A single−phase AC switch is used in between a 230 V source and load of 2 kW and 0.8 lagging power factor. Determine the rms current rating required by the SCR. Use the factor of safety = 2.
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The load current is I = P/(V·pf) = 2000/(230×0.8) ≈ 10.87 A. With a safety factor of 2, the required SCR rms current rating is about 21.74 A.
Formula / Key Relation
Iload = P/(V cosφ)
= 2000/(230×0.8)
= 10.87 A
Irating = 2 × Iload
= 21.74 A
Detailed Explanation
Concept & Reasoning
For a single-phase load, real power is P = VIcosφ. First calculate the actual rms load current from real power, supply voltage and power factor. A device rating must then include the stated safety margin; multiplying the operating current by 2 gives the minimum selected current rating.
How to Solve It in the Exam
Power → current first; safety factor last.
Important Exam Point
Use real power and power factor together; 2 kW is not 2 kVA.
Related Revision Path
This question sits in the revision path Power Electronics → AC Controllers → Thyristor Ratings .
Quick Trick
Power → current first; safety factor last.
Common Mistake
Calculating 2000/230 and forgetting the 0.8 power factor.
Exam Tip
Use real power and power factor together; 2 kW is not 2 kVA.
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