power Electronics - Unit-5 Two marks

Subject Name: Power Electronics

Subject Code: EE8552

B.E. EEE/Mechatronics Dept - semester 05

Anna University Regulation 2017 B.E. EEE/Mechatronics Dept 5th semester EE8552(Power Electronics) 2 marks for 5th unit is provided below. EE8552 all units 2 marks, Power Electronics all units notes & syllabus are listed down for students to utilize it for scoring maximum marks with our study materials.

1.What are the two types of ac voltage controllers ? Which one of these is preferred and why ?

Answer

AC voltage controllers are of two types : 1. On-off control : In this type of controller, the full input voltage is applied to output for some number of cycles and no voltage is applied to output for some number of cycles. In this control, output voltage waveform is discontinuous and contains large ripple. 2. Phase-angle control : In phase angle control, each half cycle of the source voltage is applied partially or in full to the load. Firing angle of the SCRs is charged to control output voltage. Output voltage waveform is continuous but phase controlled. In both of the above two types, phase angle control is better since it contains less ripple and discontinuities in output.

2.List the merits and demerits of ac voltage controller.

Answer

Merits : 1) They use line commutation, hence no extra commutation circuits are required. 2) They have high efficiencies since device losses are reduced. Demerits : 1) Large ripple and harmonics are present in the output. 2) Output waveforms are not sinusoidal.

3.What are the effects of load inductance on the performance of ac voltage controllers ?

Answer

i) The load inductance makes the output current to flow continuously. ii) With load inductance, the commutation becomes difficult if triac switches are used. iii) The control fails sometimes due to highly inductive load.

4.What is an ac voltage controller ?

Answer

An ac voltage controller accepts fixed or variable ac input and gives regulated ac output to the load such as fan or ac motor.

5.What do you mean by integral cycle control ?

Answer

In this technique complete cycle of the supply are applied to the load for some fixed period. These full cycles are applied by turning on the antiparallel SCRs at α =0. The voltage control is effected by switching off the antiparallel SCRs completely. In other words no voltage is applied to the load. The ‘on’ and ‘off’ periods of the SCRs are controlled to regulate the output voltage.

6.What is cycloconverter ?

Answer

1) Cycloconverter is constant or fixed AC input to variable AC output. Thus it is AC to AC converter. 2) Cycloconverters can also provide variable output frequency. It can be higher or lower than input frequency. 3) Cycloconvertes can also convert 3ϕ input to 1ϕ output. 4) Output voltage of cycloconverters can be lower or higher than input voltage.

7.What is a matrix converter ?

Answer

The matrix converter uses a matrix of switches so that any of the input phase voltage can be connected to any of the output load phase. There is exactly one switch for each of the possible connections between supply and load.

8.List out the different types of cycloconverter.

Answer

Depending upon output / input frequency relationship, a) Step up cycloconverter b) Step down cycloconverter Depending upon output/input phases, i) 1 ϕ to 1 ϕ cycloconverter ii) 3 ϕ to 1 ϕ cycloconverter iii) 3 ϕ to 3 ϕ cycloconverter

9.What are the applications of cyclo converter ?

Answer

i) High power AC motor drives. ii) High power blowers and fans. iii) Conveyor belts and lifts.

10.Write any two advantages of sequence control technique.

Answer

i) Sequence control improves power factor of AC regulator. ii) It reduces the harmonics in the source current and load voltage.

11.Mention the advantages of matrix converter over AC-DC-AC converters.

Answer

i) Inherent bidirectional power flow control. ii) Input output waveforms are sinusoidal. iii) No DC link required, hence it is compact. iv) Power factor is controlled by independent control of load current.

12.Write the principle of operation of cycloconverter.

Answer

Cycloconverters use two controlled converters connected back to back. The output voltage polarity is selected by selection of a particular controlled converter. Thus the output is ac with variable voltage and frequency. This is the operating principle of cycloconverter.

13.Give any two important applications of AC voltage controllers.

Answer

i) Fans and lighting controls. ii) Speed control of AC motors.

14.What is the control range of firing angle in AC voltage controller with RL load ?

Answer

The firing angle can vary form '0' to 'π' in AC voltage controllers.

15.What are the different control techniques for AC regulator ?

Answer

Following are the different control techniques : i) On-off control or integral cycle control. ii) PWM control or power factor control. iii) Multistage sequence conirol. iv) Matrix converter.

16.Why is half wave AC voltage regulator not used ?

Answer

Half wave AC voltage regulator is not used because of 1) Only half cycle is controlled, remaining half cycle is not controlled. 2) Not suitable for inductive loads. 3) Output voltage waveform is highly pulsating.

17.Explain the term sequence control of ac voltage regulators.

Answer

Multiple bidirectional voltage controllers are employed in parallel. Each one of them is fed from the multiple tap transformer. Depending upon the desired output voltage, particular controllers are triggered. Sequence control reduces the pulsating behaviour of the AC controller.

18.Compare On-off and Phase-angle ac voltage controllers.

Answer

* On/off control -Full supply voltage is applied across the load for few cycles of input. -Thyristor conducts 180° or half cycle. It is triggered at α = 0. -Since thyristors are switched on at α = 0, harmonics due to switching are reduced. -Load voltage will be equal to full supply voltage or zero. Hence output waveform has high voltage fluctuations. -Power control is achieved by varying the on and off times for which supply voltage is applied to the load. * Phase control -Supply voltage is partially applied to load in every cycle of input. -Thyristor is triggered at some angle depending upon output power. -Thyristors are switched on at angles other than α = 0, hence switching harmonics are present. -Supply voltage is applied partially to load. Hence there are less voltage fluctuations. -Power control is achieved by controlling the conduction period (phase) of thyristor in every half cycle.

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