POWER ELECTRONICS AND APPLICATIONS

Once SCR starts conducting a forward current, its gate loses control over

• Anode circuit current only

• Anode circuit voltage only

• Anode circuit voltage and current

• Anode circuit voltage, current and time

In a thyristor, the magnitude of anode current will

• Increase if gate current is increased

• Increase if gate current is decreased

• Decrease if gate current is decreased

• Not change with any variation in gate current

Surge current rating of an SCR specified the maximum

• Repetitive current with sine wave

• Non-repetitive current with rectangular wave

• Non-repetitive current with sine wave

• Repetitive current with rectangular wave

For an SCR, dv/dt protection is achieved through the use of

• RL in series with SCR

• RC across

• L in series with

• RC in series with

The applied sine voltage to a SCR is VM=200V and R=10Ohm. If the gate trigger lags the ac supply by 120°, the average load current is

• 15/π A

• 5/π A

• -5/π A

• -15/π A

A sine voltage of 200Vrms, 50Hz is applied to an SCR through 100ohm resistor. The firing angle is 60°. Consider no voltage drop. The output voltage in rms is

• 89.7 V

• 126.7 V

• 166.7 V

• 200√2 V

The cycloconverter require natural or forced commutation as under

• Natural commutation in both step-up and step down cyclo converter

• Forced commutation in both step-up and step-down cyclo converter

• Forced commutation in step-up cyclo converter

• Forced commutation in step-down cycloconverter

A thyristor is triggered by a pulse train of 5kHz. The duty ratio is 0.4. If the allowable average power is 100W the maximum allowable gate drive power is

• 100√2 W

• 50 W

• 150 W

• 250 W

Three-phase to three-phase cycloconverter employing 18 SCRs and 36 SCRs have the same voltage and current ratings for their component thyristors. The ratio of VA rating of 36 SCR device to that of 18 SCR device is

• 1/2

• 1

• 2

• 4

The number of SCRs required for single phase to single phase cycloconverter of the mid point type and for three phase to three phase type cycloconverter are respectively

• 4, 6

• 8, 18

• 4, 18

• 4, 36