Refrigerators and Battery Supply - Protecting Inverter Surge

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Refrigerators and Battery Supply - Protecting Inverter Surge

Postby Tracker » Mon Aug 27, 2012 10:15 am

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Plan early.. If you are thinking about attempting to connect motors, to a Solar/Battery/Inverter system, remember that motors in refrigerators etc. can draw a significant START current.
Should you have multiple motors eg. Refrigerator and freezers, you need to avoid the situation where at start-up or after some outage, they don't all try to start at the same time.

You can buy simple devices like this DELAY device, which stops power from being applied until after a set time.
Delay-ON.jpg
Delaying motor Start
Delay-ON.jpg (11.75 KiB) Viewed 9505 times

You will need a 240V relay for this device, as it's load capacity is only 1.5A..

The idea is that if you had a Fridge and two freezers and a washing machine, and a tank-pump, and you managed to get to the point where IF power was connected, then they would ALL start at the same time (because their individual thermostats has actuated), then you need to introduce a delay to each device.
Say 1Min, then 3Min, then 5Min. to consecutive devices, allowing time for recovery as house power is restored or applied. Once applied - the power continues, and then you just have to contend with the possibility that (say) three devices might thermostat-activate at the same time.. Probably NO WAY you could ever guard against that, other than a central controller..

I BLEW my inverter, when a fault condition, saw everything go off for a few hours and then when re connected - they all started together.. You simply can't guard against every possibly situation..

Anyone come up with other precautions..?
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Re: Refrigerators and Battery Supply - Protecting Inverter Surge

Postby Gordon-Loomberah » Mon Aug 27, 2012 10:48 am

Tracker wrote:Anyone come up with other precautions..?


Dont buy dodgy inverters? ;)

Seriously, all inverters should have overload protection that doesn't involve release of magic smoke.

With a good 3kW inverter, which will have a surge (5sec) rating of around 9kW, simultaneously starting a couple of fridges and a freezer should be no problem at all.
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Re: Refrigerators and Battery Supply - Protecting Inverter Surge

Postby offgridQLD » Mon Aug 27, 2012 9:05 pm

I see your point in that situation. We had similar situations with multiple large electric motors in a large dairy food processing plant .Easily overcome with soft starting of motors. Though less was a stake as it would just trip out at the panel if you had a spike.

I would have to agree that an inverter should be able to handle self prevent in a overload situation.I think my own inverter is rated for 12kw spikes for 5 seconds before going out on over load.

Its a good work around though for a small basic inverter.

A little off topic but this thread just reminded me of something regarding loads that are hard on inverters.

I found a appliance (power tool) that really messed with my inverter on the weekend. I was doing some soldering and got out my heat gun to shrink some heat shrink. Its only a 2000w. I noticed the two fluorescent lights in the garage were flickering quite noticeably when I used it . So I got out the power meter and measured the heat gun. It spiked at 2000w then settled down to 1500w. My wife then said the fridge made a funny noise every time I used the heat gun.I confirmed the fridge did get a funny harmonic shudder noise every time the heat gun was on.

Now I doubt its the load size being to big I have welded a few times after dark 5000w and start large 3hp air compressor motor along with 2700w oven grill and many other large loads that all don't have any issues with flickering lights or fridge shuddering. Why would this simple medium load (resistive load) heat gun mess with my inverter so much to make lights flicker and fridge shudder. Its like its creating a harmonic feadback. Its a cheap and nasty Chinese hot airgun. But its over my head as to why its doing this?
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Re: Refrigerators and Battery Supply - Protecting Inverter Surge

Postby Tracker » Wed Aug 29, 2012 7:04 pm

offgridQLD wrote:.....Why would this simple medium load (resistive load) heat gun mess with my inverter so much...?

I wonder if it's VARIABLE heat, like mine.. Is it the triac doing the damage..?

You would have been justified to think the heat-gun would be a very pure load
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Re: Refrigerators and Battery Supply - Protecting Inverter Surge

Postby offgridQLD » Wed Aug 29, 2012 7:11 pm

Yes, its adjustable heat (dial at the back ) I guess I should bring it back to my city house and test it on grid power and see if it messes with the lights there.

edit: just did some searching online and one u tube video showing a triac on a light dimmer was giving some interference you could actually hear.

