Inverter Dying?

[Uncle Bob]

Originally Posted by Ex-Calif

This is surreal.

What I'd like to learn is how my battery is producing AC current and if it is why do I need an inverter?

Hey I'm with you on that one. Just installed an inverter, ripped off AGAIN ??

[transmitterdan]

Originally Posted by StuM

Maybe I'm dumb, but I thought that the "alternating" in AC meant that the direction of current flow alternated, otherwise it would be called VC (variable current).

If the current is alternating, half the time you would be charging the battery instead of discharging it. Hey - perpetual motion!

Yes, we have a terminology issue. DC to an engineer is current that does not vary (i.e. the frequency is zero) . If it varies over time then it is made up of different frequencies of AC (aka time varying current).

Some lay texts teach that DC is current that always flows in one direction (say positive). Those texts teach AC must necessarily reverse direction (be positive some times and negative other times). By that definition batteries can only produce DC. But those definitions don't allow us to talk about harmonic components and do the calculus to determine how much heat a certain current profile will generate.

If you ignore the silly AC/DC comments in this thread meant to belittle someone and focus on the time varying nature of battery current in inverter applications you will see why it's not a simple DC circuit. It is more complex than that and as a result the wire losses and voltage drop calculations cannot just assume the battery produces a constant DC current. An inverter is nothing like a light bulb load to a battery.

I build inverters that produce millions of watts of AC. These issues matter there and they also matter in multi-kilowatt inverters on boats. My goal is to help enlighten so that no other inverter system melts down and burns another boat. I don't care about the belittling remarks if someone listens, uses the right wire and terminals and avoids a fire. This is a real problem in cruising. But if belittling me causes someone to ignore what I am saying then what's that say?

[Ex-Calif]

Originally Posted by transmitterdan

I build inverters that produce millions of watts of AC. These issues matter there and they also matter in multi-kilowatt inverters on boats. My goal is to help enlighten so that no other inverter system melts down and burns another boat. I don't care about the belittling remarks if someone listens, uses the right wire and terminals and avoids a fire. This is a real problem in cruising. But if belittling me causes someone to ignore what I am saying then what's that say?

There are many talented people on CF. You may have great depth of knowledge but my concern is that in explaining the science the OP loses site of some important things.

Sandero has periodically checked into this thread. He just wants "practical" advice about what happened and how he might avoid it in the future.

I don't see the harm in advising to follow the manufacturers recommendation.

My bigger concern is that I pointed out several concern areas in his power distribution photo and no one including the OP has commented.

In looking at it again I would add that the house positive cable also appears to be laying on the sharp edge of the negative bus.

This guy has several short circuits waiting to happen that will ruin his day and we are talking about whether DC is really AC by some scientific definition.

I get your point - Size the wire right. And make sure the connections are clean and tight. I don't think anyone disputes that.

However - Most "lay people" think of DC as the stuff that powers cabin lights and iPad chargers. The DC we are talking about is 100 amps - big stuff.

If you can't explain it simply to the lay people then you may as well be talking to Rex the wonder dog.

Please don't think I am picking a fight. I have made a career out of translating "science" into practical action for lay people.

http://www.cruisersforum.com/forums/...ml#post1608400

[sparrowhawk1]

Colemj. Thanks. Got it. Reading and writing are one thing. Videos another. Edit: most of time I want to keep my words to a minimum I hope I didn't sound rude

[Canibul]

Moving right along, and while the serious inverter talent is here, I ran into a situation yesterday that puzzled me a bit. I hauled the boat, and was planning to cut a hole in the cabin top to install a solar vent. I decided to try using the inverter to run a rotary cutter, a 115 VAC powered thing called a RotoZip. We have a Harbor Freight 2000 watt inverter that came with the boat. Four freshly charged six volt batteries, and over 700 watts of solar. The RotoZip barely turned. I looked at the lable on it, 115 VAC and 5 amps. I checked the voltage coming out of the inverter, no load, it was around 87 volts. I checked inputs, and it was solid 13.2 vdc measured on the input terminal bolt heads, not the cables, so I know that the dc power is directly connected via heavy battey cables a short 18 " run from the inverter. Should I assume it's time to junk the Harbor Freight inverter and buy a marine one? I don't have a manual on this thing, but is the output voltage commonly something that is adjustable on an inverter?

[transmitterdan]

Is the Harbor Freight a modified sine wave type or pure sine wave? Knowing their price structure probably it is not pure sine wave. Some volt meters cannot accurately measure modified sine wave inverters. Some power tools can't work with modified sine wave. If your meter is not true RMS type try to borrow one (Fluke is a good brand) and see if you still get 87 volts.

The output voltage is not adjustable.

[transmitterdan]

Originally Posted by Ex-Calif

There are many talented people on CF. You may have great depth of knowledge but my concern is that in explaining the science the OP loses site of some important things.

