Basic Electricity 2, May/June 2015

Question 3

A balanced three-phase delta connected system has a line voltage of 415V and a line current of 20A. If the total load on the system is 6000W, calculate the;

(i) Load power factor;

(ii) Current in each phase;

(iii) Power delivered by each phase.  


The expected answers were;

(i) The load power factor; Power (P) = √3 VLIL COSθ Pf=P/(√3 EL IL ) = 6000/(1.7×415×20) = 0.418

(ii) The current in each phase; I_P= I_L/√3 = 20/1.73 = 11.6A

(iii) Power per phase; = (Total Power)/3 = 6000/3 =2000W OR P= V_L I_Ph COSθ =415×11.6 ×0.412 =2012.3W

The candidates were to calculate load power factor, current in each phase and power per phase.

Few candidates were reported to have attempted this question. Use of incorrect formula was responsible for their weakness. √3 was found missing in the fomula the candidates used in the calculation.