142 Mains Interference 7 Mains Interference 7.1 What are Harmonics? 7.1.1 Linear Loads On a sinusoidal AC supply a purely resistive load (for example an incandescent light bulb) will draw a sinusoidal current, in phase with the supply voltage. The power dissipated by the load is: P = U × I For reactive loads (such as an induction motor) the current will no longer be in phase with the voltage, but will lag the voltage creating a lagging true power factor with a value less than 1. In the case of capacitive loads the current is in advance of the voltage, creating a leading true power factor with a value less than 1. Voltage Current Displacement angle, Fig. 7.1 Linear Load In this case, the AC power has three components: real power (P), reactive power (Q) and apparent power (S). The apparent power is: S = U × I In the case of a perfectly sinusoidal waveform P, Q and S can be expressed as vectors that form a triangle: S2 = P2+ Q2 Units: S in [kVA], P in [kW] and Q in [kVAR].
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