Distortion with a non-linear load

[dave313]

Hello all

I'm having a problem with a non-linear load driven with an AC generator.

This is an off-grid system with a generator used for charging batteries through an inverter. When the gen is charging the batteries, there is interference from the 20khz switching frequency of the inverter that causes lights to flicker and has some electronic devices not working properly. The AC waveform is distorted when fully charging and cleans up as the rate of charge decreases as the batteries come up to full.

I have heard that a tank circuit will fix this problem but the inverter manufacture can not provide me with a schematic. My system is different as I use a 240vac generator and a step down transformer to get 120vac at the inverters. The gen is 250 feet away and I use the transformer to negate some of the voltage drop.

I was once told to put a small motor run cap (1-4uf) across the input of the inverters and that works for awhile but the cap eventually fails. My problem seems to be exacerbated by the transformer. I did put a 1500 watt resistive load across the output of the gen and that reduces the interference but I don't want to waste fuel running this load.

I was thinking to use the step down transformer as part of a tank circuit but I would need a +90uf cap to resonate at 60hz if this will even work at all. I measured the inductance of the transformer output at around 4.5mH. I don't know if I need to include the transformer input inductance in the calculations and 90uf seems rather large to put across an AC line. Also, this problem has been present for 15+ years and with several different generators.

Can anyone offer me any clues on how to fix this problem?

Thank you

 

[sigmadelta]

Here's another capacitor that should be more suitable for your application than a motor running capacitor:

https://www.mouser.com/ProductDetai...lauzP%2BWPCYIRjxUd77Jm9NTjMobfp8EXFixTnckIg==

https://product.tdk.com/info/en/documents/data_sheet/20/25/ds/B33331V.pdf?ref_disty=mouser

The listed application in the datasheet is: "For general AC filtering application"
The relevant specs from the datasheet for your filtering needs are:

Dielectric Dissipation factor tan δ at +20 °C ≤ 2 • 10-4 at 1 kHz
-This low value at 1KHz helps the internal dielectric insulation within the cap from getting too hot from currents at higher harmonic frequencies. Motor capacitor tan δ is usually only spec'd and needed at 60Hz.

From the table on pg. 6 for the 4μF cap:

ESR – Equivalent Series resistance at 1KHz: 23 milliohms
-This low resistance allows more ultimate attenuation from the filter cap as well as reducing heat dissipation.

Maximum RMS current for continuous operation, including harmonics up to 20 kHz: 7A
-This allows the cap to conduct significant current from harmonics or transients without failing.

 

[dave313]

Thanks, I will order these caps. Is 4uf ok or should I get a selection of close values for testing?

 

[sigmadelta]

What value(s) of capacitor prevented the lights from flickering before the cap ultimately failed? I would use a similar value to what worked before.
4μF has a reactance of -j 663 ohms at 60Hz and so it will be drawing about 0.18 amps of reactive current with 120VAC across it, which is not all that much. So I see no real need to reduce it if that value worked well before. I would buy a couple of caps just in case, since they're relatively inexpensive.

As far as resonating with the transformer and attached generator to make a tank circuit, I would recommend against doing that. If there's not a sufficiently large applied load then the parallel resonant circuit can allow the voltage to rise up and possibly damage the inverter and/or other attached devices. Having even more capacitance to make it resonant below 60Hz is also bad, because the generator will sweep through resonance as it runs down after being turned off.