# Inverter



## Halfpint (Jan 23, 2011)

I have just built an oversize storm shelter. I am now in the process of going thru all the different inverters and your there is a bunch. The shelter is wired as if I was going to run 110 but I have decided to just go with 12v and go thru and inverter to the VERY few things I need. As of now I will be using less that 300w but I would like to have a 1000w inverter just in case. I would like to fine one that sits dormant until there is a power need; this is because the power will be used only a few times a day for venting unless someone is inside. I intend to use only 1 maybe 2 marine batteries and charge thru solar. Anyone have any suggestion on an inverter? 
Suggestion and or experence with inverters you have.
Thanks to all J


----------



## mosquitomountainman (Jan 25, 2010)

You're looking for one with a load sensor. Most load sensing units have a sensitivity adjustment to customize it for the expected load. It "idles" unless an appliance or whatever turns itself on then goes to full power when the load is sensed. We have one on our Trace Inverter but finally shut it off and just leave the inverter on all the time. There's a slight delay from the time you turn on the lights (for example) until they come on. It wasn't much of a problem except we use LED bulbs and sometimes they didn't draw enough to turn on the inverter. Then when we got the fridge we had problems with it not working properly. We finally just turned the sensitivity adjustment to maximum sensitivity and the inverter just stays on all the time. The new ones use very little current so there's no real need for the load sensor.


----------



## BlueShoe (Aug 7, 2010)

Wouldn't a large inline capacitor (like a car stereo cap) allow the sensing circuit to operate correctly since the cap would store enough power to start an appliance while giving the inverter time to come on line?


----------



## mosquitomountainman (Jan 25, 2010)

I don't know. I'm not into electronics that much. In my opinion it wouldn't be wrth the effort. Most inverters draw so little power that leaving them on is not a problem.

Steve


----------



## Halfpint (Jan 23, 2011)

Good info,,,,,can anyone put a brand / model number to your suggestions,,,Thanks


----------



## Emerald (Jun 14, 2010)

I've been looking into them to put one in my car to help keep the freezer cold during our many power outages. 
I have to say I've been Surfing Amazon for power inverters and found many I really like.. The good thing is=many folks review them all at length. Good for people like us to know which ones work for what uses etc. Even if you don't buy via Amazon you can at least scope out which ones you are interested in getting..
I often research the reviews before I buy anything either online or out of the store. 
I would also suggest(as it was suggested to me) get the biggest one you can.. it will hold up to any surges/power draws better than a smaller one that may not.


----------



## Emerald (Jun 14, 2010)

tenOC said:


> Wouldn't a large inline capacitor (like a car stereo cap) allow the sensing circuit to operate correctly since the cap would store enough power to start an appliance while giving the inverter time to come on line?


Great idea! Thanks..


----------



## machinist (Jul 4, 2012)

Link about inverters:
http://www.windsun.com/Inverters/Inverter_selection.htm

Article on choosing the right inverter:
http://www.countrysidemag.com/issues/91/91-1/Rex_A_Ewing.html

Sources:
http://www.sunelec.com/inverters-chargers-c-6.html

http://www.solar-electric.com/inverters.html

Halfpint,
I have the same sort of problem choosing inverters for our house. I want pure sine wave inverters to avoid overheating motors, since they will run a refrigerator and a freezer. I haven't found any that shut off entirely idling, but these two are pretty low draw when idling:

http://www.solar-electric.com/sa10wa12vosi.html
http://www.solar-electric.com/exxp12vol11w.html

Both operate on 12 volts.

I have decided to avoid the more expensive charging type units that function also as a battery charger with a generator hooked up. I can monitor battery condition and start up a generator myself. This can save a lot of money, if I understand all this right.


----------



## mosquitomountainman (Jan 25, 2010)

The two main advantages to the chargers that are integral with the inverter are (1) convenience, (2) they have a higher charge rate to ensure that deep cycle batteries get a thorough charge. Most automotive type chargers won't hit the voltage required to get a deep cycle battery fully charged to the core. The third advantage in non-twelve volt systems is that it's difficult to find a charger in anything except 12 volts.

We've done both types and I like the inegrated charger best. A lot will depend on your finances.


----------



## mosquitomountainman (Jan 25, 2010)

tenOC said:


> Wouldn't a large inline capacitor (like a car stereo cap) allow the sensing circuit to operate correctly since the cap would store enough power to start an appliance while giving the inverter time to come on line?


The delay is a result of the inverter's search function. It searches for a voltage draw in "pulses." Like a very fast turn signal blinking. No matter how fast it is though there's always a slight delay between the time you flip a light switch and the light comes on.

Like I've said, the normal "no-load" draw of an inverter isn't going to cause problems. It's measured in thousandths of an amp. It would take months to draw a battery down.


----------



## BlueShoe (Aug 7, 2010)

Caps don't cover the power consumed for the inverter while it's doing it's sentinel duties hunting/waiting for current draw. Caps provide a large amount of stored voltage for start-up demands for appliances and peak loads where the power source can't provide them quickly enough or in large enough quantity. They're like a battery, so you can dial your sensitivity on your inverter back down that way and the cap will provide the quick shot for start up.


