Input Power to Computers

[deeveeyes]

We are working on design of a filter to filter out 3rd harmonics generated by SMPS of computers. However, when we conducted a log on the input parameters of a computer, we found that it draws around 0.6 Amps @ 0.6 PF lead !! The filter itself is a passive filter (LC combination) and hence would deliver leading KVARs. If We connect this filter at the input of computer, will it damage the power supply of the computer ?? Pls advise.

 

[jbartos]

Suggestion: Since you selected L-C passive filter, there is a concern with potential oscillations/resonance between the L-C passive filter and the SMPS. There may be a need for some damping resistor across the inductance. The damping resistor is normally used in L-C harmonic filters for the higher order harmonics mitigation, e.g. 7th, 11th, 13th, etc.
The L-C filter can be designed with lagging power factor. It depends if Xc=1/(2pifC) is smaller or greater than Xl=2pifL.

 

[deeveeyes]

Thanks Jbartos: The LC combination is tuned near 3rd harmonic and hence how can I make it lagging ?? for ex: 36 uF and 32mH will tune to 3rd (on 50 Hz supply) and combination will be having a Leading PF!

 

[jbartos]

Suggestion:
Xl>Xc
2pifL>1/(2pifC)
L>1/(4 x pi x pi x f x f x C)= 1/(4 x 3.14 x 3.14 x 50 x 50 x 36/1000000)=28.17mH

 

[deeveeyes]

Jbartos,
This will tune it to 158 Hz and will amplify 3rd harmonic instead of reducing it ?! I want to tune it to less than 150 Hz. !!

 

[jbartos]

Suggestion: Then, the capacitance has to be increased to obtain proper frequency from:

f=1/(2 x 3.14 x sqrt(L x C))= 1/{2 x 3.14 x sqrt[(28/1000) x (100/1000000)]}= 95Hz
for capacitance C=100microF, for example; however, the capacitance value may need some fine tuning to have the power factor acceptably high.

 

[deeveeyes]

jbartos,
for xl > xc at 36 uF, the value of L is 281.7 mH and not 28.17 mH as indicated by you earlier. This is not workable. Hence there is no alternative than to use LC combination tuned to near 150 Hz, but we have to live with leading PF at the output of filter, hence pls advice what other precaution should we take to safeguard working of SMPS of computers??

 

[jbartos]

I beg your pardon for that typo. To calculate the C50 and L50 for f=150 Hz third harmonic use:
C50 x L50 = 1/(2 x 3.14 x f)**2 = 1/(2 x 3.14 x 50)**2=0.000010142, in F x H
Next, L50 = 282 milliH
C50 = 0.000010142/(0.282)=36 microF
Backcheck:
2 x 3.14 x 50 x 0.282 = 88.55 > 1/(2 x 3.14 x 50 x 36/1000000) = 88.46
If you intend to keep your existing L and C values, a check for oscillation/resonances and current rms drawn by SMPS should be performed.