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TANGEDCO AE EE 2015 Official Paper

Option 3 : 600 rpm

CT 3: Building Materials

3235

10 Questions
20 Marks
12 Mins

__Concept:__

The average output voltage is given by, \({V_1} = \frac{{2{V_m}}}{\pi }\cos \alpha \)

If SCR T_{3} is damaged and gets open circuited, the circuit acts as **half wave rectifier & only 2 SCR's conducts**

The average output voltage** **\({{\rm{V}}_0} = \frac{1}{{2{\rm{\pi }}}}\mathop \smallint \limits_{\rm{\alpha }}^{{\rm{\pi }} + {\rm{\alpha }}} {{\rm{V}}_{\rm{m}}}\sin \omega t{\rm{\;d}}\left( {{\rm{\omega t}}} \right)\)

\({{\rm{V}}_2} = \frac{{{{\rm{V}}_{\rm{m}}}}}{{\rm{\pi }}} \times \cos {\rm{\alpha }}\)

__Calculation:__

Given that, the motor is separately excited. So, the flux remains constant (ϕ), and load current is continuous

**Back Emf (E) = K _{a}ϕ ω = **V

**E ****∝**** speed (ω) **

The armature resistance is small. So, I_{a}r_{a} drop is negligible

ω_{1} = 1200 rpm

\(\frac{{{V_1}}}{{{V_2}}} = \frac{{\frac{{2{V_m}}}{\pi }cos\alpha }}{{\frac{{{V_m}}}{\pi }cos\alpha }} = \frac{{{\omega _1}}}{{{\omega _2}}}\)

\( \Rightarrow {{\rm{\omega }}_2} = 1200 \times \frac{1}{2} = 600{\rm{\;rpm}}\)

The reduced speed (ω