Philips Electromagnetic Lamp Sin, 121, Lamp factor = lamp wattage / lamp voltage . lamp current

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3.4 Power factor correction

ballast, the capacitor current is leading 90 electrical degrees to the capacitor voltage (which is the mains voltage). So the capacitor current has the opposite direction of Ilsin ϕ (see Fig. 114).

Fig. 114. Compensated circuit.

Vb Vm

Il cos ϕ

I
cap	ϕ

I	Il	Vl
	sin
l
	ϕ

Maximum compensation is achieved when the current through the capacitor Ic = Il sin ϕ ; then the power factor is 1.This is purely theoretical, as the vector diagram is only valid for the fundamentals of the currents. Due to distortion in the lamp current (see section 5.3.9: Harmonic distortion), the maximum practical power factor is between 0.95 and 0.98.This explains the difference between power factor and cos ϕ.

The power factor is the result of the quotient of the actual wattage and the product of mains voltage and mains current, including the harmonics, and can be calculated as follows:

Power factor (P.F.) = total wattage/mains voltage . mains current

The angle ϕ is the phase shift angle between mains voltage and mains current and can be found and calculated by means of the vector diagram.This is only valid for the fundamentals and does not take into account the harmonics.

The same analogy is valid for the lamp: there is practically no phase

shift between lamp voltage and lamp current: both are zero at the same time. So the phase angle α is zero and cos α = 1.

The product of lamp voltage and lamp current does not equal the lamp wattage; the difference is called lamp factor:

Lamp factor = lamp wattage / lamp voltage . lamp current

and has a value between 0.8 and 0.9. For the same lamp type the lamp factor is higher for higher wattages, identical to the lamp efficacy. Typical capacitor values for this parallel compensation (also sometimes called mono-compensation) for a 50 Hz mains are 4.5 µF for a 36 or 40 W fluorescent lamp and 6.5 µF for a 58 or 65 W lamp.

A second method for compensation is the so-called duo-circuit.This is employed for pairs of lamps, as for example in two-lamp luminaires. Here the capacitor is placed in series with one of the ballasts (see Fig. 115).

121