12V to 240V inverter discontinuous output

Hi,
I need to repair a 12V to 240V 1500W inverter connected to a 500W water pump.

Lately I am hearing that the pump is not working smoothly as it should. It seems the current is not continuous and smooth. The pump can be heard changing speed sharply in short instants.

I will give more info later when I get the inverter and test it myself.

Do you think it is something I can replace; bad cap or something?

thanks.

One possible cause is the output of the inverter. Check well if it a pure sine wave, modified sine wave or square wave inverter? Some inductive loads are too sensitive to square and modified sinewave inverter and do not work well in them.

Ogu Reginald said:

One possible cause is the output of the inverter. Check well if it a pure sine wave, modified sine wave or square wave inverter? Some inductive loads are too sensitive to square and modified sinewave inverter and do not work well in them.

Click to expand...

Oki I will check as soon as i get the inverter and let you know.

Is it best if I check the output when the inverter is loaded? If it is a pure sine wave inverter is there a possible fix?

If the inverter is a pure sine wave inverter and it is in good working condition, try powering the pump with another source of power (may be a generator) and see the behavior.

Ogu Reginald said:

If the inverter is a pure sine wave inverter and it is in good working condition, try powering the pump with another source of power (may be a generator) and see the behavior.

Click to expand...

The inverter seems to be a modified sine wave. Its make is DAK, a Chinese manufacture.

Yesterday I tested it with a relatively power full fan although I'm sure it was nothing near 500W. The result of the first few minutes of operation can be shown in the video below;

https://www.youtube.com/watch?v=HQOrSowCuyQ&list=UU5f0v6MZadFy1TDCXd2B7Mg

I left it for about 30minutes turned on while I went to wash my car and surprisingly when I went to check it again it was working smoothly with no 'glitches'. It was like something either needed to warm up or else there was like a reservoir of some kind that took time to 'fill up'.

Also, as you can observe in the video the sharp 'gliches' are irregular so I wasn't thinking about harmonics. What do you think? Is there I way I can fix it?

Could be heat-related. A hairline fracture at a solder joint, for instance.

You might be able to track it down with a hot-air gun (when the board is cool), or by using freeze spray (when components have heated up).
Etc.

Use a magnifying glass to examine every board trace and solder connection.

Push gently on components while the inverter is operating. Press up/down on the board with a non-conducting tool. (Not so hard that you break something.)

The inverter must draw 50A or more, to power the 500W pump. Look at any components in the path, and see if they show signs of deterioration.

When I was a student, one day our Head of Department called us a asked us whether we know that partial contact is worse than short circuiting. Out of surprise, I asked him to throw more light on what he meant. He then told us that when a short circuit occurs, the effect is seen immediately and subsequently the affected part located but in the case of partial contact, he said that it can trouble for a long time without you knowing that your problem is actually a partial contact.


As suggested above, take time and gently touch most of the components, trace all conductive tracks of the PCB and monitor if there is a component(s) that is burning or heating up.

BradtheRad said:

Could be heat-related. A hairline fracture at a solder joint, for instance.

You might be able to track it down with a hot-air gun (when the board is cool), or by using freeze spray (when components have heated up).
Etc.

Use a magnifying glass to examine every board trace and solder connection.

Push gently on components while the inverter is operating. Press up/down on the board with a non-conducting tool. (Not so hard that you break something.)

The inverter must draw 50A or more, to power the 500W pump. Look at any components in the path, and see if they show signs of deterioration.

Click to expand...

BradtheRad, and Ogu,

some mosfets exploded (0) (0) .. I'll have to replace them. Hope everything will be working after replacement. See pics. I think that they exploded due to absence of heat sink, or maybe somethng shorted from underneath. Tonight I will go and buy some IRF3205N. I removed the burnt mosfets and at least the inverter switched on normally and 223V ac at the output but I did not load it until i replace the mosfets.

