Power Electronics Viva 6h6767

Power Electronics(PE) Lab Viva Question-Answers Set-2 SOME BASIC QUESTION-ANSWERS:

Q1. What is Power electronics? Power electronics is one of the important branch of electronics and electrical engineering. It delas with conversion and control of electric energy. Answer Q2. What are main applications of Power electronics? Power supplies is Computers, medical equipments, Communication systems, telephone exchange, Satellite systems etc. Power control in resistance welding, induction heating, electrolysis process and etc, Power conversion for HVDC and HVAC transmission systems. Answer Q3. What is SCR? Silicon Control Rectifier (SCR) belongs to the thyristor family, it is three terminal four layer PNPN diode. Answer Q4. What are merits and applications of SCR? Merits: i) Very small amount of gate drive is required. ii) SCR’s with high voltage and current ratings are available. iii) On-state losses in SCR’s are reduced. Applications: i) SCR’s are best suitable for controlled rectifier. ii) AC regulators, lighting and heating applications. iii) DC motor drives, large power supplies and electronic circuit breaker. Answer Q5. What is TRIAC? TRIAC can be considered as antiparallel SCR. TRIAC conducts in both the directions and it has single gate. Some Important Points: i) All the power electronic devices are used as on/off switches.

ii) Power converter converts input power in the form suitable for load. iii) All power devices are solid state. Hence they are called power electronic devices. Answer Q6. Differentiate between SCR and BJT. SCR: i) Four layer PNPN device. ii) Turns on by regeneration. iii) Gate has no control once SCR is turned on. iv) External circuits are required to turn off the SCR. v) Switching frequencies are low. BJT: i) Three layer (PNP or NPN) device. ii) No regeneration exists. iii) Base has full control over the operation of BJT. iv) No external circuits are required. BJT turns off if base drive is removed. v) Switching frequencies are high. Answer Q7. What are the differences between BJT and MOSFET? BJT: i) This is bipolar device. ii) Controlled by base. iii) Current controlled device. iv) Negative temp coefficients. v) Losses are low. MOSFET: i) This is majority carrier device. ii) Controlled by gate. iii) Voltage controlled devices. iv) Positive temp coefficients. v) Losses are high than BJT’s. Answer Q8. What are the different ways to Turn-on and Turn-off the Thyristor. Turn-on: i) Gate drive.

ii) High forward voltage. iii) Dv/dt iv) Light v) High temperature. Turn-off: i) Natural commutation. ii) Forced commutation. Answer Q9. What is turn0on and turn-off time of a thyristor? Turn-on: it is defined as the time from initiation of gate drive to the time when anode current reaches to its full value. Turn-off time: it is the time required to achieve forward blocking capability after commutation is initiated. Answer Q10. List the thyristor current and voltage ratings. Current ratings: i) Average current rating. ii) RMS current rating. iii) Surge current rating. iv) I2t rating. v) Di/dt rating. Voltage ratings: i) Peak repetitive forward blocking voltage. ii) Peak repetitive reverse voltage or peak inverse voltage(PIV). iii) Non-repetitive peak reverse voltage. iv) Dv/dt rating. Answer Q11. What is DIAC, and why it used? The DIAC is the two terminal and four layer device. It is mainly used for triggering TRIAC’s. it conducts in either direction. It is low power triggering device, and there are no control terminal on the DIAC. Answer You might also like: 

