Commercial Inverter Problems

Discussion in 'General Electronics Chat' started by old codger, Apr 24, 2014.

  1. old codger

    Thread Starter New Member

    Apr 24, 2014
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    I've recently bought a 12 vdc to 230 vac pure sine wave inverter. It's rated at 1000 watts continuous, 2000 watts peak, but it trips out when loads of about 500 watts are applied. I think it needs a circuit/device to limit the inrush current. That needs to be capable of handling about 3 amps continuously and 5 amps peak, as I don't need to use the full 1000 watt continuous capability.

    Any advice ?
     
    Last edited by a moderator: Apr 24, 2014
  2. wayneh

    Expert

    Sep 9, 2010
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    Why do you think that? In other words, what is the load?

    Is the inverter able to cool itself properly, does it have adequate clearance for air flow?
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    It's probably grossly misadvertized. Inverters are one of those things they love to misadvertize, like car audio amps, shop vacs, and compressor motors. They probably take the no-load voltage output and multiply it by the peak instantaneous fault current within the first microsecond of a direct short being applied, and call that the peak power. And they probably pull the continuous power number straight out of their butthole, or it's based on some theoretical formula given operating temperature of zero kelvin, purely resistive load, in the vacuum of outer space, and with a good luck special chant administered by the pope.
     
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  4. Brainbox

    Member

    Nov 15, 2010
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    What kind of load do You apply, and does it trip immediately or after some time?
     
  5. wayneh

    Expert

    Sep 9, 2010
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    I think that's a given. But even for a "pure sine wave" inverter? I would think anyone wanting to pay a premium for one of those would not be happy with poor performance relative to the specifications.
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    is the 12 volt wireing large enough? 1000 watts (actually more due to effiency) is a lot of current and a wire too small would have quite a drop.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    An NTC inrush limiting thermistor might help, I usually salvage those from any scrap electronic stuff before it ends up in the bin, but you should be able to order them from most electronics suppliers.

    Back in the days when CFLs were expensive I used to prolong the life of filament bulbs by mounting an NTC behind the switch plate.
     
  8. old codger

    Thread Starter New Member

    Apr 24, 2014
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    Thanks for all your comments. Where do I start ?
    The connections to the bank of 12 volt batteries is about the thickness of my little finger (1/2 inch dia), so is more than adequate to provide the 1000 watts load (4A at 250V).
    It's not greatly over advertised. It will handle the rated load if it's applied gradually and incrementally, but not in one big lump of 400-500 watts from my absorption type fridge and freezer. That load is handled without problems by my similarly rated modified sine wave (square wave) inverter, but that produces interference on my radio, hence my purchase of the sine wave inverter.
    The unit has a cooling fan and it's mounted on the wall in my garage, cool, well ventilated place.
    The unit trips out immediately if the moderate load is applied in one lump.
    The unit is quite large (300mm long) and is well made externally, with a finned aluminium casing.
    I think I just need to limit the initial inrush current on the load side, so I will investigate Ian's suggestion of an NTC thermistor.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the current for 1000 watts at 12 volts is considerably greater than 4 amps. if the inverter were 100%efficient, it would be 83.33 amps. since the inverter isnt 100 % efficient, the current will be more. if you check the voltage (12 volts dc) at the inverter when the load is starting, it might be dropping. is there a starting current rating on your apliences?
     
  10. richard.cs

    Member

    Mar 3, 2012
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    Absorbtion coolers are just resistive heating elelents and they run at fairly low temperatures so I wouldn't have expected a substantial inrush from that type of load. It could be some oddity of how the inverters voltage feedback loop works I guess.

    If your only reason to change inverter is to reduce the RF interference problem it's probably not the best way to solve it. A square wave inverter will generally be more efficient than a sine wave one when powering a dumb resistive load and both inverters will have a high frequency (20-100 kHz) switching supply that will produce harmonics into the MHz. It just happens that sine wave inverters tend to be of better build quality with more filtering but adding some simple and cheap filtering to the modified sine wave inverter would do the same job.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    I think alfacliff has nailed it. Check the voltage at the DC side of the inverter at the time when it is failing. Depending on the gauge and length of that cable, and of course your battery internal resistance, that voltage may be sagging low enough to trip it out.

    You cannot start a car through normal jumper cables, because the amp draw is just too much for even the beefiest cables. You lose a volt or two and that's enough to cause a problem.
     
  12. old codger

    Thread Starter New Member

    Apr 24, 2014
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    As I stated in my original message, the inverter is connected to a BANK of batteries. They total 14 in number, each with a capacity of 120 Amp hour. They are interconnected with heavy duty cables as provided by the battery supplier. THERE IS NO PROBLEM WITH THE 12 VOLT SUPPLY SIDE TO THE INVERTER.
    I agree that absorption fridges and freezers present predominately resistive loads, but both loads trip the pure sine wave inverter when connected individually. Both these loads give no trouble when powered from the square wave inverter. Both inverters are similarly rated, and I can only conclude that the sine wave inverter is susceptible to high transient loads, whereas the square wave inverter is more immune.
    I hoped that my advisers would have noted that the load of 1000 watts I quoted was 4 AMPS AT 250 V. I am well aware that the current at the 12 V side will be considerably greater !!
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Do you have a 500W lightbulb handy? Maybe one of those work lights contractors use. It'd be interesting to see what happens when you try to fire that load. The brief heat-up current would be high and then a nice continuous 2A. It might be illuminating :D to see how different loads affect your problem. I also wonder what might happen if you pre-loaded the inverter with, say, a 100W bulb and then attempt to fire the fridge. These experiments may be somewhat random but cheap and easy.
     
  14. old codger

    Thread Starter New Member

    Apr 24, 2014
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    Thanks richard.cs. I've tried some additional filtering to the square wave inverter output, but it doesn't reduce the interference very much (on 693m medium wave). I suspect because the filtering is outside the case of the inverter, so the very short connections can still radiate before the filtering. This interference is then effectively spread around my house (and the neighbourhood) by the AC Mains lead connected to the filter output feeding the loads.
    The problem with the pure sine wave inverter may be due to its overload protection circuit being micro processor controlled with a fast response time. I need to slow that down with external circuitry, such as an inrush current limiting device (NTC Thermistor). I don't want to mess internally with the inverter in case I decide to return it to the supplier as unsuitable, and I don't have a circuit diagram of the inverter to aid modifications internally.
     
  15. old codger

    Thread Starter New Member

    Apr 24, 2014
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    Thanks wayneh. I will try that, but the pure sine wave inverter is now disconnected from my batteries and I've reverted to the square wave one, so it will take some time before I can report back.
    I have connected multiple low powered loads (TV's, computers, table lamps etc), incrementally to the sine wave inverter and it behaves ok. It's only the fridge and freezer that give trouble, even when connected individually and on their own and without any other load.
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Perhaps the fridge/freezer has a whacking great suppression capacitor in parallel with its input? That would, albeit briefly, draw a high current at switch-on.
     
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