Test: Thyristors Ratings & Thyristors Protection
20 Questions MCQ Test Power Electronics | Test: Thyristors Ratings & Thyristors Protection
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The voltage safety factor (VSF) for an SCR is the ratio of
It is peak repetitive reverse voltage/ the maximum value of input voltage(Vm).
The forward dv/dt rating of an SCR
If the temperature is high, lesser dv/dt is required to turn on the device as the higher temperature has already excited few of the holes & electrons.
Finger voltage is the minimum value of Vak (anode to cathode voltage) to turn on the device with gate triggering, it is to be avoided for accidental turn-on of the device.
Which among the following anode current waveforms will have the minimum junction temperature?
N % DC is nothing but a wave with n % duty cycle. Lower the Duty cycle lesser is the current flowing & lesser is the temperature dissipation.
An SCR has half cycle surge current rating of 3000 A for 50 Hz. Calculate its one-cycle surge current rating
By equating the energies involved in one cycle & subcycle
I2.T = Isb2.t
Isb = 3000 A
T = 1/50
t = T/2.
For a SCR the maximum rms on-state current is 35 A. If the SCR is used in a resistive circuit for a rectangular wave with conduction angle of 90°. Calculate the average & rms currents respectively.
First convert the conduction angle into N for rectangular wave.
N = 360°/Conduction angle = 4
Therefore, Iavg = I/4
Irms = I/√4 = I/2.
For an SCR the average & rms values of current are I/4 & I/2 respectively. Calculate the average on-state current rating (ITAV). Take maximum RMS on-state current = 35 A.
Form factor = RMS/Average vaules of current
Therefore, FF = 2
ITAV = 35/2 = 17.5.
The amp2-sec rating of the SCR specifies
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.
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).
ITAV = 50/FF.
The thermal resistance between junction & the SCR (θjc) has the unit
Thermal resistance always has the unit degree temperature per watt.
di/dt protection is provided to the thryistor by
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 local hot spot formation in the cross-section of the SCR is avoided by
Applying the higher gate current spreads the ions quickly and avoids hotspot formation.
The dv/dt protection is provided in order to
Accidentally some voltage spike or noise may occur in the vicinity of the device, if the magnitude is large enough it may turn on the SCR.
dv/dt protection is provided to the SCR by
Snubber circuit R-C in parallel with SCR is connected for dv/dt protection.
Figure below shows SCR having dv/dt and di/dt protection, when the switch(shown in green) is closed the current through Rl =
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 effect of over-voltages on SCR are minimized by using
Varistors are non-linear voltage clamping devices, RC circuits across the loads can also be used.
Over-current protection in SCRs is achieved through the use of
FACLF stands for Fast Acting Current Limiting Fuse.
False triggering of the SCRs by varying flux & noise is avoided by using
Shielded cables provide isolation from outside noise.
The thyristor has the following specifications
Vs = 400 V
(di/dt)max = 25 A/μsec
Find the value of L
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
Thyristors are used in electronic crowbar protection circuits because it possesses
Crowbar protection circuits have high surge current capabilities.
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