Discussion
The speed of a separately excited motor is controlled by a single-phase semi converter. The field current is also set controlled by semi converter. The AC supply voltage to the armature and field converter is single-phase, 208 V, 50 Hz.
What is the value of maximum field voltage?
Answer: A
The average output of a single phase semi converter is \(V_{dc} = \dfrac{V_m}{\pi}(1 + \cos\alpha)\). It is greatest for \(\alpha = 0\), when it equals \(2V_m/\pi\).
\[ V_m = \sqrt{2} \times 208 = 294.16\ \text{V} \]
\[ \begin{aligned} V_{dc,max} = \frac{2 \times 294.16}{\pi} \\ &= 187.36\ \text{V} \end{aligned} \]
The values 18.736 V, 1.8736 V and 1873.6 V are 10, 100 and 10 times too small or large. The answer is 187.36 V, option A.
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