Philips Electromagnetic Lamp manual 3 3 Filter coils

5

3.2 Capacitors

Capacitors for lighting applications must have a discharge resistor connected across the terminals to ensure that the capacitor voltage is less than 50 V within 1 minute after switching off the mains power. In special cases the voltage level must be 35 V within 1 second, see

IEC 598-8.2.7.

3 3 Filter coils

Fig. 108. Impedance of a filter coil, a capacitor and a coil/capacitor combination as a function of frequency.

In some countries, including Belgium, the Netherlands and France, the electric distribution network is used for transmitting messages under responsibility of the local energy supply authority.

Signals are sent over the electricity supply network for a number of purposes: to switch road lighting, to call up fire brigades and the police, to switch night-tariff kWh-meters, and so on. It is important, therefore, that this signalling system is not disturbed, which may occur when parallel power factor correction capacitors for lamp circuits are employed. Capacitors present a low reactance to the 200-1600 Hz signals employed for signalling, with the result that these are in danger of being short-circuited in a capacitive circuit.To avoid this, a coil must be connected in series with the capacitor connected parallel to the mains.This filter coil, as it is termed, presents a reactance that increases with rising signal frequency.The coil reactance is therefore chosen such as to balance out the reactance of the capacitor at 200 Hz (the resonance frequency, see Fig. 108).

For currents with a frequency of 50 Hz the circuit is predominantly capacitive, which is necessary for power factor correction.Above 200 Hz the circuit becomes predominantly inductive, which is necessary for the blocking of audio-frequency signals.At 200 Hz the impedance is only formed by the ohmic resistance, mainly of the filter coil.

As can be seen from the graph, the filter coil is effective for audio signals of 300 Hz and higher, because then the impedance of the coil/capacitor combination is higher than the impedance of the sole capacitor. Filter coils should not be used when the audio signals are 300 HZ or lower.

impedance (Z)

						impedance of filtercoil
								Z = ωL
3
10
8
6
										impedance of coil
4										and capacitor
										Z =		ωL -1
2												ωC
										impedance of capacitor
										Z = 1
2											ωC
10
8
6
4
2
1		capacitive								inductive
10					2					3						4
1	2	4	6	8		2	4	6	8			2	4	6	8
10					10					10						10
													frequency(Hz)
118