----------



## labotomi (Feb 14, 2010)

I'm probably missing something, but regarding the capacitor suggestion. Is it to be installed on the DC side or AC side of the inverter? 

If it's on the DC side in parallel, it would only help if the batteries were too small to provide the inverter enough power for the starting surge of the loads. If in series... you're not going to power anything for long.

On the load side of the inverter a capacitor would be charged and discharged continuously because of the AC signal. The power would be purely reactive, but it still causes current to flow and that leads to heat losses in the wiring. Series or parallel wiring wouldn't matter unless using more than one and then it would just change the overall capacitance. 

Feel free to let me know what I'm struggling with.


----------



## Nadja (Jan 12, 2011)

Pure sine wave for anything with a motor. Trace, Xantrax, or Outback are the best you can get


----------



## labotomi (Feb 14, 2010)

Nadja said:


> Pure sine wave for anything with a motor. Trace, Xantrax, or Outback are the best you can get


I don't feel it's a significant concern to use a pure sine over a modified sine inverter except in special circumstances. One reason is't applicable to residential use, another would be exceptionally rare and the last would only be an issue if using a very old motor.

The reduction in life of a standard motor used with a non sine inverter is exaggerated. One concern is standing wave reflections building upon themselves resulting in voltages higher than normally applied to the motors. This happens only when the cable distance is greater than approximately 20 meters. Less than that distance the standing wave effect is virtually non existent.

Inverters can produce a high peak voltage at the front of the square wave (typically 2x normal voltage). For a household motor rated at 110V or 220V, you're looking at possibly 440V peak voltage. This is well within the ratings of the insulation of the motor. When you're using it for an industrial application with an output of 575V, doubling that would be 1150V, so there's a much larger concern. This might be a concern if someone is using a very old motor.

The last problem with non inverter duty motors is lack of heat removal when running at less than rated speed. This can be a problem in industrial facilities with variable frequency drives, but it's not an issue with inverters that only output 60Hz.


----------



## LincTex (Apr 1, 2011)

labotomi said:


> If it's on the DC side in parallel, it would only help if the batteries were too small to provide the inverter enough power for the starting surge of the loads.


I thought about that... I doubt it would really help much, if the batteries and cables are big enough. It can't hurt to try, though. I grabbed one from the hatch area of a wrecked car at the scrap yard, it was for an audio amplifier. I might give it a whirl.


----------



## LincTex (Apr 1, 2011)

labotomi said:


> The reduction in life of a standard motor used with a non sine inverter is exaggerated.


I don't know... some stuff definitely "sounds different" when running on modified sine wave (versus pure).

I know that isn't a scientific answer, but it makes me think it isn't doing the motor any good if it sounds funny.


----------



## labotomi (Feb 14, 2010)

LincTex said:


> I don't know... some stuff definitely "sounds different" when running on modified sine wave (versus pure).
> 
> I know that isn't a scientific answer, but it makes me think it isn't doing the motor any good if it sounds funny.


What you're hearing is the carrier frequency or the rate at which the pulses are delivered. Even a 60Hz output frequency has to be cut up into small pulses and that is the carrier frequency or the switching speed of the transistors/SCRs

2kHz is the low range and can be heard around the inverters and motors. Those that are higher than 10K are difficult to hear especially for us that are getting older.

What the Pulse width modulated inverters are doing is using a digital signal (full on or full off.... ok really it's full positive, off and full negative so actually a 3 state *discrete*). A pure sine is an actual analog signal. It can vary from full on positive to full on negative and every state in between. You don't have the high speed pulses. A 60Hz output with an inverter using a 2kHz carrier would have the transistors switching 33 times per sinewave to achieve 2000 switches a second. I'll agree that a pure sine wave is what motors are designed to run on, but that assumes a normal source of AC power. A pure sine inverter isn't nearly as efficient as a PWM inverter. If you need the peace of mind and don't mind spending extra money upfront and using more energy with use a pure sine would be the way to go. You're not going to find pure sine inverters in industrial facilities with hundreds of motors, but the motor manufacturers will try to convince you to buy their "special" inverter duty motors and in some cases, they are needed. I just don't think it's needed in a residential setting with fairly modern motors and short distances from the inverters to the motors.

If you feel like downloading Wolfram CDF player, you can play a little with pulse width frequencies 
Here's the link to the demonstration project along with the link to download the wolfram cdf player.

http://demonstrations.wolfram.com/PulseWidthModulationPrinciple/

Or just look at the pic. While the pulses are a square wave, the portion to be concerned with is the area under the curve (integral for the calculus types)










Wolfram Alpha is a powerful tool for those who actually make use of it's capabilities.

Edit: I found another graph that I like better. The sawtooth waveform is actually a result of how the particular control circuit works.










This may help with the explanation of the jagged current graph.










I know... way to in depth. I'll shut my mouth now.


----------



## LincTex (Apr 1, 2011)

labotomi said:


> I know... way to in depth. I'll shut my mouth now.


I always love to learn!


----------

