Inverters, again.

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CALLD
CALLD Solar Expert Posts: 230 ✭✭
in Off Grid Solar & Battery Systems #1
The time has come where I'm finally looking to trade in my surprisingly reliable cheap Chinese inverter for something I can trust a little better. My Kosun 2500w TSW inverter has and is still serving me very well save for one irritating thing - what happens if I don't have at least 400watts running off it.

Those with some electronics knowledge may know what the design flaw is but I can't really put my finger on it exactly. I think it has to do with the control loop which has been poorly designed to handle light loads. By analysing the symptoms and the power going in and coming out of the inverter it would appear that the AC voltage is controlled on the DC side of the inverter by means of pulse width modulation into a capacitor. What happens when the load is small is the PWM becomes erratic and unstable and the capacitor is too small to smooth out the power adequately. The reaction time of the control loop is too slow and voltage regulation basically consists of the capacitor charging and discharging several times per second in sharp spurts. This is detectable on the battery wires as current spikes of several hundred Ampers when the average current is only a few amps. Increase the load above 400w and everything stabilises, current spikes disappear and output voltage stabilises into a near perfect sine wave without any amplitude discrepancies.

As bad as it sounds it has been running like this for 6 months now without a hitch and none of my appliances seam to mind at all, but it just doesn't make me feel any better about it when every bit of logic tells me there has to be a better way to do things.

I need an inverter that can support at least 2500 watts from time to time, but still run quietly and very efficiently at loads of only a few watts to a few hundred during daytime charging. I'm not keen on a heavy low frequency transformer type inverter as I've heard these consume a lot of power idling or on small loads. I'd like a compact light-weight design using the latest technology for maximum efficiency across a wide range of loads. Is there anything out there that ticks all those boxes and is still regarded as reasonable quality?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Inverters, again.

    It sounds like the FETs that chop the DC into AC (probably their control service)--There are "high side" and "low side" FETs (H Bridge)... And if they both turn on at the same time, you will have a virtual dead short from DC + to DC -. That can cause these very short/high current spikes.

    This is a very simple diagram--Where the load is the inductors (or transformer) used to step up the chopped DC to AC (MSW inverters look almost exactly like this, TSW inverters are more complex):

    Attachment not found.
    Normally, only the upper left/lower right is turned on at the same time (or upper right and lower left). If the two left or two right turn on at the same time, lots of issues.

    If this is what is happening--Replace the unit soon. Nothing good is going to happen.

    -Bill "just guessing" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Thom
    Thom Solar Expert Posts: 196 ✭✭✭
    #3
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    Re: Inverters, again.

    Y not get a very small invertor only use the 2500 for big loads

    Thom
    Off grid since 1984. 430w of panel, 300w suresine , 4 gc batteries 12v system, Rogue mpt3024 charge controller , air breeze windmill, Mikita 2400w generator . Added 2@ 100w panel with a midnight brat 
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #4
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    Re: Inverters, again.

    BB, Thanks for the explanation of what may be causing this issue. I've always been very curious as to why it does this and more specifically why it only happens when the load is under 400watts. The problem is more pronounced when the battery voltage is above 25volts and even more so during absorption when the battery voltage is at 29.4v.

    What makes your theory even more believable is that I've noticed that if the load is say 370watts just before the battery reaches absorb (charging voltage 28.8, current 28amps, AC load 370w), I can switch on a fan to increase the load to 420w the battery voltage suddenly goes up to absorb and the current acceptance actually goes down a bit (V=29.4, A=26.5, AC load 420w). The leap in efficiency certainly shows that something is sucking the power when the inverter is having it's jerky fits...

    It's something I wish the unsuspecting Mr Jones down the street could be made aware of before thinking he's striking a good deal when the local Walmart starts selling these inverters...

    # Thom, I need a big inverter like this because when we have rolling blackouts I still want to be able to make coffee and my wife still wants to use the hair drier and the whole house is already wired up to the current inverter. The fridges also have massive start-up surges. Remembering to switch from a small inverter to a big inverter may be a problem before my wife decides she wants to dry hair...;)
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