Don't buy this GTI

[Simon]

I bought this GTI on eBay. Well it worked fine for a couple of months until today. Now i have opened this before when I bought it so i know that things have changed somewhat.

I pluged in my lawnmower next to the GTI and pulled the trigger. At which point i heard a pop and nothing happened. The circuit breaker for the house sockets cut out. After swapping cables and checking fuses it became apparent that the GTI itslef was damaged. The photos show the results. Now I've never seen a ceramic capacitor "leak", the stuff is quite solid despite me opening it about 10 minutes after to bang. The output fuse which was a soldered in one blew the cap off (which is what the pop sound was).

Any ideas if this will work if I change the blown parts. the confusing bit is the "leaked" ceramic capacitor (rated for 400V). It looks like the control circuitry has gone wrong and let a large current in. (I only had 90W of panels connected and these normally output about 50W)

The seller of this "thing" on ebay is sungoldpower

[Zad]

It looks like a mixture of melted polyester dielectric and aluminium. Nice! I guess a voltage spike from the lawnmower caused a dielectric breakdown which led to a short. Usually semiconductors nobly sacrifice themselves to save cheap components, so I would check all the diodes and transistors before risking running it again.

[Simon]

Well the blown capacitor is right up the other end of the board near the 12V in side although it is near  a bridge rectifier so might be something to do with the first boost stage that makes the 12v into something around 240V. I'm not really looking to repair this, one failure is one too many, it is indicative of a design issue. This is not the first time I've used the lawn mower right on top of it

[Zero999]

A GTI is comprised of four parts: DC-DC converter which boosts the battery voltage to above the mains peak voltage (around 370V in this case as it's rated to 260V), the inverter part which converts the high voltage DC to AC, a filter and the control circuitry.

The output from the GTI is constant current in phase with the mains with the current being dependant on the power in more than the mains voltage, although there will be more current at lower voltages.

I have a horrible feeling that cheap and nasty inverters inject a non-sinusoidal current into the mains which is not good for appliances, especially those containing induction motors, although your lawnmower will contain a universal motor so this isn't the case here.

[Simon]

Well I was putting 50W into the inverter so it's output would hardly affect a 900W motor. The worrying bit is that it also blew a 30A circuit breaker so it took one hell of a pasting from the mains which I suppose explains the exploded 5A fuse.

The output is supposed to be a pure sine wave, it must be close as it is connected to the mains and if it does not follow that mains waveform closely it will be either trying to put out too much current or it will start to absorb current. I'm assuming (and based on the poor english in the manual) that it outputs a voltage just slightly higher than the mains voltage, just enough to get it's current to feed into the mains.

I suppose i could use the case to make a variable load device for testing stuff 

[stl]

The output is supposed to be a pure sine wave

I really doubt that, usually it's not a sine wave at all unless it's a high end equipment... Same thing happens with the most of the UPS

[Simon]

a GTI MUST follow the mains voltage or there will be a big bang, I assume that something went wrong with the control circuitry that was supposed to be tracking the mains voltage

The manufacturer replied and said that it could be my fault for misusing and and hoped I understand. I replied say no I don't understand and neither do they. They have another week and I'll be kicking off big time.

[Zero999]

Why do you think injecting a low current non-sinusoidal waveform into the mains will cause a huge problem?

It's no worse or better than a non-sinusoidal load such as a compact fluorescent or DC power supply without active power factor correction.

There won't be any big bang, all that will happen is the voltage will be distorted very slightly and as the impedance is so low, the current would have to be high to skew the waveform significantly.

Drawing or injecting a non-sinusoidal current into the mains is not good but a tiny 50W inverter will probably contribute less of a harmonic current than all of the compact fluorescent lamps in your house.

Suppose it's a cheap and nasty modified sinewave inverter giving an RMS current of 24% of the output and the impedance of the cable between the socket and the regulated substation transformer is 0.1 Ohm. The total output current from the inverter is 50/230 = 217mA, the RMS harmonic current will be 52.2mA so the harmonic voltage is just 5.22mV which is nothing compared to 230V. Of course, this is overly simplistic as the impedance of the mains won't be purely resistive.

This is only a problem if your inverter is huge (something like >2kW), it's connected to a long extension lead (high source impedance) and you have a really sensitive appliance plugged into the same extension lead.

[Simon]

I think the inverter may actually be using PWM at higher frequency to replicate the "pure sine wave" whether or not it is then low pass filtered is another matter

[Zero999]

Maybe so but I don't think you understand the basic principle behind a GTI.

To the mains a GTI looks like a constant current source. The output voltage is equal to that of the mains, regardless of whether the current is sinusoidal or perfectly in phase with the mains or not. In theory the current will be the same regardless of the voltage, meaning if the output terminals are shorted the voltage will be zero, if the output terminals are open circuit, the output voltage sill be infinite. In practise it's not a perfect current source, the mains needs to the present to act as a frequency reference and the protection circuit limits the maximum and minimum voltage.

Think of your solar panel acting like a current source when it's charging the batteries, you could short circuit it or connect a 6V battery, rather than a 12V batter and the current will be the same. A GTI is similar, except it's AC not DC.

[Simon]

surely if the output of the GTI falls below the mains voltage it will start to take current into it. Like if you have two batteries in parallel, if one is a higher voltage it will srat to feed into the other.

The sellers description of the GTI working (which i admit is very simplistic) is that it outputs a slightly higher voltage than the mains (well they say volume but meant voltage  ) which sounds logical

[Zero999]

surely if the output of the GTI falls below the mains voltage it will start to take current into it. Like if you have two batteries in parallel, if one is a higher voltage it will srat to feed into the other.

In theory yes but if they output transistors off at that point of the cycle, no current will flow into the inverter, just like most solar panels come with a built-in blocking diode to stop backfeeding when the sun goes down.

The sellers description of the GTI working (which i admit is very simplistic) is that it outputs a slightly higher voltage than the mains (well they say volume but meant voltage  ) which sounds logical

The Seller dumbed it down.

Of course the output voltage is going to be slightly higher than the mains but it's negligible. It'll simply be equal to the cable resistance multiplied by the current flowing through it so for all intents and purposes the output voltage is equal to the mains. If the output voltage is significantly higher than the mains (by more than tens of millivolts in this case) the cable resistance is too high. It's just Ohm'd law.

[Simon]

oh yes I'd only expect it to be a few mV abobe the mains. WEll if they don't reply soon I'll be creating some listings of my own on eBay for this inverter. And i wouldn't be the first

[Polossatik]

sexy destruction.
congrats