When a separately excited d.c. motor is to be controlled from a three-phase supply led from controlled rectifier in only first quadrant, which one of the following converters is used to serve the purpose ?

Solution:-
In the first quadrant operation of the dc motor, both voltage and current should be positive therefore, we have to choose a convert whose output voltage and current is always positive.

→ For 3 - ϕ half wave converter,

\(\rm V_0=\frac{3Vml}{2\pi}\cos α\)

when firing angle α > 90°, then V₀ is negative.

→ For 3 - ϕ full wave converter,

\(\rm V_0=\frac{3Vml}{\pi}\cos α\)

when firing angle α > 90°, then V0 is negative.

→ For 3 - ϕ full wave semi converter

\(\rm V_0=\frac{3Vml}{\pi}(1+\cos α)\)

For any firing angle, V₀ is always positive.

→ Dual converter operates in all four quadrants.

Therefore most suitable converter is a 3-phase semi-convertor.