Kurt
Last edited by offgridQLD on Wed Aug 29, 2012 7:23 pm, edited 1 time in total.
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Re: Refrigerators and Battery Supply - Protecting Inverter Surge

Postby Tracker » Wed Aug 29, 2012 7:17 pm

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Mine is a Ryobi, and it sorta spits and carries on when on reduced power.. Not a smooth sound, but works great

I would suspect that random "Control" is a likely culprit for what you see...
Wonder if it's more the random nature of current spikes, that the Inverter can't settle down to, rather than the actual power level..
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Re: Refrigerators and Battery Supply - Protecting Inverter Surge

Postby offgridQLD » Wed Aug 29, 2012 7:28 pm

Well I always just use it on full 100% heat setting, as i just want to give the heatshirk a quick blast. Mine is a ozito rubbish $30 job. I had a makita one for years but knocked it off the bench once just after using it for a long time . It was still hot and blew the element.

Perhaps I am better off just getting a basic unit like my old makita with high-medium-low.

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Re: Refrigerators and Battery Supply - Protecting Inverter Surge

Postby Andrew_electrix » Thu Aug 30, 2012 10:29 pm

Heat guns, light dimmers and vacuum cleaners with a speed control all use phase control, which varies the RMS voltage to load by means of switching the load off for part of each mains half cycle.
This is normally accomplished by a TRIAC which triggered at some angle from 0-180 degrees past a zero crossing.
The problem is that the switching or discontinuites in the mains waveform by definition generate harmonics in current waveform.
These harmonics come back in the face of the inverter which create extra heating and noises, also the inverter has to regulate it's RMS voltage where the current is varying dramatically through each half cycle.
This is why the lights are flickering when one of these loads are used with a inverter.
The grid supply is so low impedance that it can deal with these harmonics without it effecting the voltage waveform too much.

Regards

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Re: Refrigerators and Battery Supply - Protecting Inverter Surge

Postby Tracker » Fri Aug 31, 2012 6:54 am

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BTW - Seriously - the WORST situation with multiple refrigerators, is if you get this situation... ;)
Power OFF after a failure for a long time. (All thermostats reset, to start cooling process..)
Power BACK for a few seconds, and then drops out. ( Compressors charge the HP lines causing Back Pressure)
Power BACK for good.... Now you have stalled compressors, each drawing MAX current for something like 5 to 10 seconds, before the thermal cutout on the motors kills the current.
... :twisted: .... TOO LATE - Potential damage done...

PS - I pick up a real inverter next week... So hopefully it will be more tolerant and less prone to "Magic Smoke".. :roll:
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Heat Guns and erratic inverter power

Postby Tracker » Fri Aug 31, 2012 7:07 am

offgridQLD wrote:...Perhaps I am better off just getting a basic unit like my old makita with high-medium-low...

One would ASSUME that 100% was NO CONTROL ie 100% waveform. The problem with Medium and Low, is you can be sure they are still triac controlled.. The problem with even 100% is that the triac might actually fire at say 5deg... Not enough to make a practical difference to the heating, but enough to create current "Noise".. :(
The control would need to be true Zero-Crossing, to be noise free..

As per Andrew's observations, the poor inverter has no way of responding to the chaotic currents being asked..

I would think that the MOST basic would be best.. ie.. ON/OFF, and learn to control the heat by movement and distance, as we had to when using Propane-Torches..
And that is the other thought... Design a head for a Propane Torch that kills the flame to leave just heat.. :oops:
I think they use inner wire grills... I have a Weller Gas Soldering torch, that has a heat Head...

PS - It's good that this is turning into a general thread on Do's and Dont's , as it might give those on limited budgets, the thoughts to protect what they have... Even our friend with the Torroidally challenged M7 , has produced that magic smoke.. I still have to FIX mine, but WILL do so, if only to use it as a 50A charger..
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