GATE EE 2024 Set 1 — Question 28
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Power Electronics → Harmonics & AC-AC Converters → AC Voltage Controllers
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Question
A single-phase triac based AC voltage controller feeds a series RL load. The input AC supply is 230 V, 50 Hz. The values of R and L are 10 and 18.37 mH, respectively. The minimum triggering angle of the triac to obtain controllable output voltage is
Correct answer
(B) 30°
Solution
Load data: , mH, Hz.Load (impedance) angle:Why control starts at : With a series RL load, once the triac is fired at angle the load current lasts until the extinction angle , where grows as falls. When , the current is a continuous sinusoid lagging the voltage by (), and the whole supply voltage appears across the load. Firing earlier than cannot increase the output any further: the triac is still carrying the previous half-cycle's current and the load just sees the full sinusoid, so the output voltage does not change with . Varying the firing angle changes the output voltage only forTherefore the minimum triggering angle for controllable output voltage is .Answer: (B)
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