Question

It is required to operate 250 A SCR in parallel with 350 A SCR with their respective on-state voltage drops of 1.6 V and 1.2 V. The value of resistance to be inserted in series with each SCR so that they share the load of 600 A in proportion to their current ratings is ________m$\Omega$.

Answer 1

Dynamic resistance of 250 A SCR is $r_1=\frac{1.6}{250}=6.4m\Omega$

Dynamic resistance of 350 A SCR is $r_2=\frac{1.2}{350}=3.43m\Omega$

Let R be the resistance inserted in series with each SCR

Current shared by SCR $\left(I_1\right)=600\times \frac{(3.43m\Omega +R)}{(3.43m+6.4m+2R)}$ ....(i)

Similarly, $I_2=600\times \frac{(6.4m\Omega +R)}{(6.4m+3.43m+2R)}$ .....(ii)

From (i) and (ii),

$\frac{I_1}{I_2}=\left(\frac{3.43m\Omega +R}{6.4m\Omega +R}\right)=\frac{250}{350}$

$\frac{3.43m\Omega +R}{6.4m\Omega +R}=\frac{5}{7}$

$R=3.995m\Omega$