'Glitch' Problem

I resoldered all the joints at the underside. I also resoldered the pins on the SMD ICs (TL494C), and SMD diodes. I didn't try resoldering smd resistor and caps yet. When I tried to connect the fan, still hear about '3' glitches a minute although now they are less frequent.

I went a step further and I think I have a good hint what is causing this. I put my DMM on frequency measurement and when ever I hear this change in speed, the freq output goes from 50.6Hz to 52.5Hz and it is the same everytime. The DMM is true RMS and the voltage output is quite steady. This says it all why the probelm arises only with inductive loads such as this fan. (inductive motor).

I tried to trace the problem. The crystal to the PIC micro controller is very steady. I cannot detect any change with my DMM. Probably, this frequency has nothing to do with the mains output frequency. There are the LT494C which are controlled by the PIC. I think they are controlled by the DAC of the PIC.

NAME: GMT-INVERTER POWER
VER: GMT-8809.07.V1.0

Attached are the photos maybe you can give me some hints or tell me where;

Mosfets: **broken link removed**
Inverter:

**broken link removed**

Control circuit:

**broken link removed**

**broken link removed**

**broken link removed**

thanks a lot for your help

johnny

Yes, for now don't touch any SMD components, replace the MOSFETS and give me a feedback.

still hear about '3' glitches a minute

Click to expand...

This brings to mind static charge buildup, discharging suddenly every several seconds.

However it would be odd to have that happen in your circumstances.

But maybe it doesn't have to be kV. It could be peaks of 340V, arcing between two points very close together. Perhaps inside a capacitor. You might even be able to hear it. Or (less likely) between windings of an inductor/ transformer, etc.

BradtheRad said:

This brings to mind static charge buildup, discharging suddenly every several seconds.

However it would be odd to have that happen in your circumstances.

But maybe it doesn't have to be kV. It could be peaks of 340V, arcing between two points very close together. Perhaps inside a capacitor. You might even be able to hear it. Or (less likely) between windings of an inductor/ transformer, etc.

Click to expand...

I replaced the mosfets and everything is working as before. (glitch still there).

I stopped the cooling fans to hear a little better but there was no arking noise :s.

I noticed that from one of the LT494 there was a steady freq of 34k on 2 pins while on the other lt494 there was nothing steady. Is it that one of them is mulfunctioning?

Also, with regards to the capacitors, i think you are refering to those at the high voltage side.. (coin shaped). Shal i take them out and test them?

i think that a capacitor wizard is very usefull for these type of repairs. what do you think. I read that you can test them in circuit.

Also if the arcing is in the transformers there is not much i can do i think.

jonnybgood said:

I noticed that from one of the LT494 there was a steady freq of 34k on 2 pins while on the other lt494 there was nothing steady. Is it that one of them is mulfunctioning?

Click to expand...

Parasitic oscillations might account for the 34kHz.

Unless that is supposed to be the operating frequency of the switching IC. Then the problem might be the other LT494 which lacks the 34kHz. (I have no direct experience with this IC.)

Parasitic oscillations tend to result in components heating up. Suppose a safeguard in your circuit kicks in, and turns something off for a split second? That would create the glitch. This would happen regularly, same as your glitch happening regularly.

A strategically placed capacitor is often a solution to stop parasitic oscillations. This means a lot of experimenting. It carries risks of making something worse.

Also, with regards to the capacitors, i think you are refering to those at the high voltage side.. (coin shaped). Shal i take them out and test them?

i think that a capacitor wizard is very usefull for these type of repairs. what do you think. I read that you can test them in circuit.

Click to expand...

Capacitors are frequent weak points. An ESR meter is handy to test them in circuit.

It's a question whether a deteriorating cap is likely to allow normal running except for a glitch every several seconds?

Also if the arcing is in the transformers there is not much i can do i think.

Click to expand...

Maybe not. It is not likely to happen anyway. If arcing occurs at all, it would tend to be constant.

Nevertheless just in case, turn off all lights and look for any arcs in the dark.

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Your photographs show a lot of components affected by the burned mosfets. If possible you should test them. This won't be easy since they're surface mount.