Question

Find the expression for average output voltage for the given circuit if firing angle is greater than 30°. Take Vmp = secondary side maximum value of phase voltage.

(a) (3√3Vmp/2π) x cosα

(b) (3√3Vmp/2π) x (1+cosα)

(c) (3√3Vmp/2π) x [1+cos(30+α)].

(d) (3√3Vmp/2π) x [3+cos(30+α)].

This question was posed to me in semester exam.

My doubt stems from Three Phase Converters-1 topic in chapter Converters of Power Electronics

Answer 1

Correct option is (c) (3√3Vmp/2π) x [1+cos(30+α)].

Easiest explanation: Vo = 3 x [ 1/2π ∫ Vmp sin⁡ωt d(ωt)  ]

Where, the integration runs from α+π/6 to π. Because conduction takes place from 30 + α to 180° for  T1 and than the waveform is symmetrical for all other SCRs.

Vo = (3√3/2π) x Vmp x [1+cos(30+α)].