The method
Before using the result
- Sinusoidal lagging load; target PF must be at least existing PF. Capacitor harmonics, resonance, switching and equipment selection are excluded. Capacitance outputs are per branch.
Worked examples
These original training examples illustrate the method for Third-Class Electric Chief Engineer 電験三種 (Theory/Power/Machine/Law). Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.
1. Worked example
Calculate reactive compensation for an improved lagging power factor. Specify capacitor connection before converting total reactive power to capacitance per branch.
- Real power
- 10 kW
- Existing lagging power factor
- 70 %
- Target lagging power factor
- 95 %
- Calculate capacitance
- No
- Qc = 10000 × [tan(acos(0.7)) − tan(acos(0.95))] = 6915.199560415442 var.
- Capacitance not requested.
- Reactive compensation: 6,915.2 var
2. Comparison example
Compare a different supplied condition using the same method. For Denken Exam, the important check is that the input meaning, units and method assumptions still match the problem.
- Real power
- 15 kW
- Existing lagging power factor
- 70 %
- Target lagging power factor
- 95 %
- Calculate capacitance
- No
- Qc = 15000 × [tan(acos(0.7)) − tan(acos(0.95))] = 10372.799340623162 var.
- Capacitance not requested.
- Reactive compensation: 10,372.8 var
Check your understanding
1. Which relationship is used?
Qc = P[tan(acos(PFold)) − tan(acos(PFtarget))]. Single-phase C = Qc/(ωV²); star/delta use per-phase voltage and Qc/3.
2. Which assumption should you check first?
Sinusoidal lagging load; target PF must be at least existing PF. Capacitor harmonics, resonance, switching and equipment selection are excluded. Capacitance outputs are per branch.
3. How should you verify a different unit system?
Convert every input to the displayed calculation units first. Equivalent physical inputs must give equivalent results; a unit label alone does not convert a number.
Continue with the matching exam topic
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