SHORT QUESTIONS AND ANSWERS
1. Why IGBT is very popular nowadays?
- Lower hate requirements
- Lower switching losses
- Smaller snubber circuit requirements
2. What are the different methods to turn on the thyristor?
- Forward voltage triggering
- Gate triggering
- dv/dt triggering
- Temperature triggering
- Light triggering
4. IGBT is a voltage controlled device. Why?
- Because the controlling parameter is gate-emitter voltage.
5. Power MOSFET is a voltage controlled device. Why?
- Because the output (drain) current can be controlled by gate-source voltage.
6. Power BJT is a current controlled device. Why?
- Because the output (collector) current can be controlled by base current.
7.
8. What are the different types of power MOSFET?
- N-channel MOSFET
- P-channel MOSFET
9. How can a thyristor turned off?
- thyristor can be turned off by making the current flowing through it to zero.
10. Define latching current.
- The latching current is defined as the minimum value of anode current which it must attain during turn on process to maintain conduction when gate signal is removed.
11. Define holding current.
- The holding current is defined as the minimum value of anode current below which it must fall to for turning off the thyristor.
12. What is a snubber circuit?
- It consists of a series combination of a resistor and a capacitor in parallel with the thyristors. It is mainly used for dv / dt protection.
13. What losses occur in a thyristor during working conditions?
- Forward conduction losses
- Loss due to leakage current during forward and reverse blocking.
- Switching losses at turn-on and turn-off.
- Gate triggering loss.
14. Define hard-driving or over-driving.
- When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or over-driven. Hard-firing of a thyristor reduces its turn-on time and enhances its di/dt capability.
15. Define circuit turn off time.
- It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.
16. Why circuit turn off time should be greater than the thyristor turn-off time?
- Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.
17. What is the turn-off time for converter grade SCRs and inverter grade SCRs?
- Turn-off time for converter grade SCRs is 50 – 100 ms turn-off time for converter grade SCRs and inverter grade SCRs and for inverter grade SCRs is 3 – 50 ms.
18. What are the advantages of GTO over SCR?
- Elimination of commutation of commutating components in forced commutation, resulting in reduction in cost, weight and volume.
- Reduction in acoustic noise and electromagnetic noise due to elimination of commutation chokes.
- Faster turn-off, permitting high switching frequencies.
- Improved efficiency of the converters.
19. What is meant by phase controlled rectifier?
- It converts fixed ac voltage into variable dc voltage.
20. Mention some of the applications of controlled rectifier.
- Steel rolling mills, printing press, textile mills and paper mills employing dc motor drives.
- DC traction
- Electro chemical and electro-metallurgical process
- Portable hand tool drives
- Magnet power supplies
- HVDC transmission system
- It serves two process.
- It prevents the output voltage from becoming negative.
- The load current is transferred from the main thyristors to the freewheeling diode, thereby allowing all of its thyristors to regain their blocking states.
- Input power factor is improved.
- Load current waveform is improved and thus the load performance is better.
- The delay angle is defined as the angle between the zero crossing of the input voltage and the instant the thyristor is fired.
- SCRs are subjected to a peak-inverse voltage of 2Vm in a fully controlled bridge rectifier. Hence for same voltage and currnt ratings of SCrs, power handled by mid-point configuration is about
- In mid-point converter, each secondary winding should be able to supply the load power. As such, the transformer rating in mid-point converter is double the load rating.
- The commutation period when outgoing and incoming thyristors are conducting is known as overlap period. The angular period, when both devices share conduction is known as the commutation angle or overlap angle.
- UJT firing circuit.
- The cosine wave crossing pulse timing control.
- Digital firing schemes.
- Average output voltage
- The input power factor is defined as the ratio of the total mean input power to the total RMS input volt-amperes.
29. What are the advantages of six pulse converter?
- Commutation is made simple.
- Distortion on the ac side is reduced due to the reduction in lower order harmonics.
- Inductance reduced in series is considerably reduced.
- It is the process of changing the direction of current flow in a particular path of the circuit. This process is used in thyristors for turning it off.
- Natural commutation
- Forced commutation
30. What is meant by commutation?
- It is the process of changing the direction of current flow in a particular path of the circuit. This process is used in thyristors for turning it off.
- Natural commutation
- Forced commutation
- Here the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off.
- In this commutation, the current flowing through the thyristor is forced to become zero by external circuitry.
- A dc chopper is a high speed static switch used to obtain variable dc voltage from a constant dc voltage.
a. Battery operated vehicles
b. Traction motor control in electric traction
c. Trolly cars
d. Marine hoists
e. Mine haulers
f. Electric braking.
36. What are the applications of dc chopper?
Chopper provides
a. High efficiency
b. Smooth acceleration
c. Fast dynamic response
d. Regeneration
37. What is meant by step-up and step-down chopper?
In a step- down chopper or Buck converter, the average output voltage is less than
the input voltage. In a step- up chopper or Boost converter, the average output voltage is
more than the input voltage.
38. Write down the expression for average output voltage for step down chopper.
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