Calculation Of Reactive Power

Practical calculation of reactive power

Jan 08, 2018

Please refer to the following calculation examples

Type of circuit	Apparent power S (kVA)	Active power P (kW)	Reactive power Q (kVAr)
Single phase (Ph + N)	S = V x I	P = V x I x cos φ	Q = V x I x sin φ
Single phase (Ph + Ph)	S = U x I	P = U x I x cos φ	Q = U x I x sin φ
Example: 5 kw load Cos φ= 0.5	10 kVA	5 Kw	8.7 kVAr
Three-phase (3Ph or 3Ph+N)	S = √3 x U x I	P = √3 x U x I x cos φ	Q = √3 x U x I x sin φ
Example of Motor with Pn = 51kW cos φ = 0.86 efficiency = 0.91	65 kVA	56 kW	33 kVAr

Calculations in the three-phase example were as follows:
Pn = power supplied to the rotary axis = 51 kW
P = active consumed power = Pn/ρ = 56 kW
S = apparent power = P/cos φ = P/0.86 = 65 kVA
Hence:
Q = √(S² - P²) = √(65² - 56²) = 33 kVAr
The average power factor values for various loads are given below.

Categories