MCQs on Power Electronics

Page 10 of 10. Go to page
01․ The latching current of GTO should be of order
100 mA.
500 mA.
1 A.
2 A.

Latching current for GTO is 2 A where latching current for a conventional thyristor of same rating is 100-500 mA.

02․ The maximum di/dt in a SCR is
Directly proportional to supply voltage.
Directly proportional to inductance in the circuit.
Inversely proportional to supply voltage.
Both A and B.

The maximum rate of rise current from cathod to anode is directly proportional to supply voltage and inductance in the circuit.

03․ Switching frequency of SITH is
5 KHz.
10 KHz.
60 KHz.
100 KHz.

The switching frequency of a static induction thyristor is 100 Khz because of their lower turn off current gain.

04․ Practical way of obtaining static voltage equalization in series connected SCRs is by the use of
one resistor across the string.
resistors of different values across each SCR.
resistors of the same value across each SCR.
one resistor in series with each SCR.

By connecting the resistors of the same value across each SCR, then we can be eliminated the unequal distribution of voltage across each SCRs. Hence, we can obtain the static voltage equalization in series connected SCRs by doing the above thing.

05․ A single phase full bridge inverter can operate in load commutation mode in case load consists of
RLC underdamped.
RLC overdamped.
RLC critically damped.

In a load commutation mode single phase full bridge inverter, if the load is not connected to the inverter, the SCRs would never be tuned off and after half cycle a short circuit would develop through SCR1 and SCR4. If it is not possible to guarantee that a load will always provide the proper counter voltage for commutation. This counter voltage can be provided by the RLC under damped loads for achieving the proper commutation.

06․ The function of snubber circuit connected across the SCR is to
suppress dV / dt.
increase dV / dt.
decrease dV / dt.
decrease di / dt.

Snubber circuit is basically series connected resistor and capacitor connected across the power electronics switch like thyristor, power transistor, and etc. The impedance of the device abruptly jumps to a very high level and it is blocking the current when a switching device changes its state from ON-state to OFF-state. But the current still tends to keep flowing through the switch, which induces a high voltage across the switch because if the current decreases faster, then the induced voltage (dV / dt) becomes higher. It may reach to sufficiently high level to destroy the switch. Snubber circuit is required in order to suppress the rate of rise of forward voltage dV / dt across the switch.