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The thyristor has the following specifications Vs = 400 V (di/dt)max = 25 A/μsec Find the value of L https://www.sanfoundry.com/wp-content/uploads/2017/06/power-electronics-questions-answers-thyristor-protection-q5.png a) 8 μH b) 80 μH c) 16 μH d) 160 μH
: c Explanation: As soon as the switch is closed, C acts like a S.C & the voltage equation gives, Vs = (Rs + Rl) I + L di/dt Solve the above D.E. di/dt = Vs/L e-t/τ di/dt is maximum at t = 0, substitute the above given values & find L
: c
See lessExplanation: As soon as the switch is closed, C acts like a S.C & the voltage equation
gives,
Vs = (Rs + Rl) I + L di/dt
Solve the above D.E.
di/dt = Vs/L e-t/τ
di/dt is maximum at t = 0, substitute the above given values & find L
False triggering of the SCRs by varying flux & noise is avoided by using a) F.A.C.L.F & C.B b) Shielded cables & twisted gate leads c) Snubber circuits d) di/dt inductor in series with the gate terminal
b Explanation: Shielded cables provide isolation from outside noise.
b
See lessExplanation: Shielded cables provide isolation from outside noise.
Over-current protection in SCRs is achieved through the use of a) Varistors b) Snubber Circuits c) F.A.C.L.F & C.B. d) Zener diodes
: c Explanation: FACLF stands for Fast Acting Current Limiting Fuse.
: c
See lessExplanation: FACLF stands for Fast Acting Current Limiting Fuse.
The effect of over-voltages on SCR are minimized by using a) RL circuits b) Circuit breakers c) Varistors d) di/dt inductor
c Explanation: Varistors are non-linear voltage clamping devices, RC circuits across the loads can also be used.
c
See lessExplanation: Varistors are non-linear voltage clamping devices, RC circuits across the
loads can also be used.
Figure below shows SCR having dv/dt and di/dt protection, when the switch(shown in green) is closed the current through Rl = https://www.sanfoundry.com/wp-content/uploads/2017/06/power-electronics-questions-answers-thyristor-protection-q5.png a) I(1 – e-t/τ) b) I(1 + e-t/τ) c) I(1 – et/τ) d) I(1 + e-t/τ)
a Explanation: As soon as the switch is closed, C acts like a S.C & the voltage equation gives Vs = (Rs + Rl) I + L di/dt Solve the above D.E.
a
See lessExplanation: As soon as the switch is closed, C acts like a S.C & the voltage equation
gives
Vs = (Rs + Rl) I + L di/dt
Solve the above D.E.
The local hot spot formation in the cross-section of the SCR is avoided by a) reducing the junction temperature b) applying gate current nearer to the maximum gate current c) using only R loads d) proper mounting of the SCR on heat sink
b Explanation: Applying the higher gate current spreads the ions quickly and avoids hotspot formation.
b
See lessExplanation: Applying the higher gate current spreads the ions quickly and avoids
hotspot formation.
di/dt protection is provided to the thryistor by a) connecting an inductor in parallel across the load b) connecting an inductor in series with the load c) connecting an inductor in parallel across the gate terminal d) connecting an inductor in series with the gate
b Explanation: By placing the di/dt inductor (L) in series with the load, the change in the anode current can be limited to a small value.
b
See lessExplanation: By placing the di/dt inductor (L) in series with the load, the change in the
anode current can be limited to a small value.
The thermal resistance between junction & the SCR (θjc) has the unit a) Ω/°C b) W/Ω c) °C/W d) ΩW/°C
c Explanation: Thermal resistance always has the unit degree temperature per watt.
c
See lessExplanation: Thermal resistance always has the unit degree temperature per watt.
The maximum rms current of an SCR is 50 A. For a 120° sine wave conduction the form factor (FF) = 1.878 Find the average on-state current rating (ITAV). a) 93.9 A b) 174 A c) 26.62 A d) 68.52 A
c Explanation: ITAV = 50/FF.
c
See lessExplanation: ITAV = 50/FF.
The amp2 -sec rating of the SCR specifies a) The power dissipated by the device when fault occurs b) The energy dissipated by the device when fault occurs c) The energy that the device can absorb before the fault is cleared d) The energy that the device can absorb while operating in the forward blocking mode.e
c Explanation: The amp2 -sec is the energy that the device can handle before the fault is cleared. Its value decides how fast the fault has to be cleared to avoid damage to the device.
c
See lessExplanation: The amp2
-sec is the energy that the device can handle before the fault is
cleared. Its value decides how fast the fault has to be cleared to avoid damage to the
device.