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Old 20-08-2014, 15:38   #61
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Re: Inverter Dying?

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Originally Posted by transmitterdan View Post
There is always AC current at the battery. Inverters draw both DC and AC from the battery. If you put a clamp on meter on the negative cable you will be able to measure a DC current and if you switch to AC mode you will also measure some AC current. This is normal.
If we could draw AC current from a battery we wouldn't need an inverter, just a transformer. I think you are confused.
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Old 20-08-2014, 15:58   #62
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Re: Inverter Dying?

Try measuring what I described. You will find there is a lot of AC current on the DC wires. That's how inverters maintain their efficiency. As the output current increases following the AC waveform most of that current comes from the battery. When the AC wave form goes negative the inverter "inverts" itself and supplies the negative current from the battery positive. The average current is still DC but if you look at it in real time the battery current looks like a rectified sine wave. So it has both DC and AC characteristics.
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Old 21-08-2014, 07:14   #63
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Re: Inverter Dying?

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Originally Posted by transmitterdan View Post
Try measuring what I described. You will find there is a lot of AC current on the DC wires. That's how inverters maintain their efficiency. As the output current increases following the AC waveform most of that current comes from the battery. When the AC wave form goes negative the inverter "inverts" itself and supplies the negative current from the battery positive. The average current is still DC but if you look at it in real time the battery current looks like a rectified sine wave. So it has both DC and AC characteristics.
It doesn't work like that. Take a look at an inverter schematic. Start from the oscillator and work your way through the circuit diagram in both directions.
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Old 21-08-2014, 12:12   #64
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Re: Inverter Dying?

It does work like that. No matter what the schematic an inverter has to deliver AC current to the AC load. It has to get that current from somewhere as it has no local source of any current. It draws the current from the battery. The inverter "rectifies" the battery current so that it draws rectified current from the battery. The average is DC but it varies in time from near zero to peaks well above the average DC level. If you don't think it works that way try clipping a 300A RMS AC clamp on meter around one of the DC wires. Turn on a 1000W coffee pot or microwave oven. See how much AC current the meter displays. It will not be zero.

This is just one reason it's a good idea to run the plus and minus wires feeding an inverter close together so as to not radiate that AC field. Another is to help reduce interference to a compass.
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Old 21-08-2014, 12:39   #65
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Re: Inverter Dying?

The inverter chops the current from the battery (turns it on and off). This pulsating DC current flows through the primary of a transformer and the output from the secondary of the transformer is alternating current (AC). The current from the battery is not alternating current, it varies (in this case) from zero volts DC to 12 volts (nominal) DC.

If you are measuring an AC voltage on the DC conductors it's because you are not using your test instruments correctly.
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Old 21-08-2014, 12:44   #66
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Re: Inverter Dying?

Read what I wrote. We are talking about current not voltage. The current varies from near zero when the AC output current is near zero to a value much higher than the average DC when the AC output is at a positive or negative peak. There is AC current flowing in the DC battery cables.
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Old 21-08-2014, 14:58   #67
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Re: Inverter Dying?

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Read what I wrote. We are talking about current not voltage. The current varies from near zero when the AC output current is near zero to a value much higher than the average DC when the AC output is at a positive or negative peak. There is AC current flowing in the DC battery cables.
Nope. There is no way to get alternating current out of a battery. I studied this stuff in school and made a career out of it. The current can vary from zero to one hundred amps or more but it's still not "alternating current".

If your test equipment is showing you alternating current you are using it incorrectly.
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Old 21-08-2014, 15:07   #68
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Re: Inverter Dying?

Dan, I think what you are describing is more a pulsating DC current. It varies in magnitude, but not in direction as an AC current does. This will show up on a clamper as an AC reading, but is not one in fact.

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Old 21-08-2014, 15:23   #69
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Re: Inverter Dying?

No, it's not pulsating DC I am referring too (although cheap inverters do that too). It is the real 60 (or 50) Hz current waveform that flows from the battery. If you could see it, it would look like a sine wave except every other half cycle would be inverted. I'm not at a place where I can draw a picture easily but I will try to when I can.
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Old 21-08-2014, 15:37   #70
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Re: Inverter Dying?

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Nope. There is no way to get alternating current out of a battery. I studied this stuff in school and made a career out of it. The current can vary from zero to one hundred amps or more but it's still not "alternating current".

If your test equipment is showing you alternating current you are using it incorrectly.
Um, an inverter's only purpose is to get alternating current out of a battery.

Try it with your quality clamp on RMS AC probe. It will read an AC value and it will probably be correct. If you have a DC setting that should show the DC component only but not the AC. Or if your inverter has a shunt put an oscilloscope on the shunt and see what the current looks like. It will not be constant DC.

Any current that is not steady but varies over time has an AC component by definition. Here is a thought problem; if a current waveform varies and you subtract the DC value what is left? The answer is the AC part. You can have an AC current riding on a DC current. The sum of the 2 is the total current.

Inverters draw a lot of AC current from the batteries thus the RMS current is much higher than the DC average. This is often ignored when computing voltage drop in cables. It's why some (most) inverter installation guides specify wire much larger than people expect. And why inverter installations overheat the wires in many cases.
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Old 21-08-2014, 16:23   #71
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Re: Inverter Dying?

AC power from a battery.....Wow and this is why you should never get electrical advice from the Internet.

AC power is created from a spinning generator. As the rotor passes the stator the current switches poles. This is why the speed of the generator sets the cycles of the power as in 60 htz

The batteries do not make AC [power despite what you see from your meter. It can't it simply defies physics.

An inverter works by electronically switching the the DC poles this is why many produce a square wave rather than a sine wave. It is hard to produce a true sine wave electronically.

If you truly believe batteries produce AC power I suggest you take a basic course in electricity. I am not saying that in a mean way just that you would learn some basic things.
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Old 21-08-2014, 17:40   #72
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Re: Inverter Dying?

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An inverter works by electronically switching the the DC poles this is why many produce a square wave rather than a sine wave. It is hard to produce a true sine wave electronically.
Producing a pure sine wave electronically is quite trivial. It's done in millions of devices every day. Listen to your local AM radio station. There is a 100% probability that those radio sine waves were produced electronically. No poles (except the antenna mast) required.
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Old 21-08-2014, 17:49   #73
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Re: Inverter Dying?

Before everyone starts foaming at the mouth about this please understand I am not looking for a fight nor to teach AC/DC circuits. This got started because I was trying to explain to why the DC battery cables to an inverter can overheat. The current is a lot higher than most people expect even higher than simple DC calculations will show. So make the cables per the manufacturers instructions and if they are vague then go as big and as short as you can. Make the crimps with good hydraulic crimpers and bolt everything as tight as possible. Then set a reminder to check for heating and retighten every month or so.

If you want to understand what I am saying in technical detail I am happy to discuss via PM (including my credentials on the subject).
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Old 21-08-2014, 18:33   #74
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Re: Inverter Dying?

Very interesting discussion. I wonder why more people don't have problems with their ProWatt1000 installations from overheating of wires and corrosion if the recommendations for wiring sizing are inadequate?

It's difficult to accept that the neg connection and pos connection were done differently and that one failed and the other remained perfect.
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Old 21-08-2014, 18:44   #75
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Re: Inverter Dying?

Sandero,

I see this all the time. It takes a tiny difference for heat to build up and run away. Please don't be too surprised by what happened to you. It happens very often the way you described.

Good luck with your new installation. But don't be too surprised if the same thing happens again.
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