SMPS inverter starts giving low output voltage

[sam781]

I have been using a SMPS inverter for one years. Rating is as below -
Input voltage: 12V dc
Output voltage: 230V ac
Power: 500VA

I measured the output voltage before which was around 230-240v ac. All the loads were running properly. Suddenly, we noticed that Ceiling Fan moved slow than usual. I measured the output voltage ad 170-180v ac. Low output voltage is causing the low fan speed. I also measured the input battery voltage which is okay (around 13v DC). I cannot find the problem of this low output voltage. Where I can look in to resolve the problem? Any idea?

 

[tpetar]

Did you try to add some load such as light bulb for example and measure voltage ?

Inverter is commercial (what model ID and manufacturer?) or DIY ?



Best regards,
Peter

 

[sam781]

Non branded china inverter cum charger

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Model: KBM12-500B
I googled the model number and found it in the following link -
https://vivianhuiyi.en.made-in-china.com/product/CbuElYdcADVW/China-Power-Inverter-KBM12-500B-.html

 

[tpetar]

This is probably cheap and bad quality device. I see this like Chinese SMPS inverter devices on every corner.

For start try to open and look inside, inspect parts, check for PCB black or brown colored areas due to longer overheating,...

Make some quality photo and post here.


Be careful dont touch any internal component with hands, first disconnect battery, and avoid touching HV capacitors.


Did you try with some load, what voltage you have ?


Best regards,
Peter

 

[sam781]

I found almost same voltage with load (tested with around 120w load) and without load. It is very hard to unscrew the PCB from the body as its MOSFETS are tightly attached with the body with screws. Very difficult to use screwdriver in the congested areas to unscrew. I'll try to take some photos and upload here...

 

[sam781]

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Some photos have been attached. Please check...

 

[sam781]

Any suggestion?

 

[mtwieg]

Likely a problem with the DC-DC stage, maybe a bad optoisolator or an out of spec resistor. It's impossible to tell from here without a schematic, but I would start by cleaning off as much dust and dirt as possible, preferably with isopropyl alcohol.

 

[sam781]

Ferrite Core of one of the transformers is lose. Can it be an issue? I also found a Varistor near a fuse (3rd photo), two pins are short circuited.

 

[sam781]

May be secondary winding of dc-dc stage transformer is faulty (winding short circuited at somewhere causing lesser turns). Only the Fan speed is impacted for such low voltage. Lights are okay...

I faced another problem with this inverter cum charger at the time of installation. Cannot connect inverter one output terminal to ground to make common ground for power line and inverter line. While connecting charger and power line with a common ground fuse burnt. I saw some other UPS (IPS) uses common ground. What is my fault here? Any idea?