Beware modified sine inverters!

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AccordGuy
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Joined: Tue Jul 01, 2008 11:47 am
Location: West Sussex, UK
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Beware modified sine inverters!

Post by AccordGuy »

I'm looking to take back my 600W inverter to the shop I bought it from a couple of weeks ago.

It works ok (in so much as modified sine ones can) but while the lights and the laptop work on on it, the LCD TV doesn't care much for it. The TV works but makes a loud mechanical 100Hz buzzing from it's power unit. On mains it works silently.

Then today I tried out the plug-in kWh meter I bought at another car boot sale last week. It too made a loud buzzing noise and gave some weird results on its meter. With no load plugged in it was saying that there was a 3W load (or 25VA) at 120mA and the power factor was 0.00 instead of 1.00. It also said that the mains frequency was "DC"! Plugged into the house mains it read normal (no current, no power, 1.00 PF, 50Hz).

Worst was the burning smell it made! Taking it apart I found a badly designed power supply to get from 230VAC to about 17VDC to run the electronics. It had a 100R 3W power resistor in series with a u68 cap on the live rail going into a couple of diodes and then some more resistors. On the mains, the cap limited the 50Hz power that could pass through the rectifiers and the resistor sat at 40 degrees C. On the modified sine supply the squarish wave HF harmonics went straight through the cap and started to fry the resistor - I measured 180C before the insulation on the thermocouple started to melt!

Who knows... maybe that could be happening in my TV (although I've not noticed any burning smells from that yet).

There's a reason why modified sine inverters are cheap...

I've found a seller who has a new range of Cotek pure sine inverters that are better than older types and quite a bit cheaper. These ones are 1kW continuous with 2kW surge rating and have a load seeking standby mode to reduce their idle current to about 0.25A. When you turn "on" a load the inverter wakes up from standby to live. Otherwise it would sit there drawing about 1.2A DC while doing nothing. Of course you can still turn it off manually as well. It's been a bit of a pain to have to turn on the light switch on the lamp and remember to turn on the DC relay for the inverter as well (and remember to turn it off before going to bed or else leave it sitting there pulling 0.8A all night).
Sharkey
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Post by Sharkey »

Yes, "modified sine-wave" inverters are misnamed. They should be caled "modified square-wave". For all intents, they have been replaced by true sine wave inverters. I have a couple around, but am very careful what I plug into them.

Some years back at the Oregon Counrty Fair, we built four "energy carts" so that construction crews could use pwer tools out in the woods to put up the stages, build booths, etc. without running petrol generators. Each had two golf car batteries and a Trace DR 1512 inverter. This did fine for corded circular saws, recriprocating saws, and hand drills, but we found out pretty fast that it would cook certain cordless tool chargers if plugged into the inverters.

Live (?) and learn.
AccordGuy
Posts: 144
Joined: Tue Jul 01, 2008 11:47 am
Location: West Sussex, UK
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Post by AccordGuy »

Pure sine ones might have replaced mod-sine inverters technically but the stores round here only stock mod-sine ones (all the way up to 5kW) as "there's no demand for pure sine ones". I asked about the Cotek one they claimed to stock but it's a 4 week pre-order to get one...

She said they've never had a come-back on the mod-sine ones. Presumably, the people whose TVs and power tools broke while using them just figured the appliance was faulty and never suspected the power waveform was to blame.

The manufacturers all claim that mod-sine inverters will work 95% of appliances. They don't mention that the other 5% will catch fire! :roll:
AccordGuy
Posts: 144
Joined: Tue Jul 01, 2008 11:47 am
Location: West Sussex, UK
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Post by AccordGuy »

Well, I got me a new invertor. I found an electrical contractor not far off the path I had to take to work out in Reading (some 2 hours drive away). It wouldn't have been worth going out there otherwise but I get expenses paid on mileage away from my usual office.

It's the same Cotek SK1000 1kW pure sine invertor that the other vendor couldn't get but this lot had re-badged it as their own. How did I know it was really a Cotek? They didn't bother to reprint the manual and that still says Cotek all over it and you can even see the Cotek legend under the "Antares" sticker on the invertor!

Anyway, hopefully this means that they will honour their 2 year warranty rather than other suppliers who just offer 1 year and if it goes wrong in the second year you have to send it back to Cotek in Taiwan!

Once I started to read the cabling instructions I began to wonder if I should have gone for a 24V system... It's not easy to find 2 AWG cable in my home town. The local auto shop only sold cut lengths of 4 and 8 but then I noticed that it was actually cheaper per meter to buy a diesel truck booster cable set as that is 35mmsq cable so was good enough for the 150A required. As I only needed 80cm of the 4.5m boosters I'd also have lots left over for battery jumpers and so on if I get more batteries later, so it seemed good value. I also bought some 8 AWG for wiring up the in-line fuses. I couldn't find anything like a 150A fuse & holder - the closest thing was a dubious looking blade fuse holder and a 80A maxi-fuse. Instead, I got three gold plated 5AG fuse holders from a car hi-fi shop and 40A fuses (ok that only makes 120A but it was either that or 60A fuses). Realistically, the pair of 70Ah batteries can't deliver 120A continuously so it doesn't matter and I haven't got that much solar power to keep such loads running anyhow.

Wiring up the invertor lugs was easy as it has #2 size cable rings. Wiring up the battery posts was a nightmare as the post holes are only chamfered and of a big enough diameter on one end so I had to turn the post terminals round and still then try to carefully push the stranded cable in without loads of it getting snagged and scrunching up outside the terminals.

I welded the fuse holders together into a single unit using my soldering iron to melt the plastic mounting plates and then soldered the 8 AWG tails together, joining to the massive 2-ish AWG booster cable - that much wire absorbs a lot of solder! Another useful component from the booster cable set was some copper lugs from the battery clamps. I re-used those to connect the massive cable to a 180A DC isolator relay.

Happily, now all the high in-rush hi-fi kit works just fine. This invertor can put out 2kW surges and so starting the power amp with the massive toroidal transformer no longer causes everything else to flicker. Nothing makes any buzzing noises and even the temperamental power meter that almost caught fire on the mod-sine invertor is behaving itself.

Last weekend's car boot sale hunting scored a new master-slave plug adaptor for a couple of Pounds. It's meant for turning on and off your peripherals automatically when you start / stop your computer but the slave sockets on it have a 5 second delay. This was ideal for the two big amps. Now I can turn the main one on and it has 5 seconds to settle before the other big amp kicks in. Starting both at the same time caused the previous 600W invertor to trip out.

This new invertor uses a lot more power when idle (1.4A) but it does have a sleep mode (programmable by DIP switches) to sleep when less than 15W of load is turned on. In that state it draws 0.3A and sends a short pulse of mains every second and if it detects a big enough load it wakes up. It sometimes doesn't wake up when I just have one CFL light turned on as the little table lamps are only 8W and 9W tubes. Once they do start their load is enough to hold the invertor in it's live state so I just have to turn two on and then one off after the invertor has woken up.
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