1). The typical value of SCR formodern alternator is
Answer: b) Always the typical value of SCR for modern alternator is 0.5
2). An SCR is considered to be a semi controlled device because
a) it can be turned OFF but not ON with a gate pulse
b) it conducts only during one half cycle of an alternating current wave
c) it can be turned ON but NOT with a gate pulse
d) it can be turned ON only during one half cycle of an AC
Answer: c) during positive half cycle SCR is in forward blocking mode. by applying gate pulse the SCR can be turned ON during forward blocking mode. but SCR can not be turned OFF by applying gate pulse. It automatically turns off when anode current is below the holding current . that is why it is called semi controlled rectifier.
3). An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. one cycle surge current will be
a) 1500 A
b) 6000 A
c) 2121.32 A
4). A single phase one pulse controlled circuit has a resistance R and counter EMF E load 400 sin(314 t) as the source voltage . For a load counter emf of 200 v, the range of firing angle control is
a) 30° to 150°
b) 30° to 180°
c) 60° to 120°
d) 60° to 180°
Answer: a) 400sinΘ = 200, Θ = 30° so, control range is Θ to (π-Θ) i.e. 30° to 150°
5). A single phase full bridge Inverter can operated in load communication mode in case load consist of
b) RLC underdamped
c) RLC overdamped
d) RLC critically damped.
Answer: b) In this case reading current will flow in circuit and it will become zero so thyristor will be load communication.
6). The latching current of SCR is 20 mA its holding current will be
Answer: c) As holding current is always less than leaking current.
4). Thyristor can be protected from over voltages by using
a) Voltage clamping device
c) heat sink
d) snubber circuit
Answer: a) non linear or voltage clamping device is used to protect thyristors under surge conditions by providing low impedance path.
8). Example of a voltage clamping device
a) fast acting fude
b) snubber circuit
c) metal oxide varistor
d) aluminium block
9). Under over voltage condition impedance offered by the voltage clamping device is