5
3.3 Filter coils
There are other advantages to be gained from employing filter coils. The parallel capacitor can cause troublesome switching phenomena to occur, which can give rise to very large current surges.Although these surges are of only very short duration (a few milliseconds), they are nevertheless sufficient to cause switching relays to stick or circuit breakers to switch off.The filter coil serves to prevent this problem by damping the very short, high amplitude pulses in the current.
The type of filter coil needed depends on the capacitance of the capacitor employed. So, in fact every capacitor needs its own filter coil. But in some cases it is possible to group the capacitors and match them with the corresponding filter coil. For example: two capacitors of 4 µF parallel can be connected in series with one filter coil for 8 µF (see Fig. 109).
Also central filter coil systems exist where a filter system in the supply system is blocking the applied signalling frequencies.
Although the voltage across the filter coils is rather low (approx. 14 to 20 V), the filter coils have to be regarded as ballasts, as they are directly connected to the mains.They also cause some additional watt losses.
The amount of third and fifth harmonics in the mains current will rise in cases where the mains supply voltage is disturbed with third or fifth harmonics, when applying a filter coil.The total impedance for the combination of capacitor and filter coil is lower than the impedance of the sole capacitor for these frequencies (see section 5.3.9: Harmonic distortion and Fig. 108).
Fig. 109. Different ways of grouping capacitors to match them with the corresponding filter coil.
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2 x 4 ∝F | = |
capacitors | |
2 x 4 ∝F | |
coils |
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N 
4 x 4 ∝F capacitors
1 x 8 ∝F filter coil
3 
4 
Power factor correction
Circuits with
Without the capacitor the inductive ballast causes a phase shift of the current, which is lagging behind the applied voltage.
Fig. 110. Power factor correction with a parallel compensating capacitor.
B
L 
C
La
N 
119
