Figure shows four electronic switches $$(i), (ii), (iii)$$ and $$(iv).$$ Which of the switches can block voltages of either polarity (applied between terminals $$‘a’$$ and $$‘b’$$) when the active device is in the OFF state?

In the figure shows on period of the output voltage of an inverter. $$\alpha $$ should be chosen such that $${60^0} < \alpha < {90^0}.$$ If $$rms$$ value of the fundamental component is $$50$$ $$V$$, then $$\alpha $$ in degree is _________.

Figure (i) shows the circuit diagram of a chopper. The switch $$S$$ in the circuit in figure (i) is switched such that the voltage $${V_D}$$ across the diagram as the wave shape as shown in figure (ii). The capacitance $$C$$ is large so that the voltage across it is constant. If switch $$S$$ and the diode are ideal, the peak to peak ripple (in A) in the inductor current is ________.

In an unbalanced three phase system phase current $${{\rm I}_a} = 1\angle \left( { - {{90}^0}} \right)\,\,pu,\,\,$$ negative sequence current $$\,{{\rm I}_{b2}} = 4\angle \left( { - {{150}^0}} \right)\,\,pu,\,\,$$ zero sequence current $$\,\,{{\rm I}_{c0}} = 3\angle {90^0}\,\,pu.\,\,\,$$ The magnitude of phase current $${{\rm I}_b}$$ in $$pu$ is