Help Me Understand This Harmonic Filter

C

CoolWill

Senior Member
Location
Alabama
Occupation
Electrical Contractor
I have an Eaton 3 kVA UPS that is playing havoc with my inverter. The UPS causes the inverter to fault and kick off. Apparently it's a problem many people have with devices with rectifiers. Hair dryers that use a half wave rectifier for the low setting does it as well. So I investigated using my oscilloscope and found both the UPS and the hair dryer have significant harmonics around 180 Hz.

So I built an LC shunt filter with a 4.83 mH inductor and 160 uF capacitor, which gets me pretty close to 180 Hz. And it works! Both devices that were causing problems now work just fine with the filter in the circuit. But the current flowing in the filter is pretty significant. 16-ish amps. It is reactive power, so it isn't drawing much from the battery, but it the amps are still flowing so some is lost as heat. My question is, when I attempted to bring that current down by changing L to 9.66 mH and C to 80 uF, it didn't work as well and the inverter would still fault as before. The math works out the same for the resonant frequency, but I don't understand why it doesn't work the same.

Any thoughts?
 
CoolWill said:
Any thoughts?
Click to expand...
Source impedance of the 3rd harm of the UPS is apparently relatively high (e.g > 100 ohms), thus the capacitance is the main contributor to EMI reduction. The inductance is relatively negligible factor.

try fiipping the filter to reverse input and output. Of course, not you may have too high of source impedcance and you will have instability.

usea couplt of CTs with your scope to read the line and capacitor 3rd harmonic levels to gain understanding and provide 'education'
 
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not you may have too high

edit: NOW your many have... e.g. Middlebrook criteria violated...
 
junkhound said:
Source impedance of the 3rd harm of the UPS is apparently relatively high (e.g > 100 ohms), thus the capacitance is the main contributor to EMI reduction. The inductance is relatively negligible factor.

try fiipping the filter to reverse input and output. Of course, NOW you may have too high of source impedcance and you will have instability.

use a couple of CTs with your scope to read the line and capacitor 3rd harmonic levels to gain understanding and provide 'education'
Click to expand...
CoolWill I think this is what he meant to say.
 
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