How can a dimmed incandescent lamp cause fundamental reactive power?

How can a dimmed incandescent lamp cause fundamental reactive power, while there are no energy stores in the circuit and no sections of adverse polarity of voltage and current occur?

Indeed, this seems awkward, and yet it is a fact. If Fourier analysis is carried out on the current of a phase angle controlled dimmer with ohmic load, the fundamental current turns out to be lagging behind the feeding voltage. This is an instance of fundamental reactive power which indeed can be offset with a compensation capacitance. This is fairly easy to imagine, since during the first section of each semi wave before firing the thyristor no current flows and after firing there is the normal ohmic lamp current. Attributing a sinusoidal constituent of same frequency to such current must evidently lead to a lag of same current, as mentioned unsymmetry of current flow cannot be concealed in constituents the frequencies of which are integer mutiples of the voltage frequency.

Yet a harmonics analyser will display presence of fundamental reactive power only if triggered with the line voltage. When the phase angle controlled lamp voltage (behind the dimmer) is used, voltage and current in the pure ohmic load, even if not sinusoidal, are proportional to each other, and there is no fundamental reactive power recorded, only so-called wattless power contained in the harmonics.

Mind: It is not necessarily only the presence of energy stores that causes / compensates reactive power but also the disproportionality of voltage and current.


Harnaak Khalsa's picture

On a similar note, what is the status of reactive power for a diode resistor load connnected to a sinusoidal supply when viewed before the diode? During the forward conducting cycle the half wave current is in phase relationship with the applied voltage (assume lossless diode). The diode-R load would be fully absorbing and reactive power is not expected to be flowing.

By Harnaak Khalsa 21/06/2007
Log in to post comments

Follow us