Capacitors in inverter circuitry...

[Externet]

Hello all.
What are the numerous capacitors inside inverters for ? Are they in the DC input circuitry to smooth insolation variations or there is another reason ? Or are they in the AC section to supply current during demand surges from sudden equipment starting ?

 

[Risky Rob]

I'm not an expert, but have been told the big capacitors on the main board of my inverter store charge and help with surge demand, such as starting motors. I only have 2, but there are spots to solder in 3 more.

 

[nc73]

They store energy. Did help prevent huge voltage dips when my high loads come on. Fixed my heat pump because it smoothed out the crazy voltage dips when it was running. I put a 20uf at the ac output.

 

[chess-equality]

They store energy. Did help prevent huge voltage dips when my high loads come on. Fixed my heat pump because it smoothed out the crazy voltage dips when it was running. I put a 20uf at the ac output.

Do you also know how to size the DC side? I'm intending to add a super cap in my 12v system, parallel to the battery. I just don't know what size.

 

[Externet]

They store energy. Did help prevent huge voltage dips when my high loads come on. Fixed my heat pump because it smoothed out the crazy voltage dips when it was running. I put a 20uf at the ac output.

Thanks. This I do not understand. A capacitor at the AC output connection line cannot prevent dips from AC load lines becoming 'filled'

 

[Externet]

Do you also know how to size the DC side? I'm intending to add a super cap in my 12v system, parallel to the battery. I just don't know what size.

The formula is time in seconds = Q in Coulombs divided by Intensity in Amperes t=Q/I
And Charge in Coulombs = Capacity in Farads times Voltage in volts; Q=CV

A 1 Farad super capacitor charged at 12VDC can provide 1F x 12V = 12 Coulomb
Those 12 Coulomb from the capacitor can smooth/fill/ source extra 10 Amperes sag, shortage or dip in the demand during
time = 12 Coulombs divided by 10 Amperes - 1.2 seconds.

 

[BentleyJ]

A 1 Farad super capacitor charged at 12VDC can provide 1F x 12V = 12 Coulomb

Super Caps that I'm aware of have an upper voltage limit of 3.65V each so you would need 4 or 5 in series for a 12V system.

 

[HighDesertOffgrid]

Thanks. This I do not understand. A capacitor at the AC output connection line cannot prevent dips from AC load lines becoming 'filled'

Smoothing caps on the ac side keep the sine wave siney.

 

[Hedges]

The big caps in an inverter smooth out pulses of current drawn by high frequency step up SMPS (HF inverters) and store the boosted voltage (HF inverters), They smooth the high frequency pulses used to synthesize a sine wave (HF and LF inverters.)

From my calculations and measurements, they are not big enough on the battery side to smooth out a single 8.3 millisecond half of a 60 Hz sine wave. Just the higher frequency switching. The 60 Hz ripple current is drawn from battery; voltage ripple isn't enough to extract much energy from the capacitors.

Battery ripple current

After reading of someone tripping battery breakers at well below rating, I realized that could be due to pulses of current drawn by the inverter. Ideally capacitors would smooth out current draw so high frequency switching pulses and 60 Hz draw all came from the caps, and current from batteries...

diysolarforum.com

Supercapacitors might do that, much greater capacity. If battery voltage sags, or wire resistance causes voltage drop, then the capacitors would do their thing. Or, if an inverter had a big inductor on its input as 60 Hz EMI filter. But you wouldn't believe how massive that would need to be.


From the boosted high voltage of an HF inverter, or the PV input of a grid-tie inverter, they do smooth out the 60 Hz. Several volts ripple of the capacitor supplies that energy.

High Frequency Inverter battery current waveform

In the interest of science ;) I have captured waveforms drawn by a high frequency inverter from the battery. This could give some insight to how a BMS might react. About 25 years ago I found myself with idle hands and some money ($800?) burning a hole in my pocket. I wandered into Quement...

diysolarforum.com