Power Electronics(PE) Lab Viva Question-Answers

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Power Electronics(PE) Lab Viva Question-Answers Q1. What are the difference between transistor and SCR? 1. SCR is four layer PNPN device, whereas transistor is three layer (PNP or NPN) device. 2. SCR turns on by regeneration but there is no regeneration in BJT. 3. Gate has no control over the conduction once SCR turns on. BJT can be turned off by base drive. 4. BJT’s have higher switching frequencies than SCR’s. Answer Q2. Explain latching current and holding current of the thyristor? Latching current: It’s the minimum forward current that flows through the thyristor to keep it in forward conduction mode at the time of triggering. If the forward current is less than latching current, thyristor doesn’t turn on. Holding Current: It is the minimum forward current that flows through the thyristor to keep it in forward conduction mode. When forward current reduces below holding current, thyristor turn off. Answer Q3. What are the advantages of MOSFET’s over BJT’s? i) MOSFET’s have simpler gate drive circuits. ii) They have high switching frequencies. iii) MOSFET’s are majority carrier devices. iv) MOSFET’s have positive temperature coefficients. Answer Q4. Why pulsed gate drive is used for SCR? Once SCR is turned on, gate drive is no more required. Hence if pulsed gate drive is used, SCR turns on at particular pulse. The remaining pulses are not used. But pulsed drive reduces losses in gate cathode of the SCR. Answer Q5. Define the delay angle of phase-controlled rectifier.

It is the angle at which thyristors are triggered after zero crossing. After zero crossing of supply voltage, one pair of thyristor is forward biased. After delay angle (α), the SCR’s are triggered. Answer Q6. Why is power factor of semiconverter better than full converter?

Answer Q7. What are the differences between freewheeling diode and diode?

Freewheeling diode is used to circulate the load energy in the load itself. diode is used to transfer the load energy to the source. diode is normally used in inverters. Freewheeling diode is normally used in controlled rectifiers. Answer Q8. What is inverting operation of the converter? Converters are basically AC to DC controlled rectifiers. The power flows from AC supply to DC load. In inverting operation, the power flows from DC load to AC supply. Under such situation, the DC load is normally a battery. Thus the power of DC battery load is given to AC supply through controlled rectifier. For this operation, firing angle is greater than 90˚. Answer Q9. What are control strategies of chopper? Choppers have following control strategies: 1. Fixed frequency operation: in this type of control, the frequency of chopper remains fixed , but on and off periods of chopper change as per the duty cycle. 2. Variable frequency operation: in this type of control, the on or off period remains fixed. Hence frequency of the chopper changes when duty cycle is changed. 3. Current limit control (CLC) : in this type of control, the output is sensed, when output current falls below reference level, chopper is turned on. And when output current rises above reference level, chopper is turned off. Answer Q10. Explain the use of step-up chopper.

Step-up chopper is normally used for energy transfer. The load energy is transferred to source by stepup operation.

Answer Q11. What is four quadrant chopper? Four quadrant chopper operates in all the four quadrants of Vo – Io plane. It can perform forward and reverse motoring as well as forward and reverse regeneration. Answer Q12. What is the main difference between voltage source inverter and current source inverter?

1. In the voltage source inverter, input voltage remains fixed and current varies according to load variations. But in current source inverter, input current remains fixed and voltage varies according to the load. 2. In voltage source inverters, short circuit of the devices can damage them. In current source inverter short circuit cannot damage the circuit. 3. Voltage source inverters need freewheeling diodes. But current source inverters doesn’t need freewheeling diodes. Answer Q13. What are PWM inverters? PWM inverters are basically pulse width modulated inverters. Voltage control in such inverters is achieved by varying width of the pulse. In one half cycle there can be single or multiple pulses. Number of pulses determine lowest order harmonic present in inverter output voltage. Thus PWM inverters provide voltage control as well as harmonic reduction simultaneously. Answer Q14. What is cycloconverter? Cycloconverter is AC to DC converter. It has 1ф or 3 ф AC input. Output is variable voltage and variable frequency. Cycloconverters are used for induction motor drives. They have an advantages of variable frequency output over other AC voltage. Answer Q15. What are the advantages and disadvantages of AC voltage controller? Advantages: 1. They use line communication, hence no extra communication circuits are required. 2. They have high efficiencies since device losses are reduced. Disadvantages: 1. Large ripple and harmonics are present in the output. 2. Output waveforms are not sinusoidal.