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.

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.

Q3. What is SCR?

Silicon Control Rectifier (SCR) belongs to the thyristor family, it is three terminal four layer PNPN diode.

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.

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.

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.

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.

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.

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.

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.

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.