APPROX. DIAGRAM OF THE POWER FACTOR (COSϕ) AGAINST
THE OUTPUT D.C. VOLTAGE BY A.C./D.C. SCR CONVERTER
The power factor doesn’t depend on the output current but on the output dc voltage only
Cos ϕ
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0
10
20
30
25
40
60
50
70
75
80
Vc %
90
100
Power factor correction
The capacitors for the power factor correction must be choosen properly, due to the current (voltage)
harmonics supplied by the ac/dc converters, that can give cause for:
- high current value flowing through the capacitors
- dangerous resonance effect, that can damage some electronic components.
Keeping in mind what above it must be properly considered what follows:
a) the capacitors must withstand a max voltage 30% approx. higher in comparison with the network
voltage.
Sometimes proper inductors are series connected with the capacitors in order to limit the currernt
flowing through the capacitors.
b) In any case the capacitors set must not be placed close to the converter, in order to avoid dangerous
resonance effect that could damage some electronic components.
c) Finally, as regards the design of the reactive power of the capacitor set the average power factor of the
plant must be considered.
Only one capacitor set, provided with automatic control, can be provided in order to obtain the
maximum power factor value.
IN ORDER TO AVOID DANGEROUS RESONANCE EFFECT THE CAPACITORS MUST NOT
BE CONNECTED CLOSE TO THE CONVERTER