If 'α' is the firing angle of thyristor, the average value of output voltage of a single phase halfwave AC voltage controller with R load shown in Fig. is ________.
1
\({V_o} = \frac{{{V_m}}}{{2\pi }}\left( {\cos \alpha - 1} \right)\)
2
\({V_o} = \frac{{{V_m}}}{{2\pi }}\left( {\cos \alpha + 1} \right)\)
3
\({V_o} = \frac{{{V_m}}}{\pi }\left( {\cos \alpha + 1} \right)\)
4
\({V_o} = \frac{{{V_m}}}{\pi }\left( {\cos \alpha - 1} \right)\)