Showing posts from May, 2018

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Loads - Scenario 1

SITUATION An induction motor, rated 240 Volts, draws a current of 150 Amps and brings the system power factor down to 70.7%. What capacitor size is needed to raise system power factor to 95%? ANALYSIS 1.) Complex power equations, vector: S-> = P-> + Q-> S<a = P + jQ S<a = S cos(a) + j S sin(a) 2.) System power factor angle: P = S cos(a) pf = P/S = cos(a) = 0.707 a = arccos(0.707) = 45deg 3.) Motor apparent power, scalar: S = Vi = 240V * 150A = 36kVA 4.) Motor real and reactive power: P,load = S cos(a) P,load = 36k cos(45) = 25,455.84W Q,load = S sin(a) Q,load = 36k sin(45) = 25,455.84VAr 5.) System power factor angle after capacitor correction: pf = cos(a) = 0.95 a = arccos(0.95) = 18.19deg 6.) Real power, unaffected: P,load = 25,455.84W 7.) Desired system apparent power: P = S cos(a) 25,455.84 = S cos(18.19) S = 25,455.84 / cos(18.19) S = 26,794.87VA 8.) Desired system reactive power: Q,sys = S sin(a) Q,sys = 26,794.

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Why "Design Analysis"?

According to the Philippine Electrical Code (PEC), Article 1.3 (Electrical Plans and Specifications) states that the Plan Requirements (Section are as follows: (A) Location and Site Plans (B) Legend or Symbols (C) General Notes / Specifications (D) Electrical Layout (Floor Plan) (E) Schedule of Loads (F) Design Analysis (G) One Line Diagram Details of each part shall be covered on a later post, but together, they provide a complete picture of the electrical system of a given project. Parts A, B, D and G provide a "schematic overview" of the electrical system, while parts C and E provide an "elaborate description" of the nature and components of the electrical project. Design Analysis (F), however, is the part that provides justification for the decisions made in the schematic arrangements and the component choices for the project. As such, it requires a quantitative approach to prove beyond any doubt that the electrical system is establ