Sandero has periodically checked into this thread. He just wants "practical" advice about what happened and how he might avoid it in the future.

I don't see the harm in advising to follow the manufacturers recommendation.

My bigger concern is that I pointed out several concern areas in his power distribution photo and no one including the OP has commented.

In looking at it again I would add that the house positive cable also appears to be laying on the sharp edge of the negative bus.

This guy has several short circuits waiting to happen that will ruin his day and we are talking about whether DC is really AC by some scientific definition.

I get your point - Size the wire right. And make sure the connections are clean and tight. I don't think anyone disputes that.

However - Most "lay people" think of DC as the stuff that powers cabin lights and iPad chargers. The DC we are talking about is 100 amps - big stuff.

If you can't explain it simply to the lay people then you may as well be talking to Rex the wonder dog.

Please don't think I am picking a fight. I have made a career out of translating "science" into practical action for lay people.

http://www.cruisersforum.com/forums/...ml#post1608400

I have told Sandero his wire is too small. His experience will be repeated if he doesn't use the proper size wire. He thinks that because it worked for years it is ok. But he also said he had recently rewired and so it couldn't be the wiring. But it was the wiring. Because others with knowledge told him his wire was fine I tried to explain why it is not fine. Most people don't let inverters run at 100A draw long enough for the wire to reach maximum temperature. But when they do they are shocked that the wire melted the insulation.

I can't tell from his picture whether he is in danger of short circuits. So I did not comment on that part. But with 2 AWG wire it will melt again if the inverter runs a few times for 15-30 minutes each. That assumes there is enough battery to let it run that long. He needs to use minimum 0 AWG wire and preferably 2/0 from inverter all the way to the battery per vendor recommendation. The lugs should be closed end hydraulic crimped with no solder.

[Auspicious]

Originally Posted by transmitterdan

It does work like that. No matter what the schematic an inverter has to deliver AC current to the AC load.

We agree on that. All the AC is generated in the inverter. Period dot.

Originally Posted by transmitterdan

It draws the current from the battery. The inverter "rectifies" the battery current so that it draws rectified current from the battery.

No. Rectification is how you get AC to DC, often with a diode bridge, sometimes with a single diode. That isn't necessary when drawing from a battery which always, and can only, deliver DC.

Originally Posted by transmitterdan

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.

No. The inverter draws DC at varying levels. The time is based on the inversion circuitry. Old electromechanical inverters had current limited by the inductive reactance of the switched transformer. Newer ones are limited by the MOSFET or other switching circuitry. Regardless the battery is delivering current in one and only one direction albeit with changing current values. Current runs from 'plus' to 'minus' (electrons migrate from 'minus' to 'plus' for those so inclined to physics).

Originally Posted by transmitterdan

This got started because I was trying to explain to why the DC battery cables to an inverter can overheat.

They generally overheat because people cheap out and buy wire that isn't big enough.

Originally Posted by transmitterdan

Yes, we have a terminology issue. DC to an engineer is current that does not vary (i.e. the frequency is zero). If it varies over time then it is made up of different frequencies of AC (aka time varying current).

Not true. 1. I am an engineer. 2. DC is current that flows in one direction although it may change in value. AC is current that flows in alternating directions.

Originally Posted by transmitterdan

Some lay texts teach that DC is current that always flows in one direction (say positive). Those texts teach AC must necessarily reverse direction (be positive some times and negative other times). By that definition batteries can only produce DC. But those definitions don't allow us to talk about harmonic components and do the calculus to determine how much heat a certain current profile will generate.

You are missing something. AC by definition reverses direction. That doesn't change at all the value of integration (easier for variable DC than AC and much easier than variable AC) in determining the energy that resistance turns into heat.

Originally Posted by transmitterdan

If you ignore the silly AC/DC comments in this thread meant to belittle someone and focus on the time varying nature of battery current in inverter applications you will see why it's not a simple DC circuit.

You seem to be missing the component of thermal inertia. Variable DC at 50 or 60 Hz has little impact on the heat generated by resistive loss. RMS measurements of DC can be plugged directly into P=I^2*R for heat.

Originally Posted by Canibul

Moving right along, and while the serious inverter talent is here, I ran into a situation yesterday that puzzled me a bit. I hauled the boat, and was planning to cut a hole in the cabin top to install a solar vent. I decided to try using the inverter to run a rotary cutter, a 115 VAC powered thing called a RotoZip.

From your voltage measurements it would seem you have a modified sine wave inverter. Many inductive loads (like the motor in your Roto-Zip) don't operate well with the wave form generated by a modified sine wave inverter. The motor will just sit there and buzz.

[irwin37]

Thank you Auspicious, I have been grinding my teeth but did not want to get in a fight about this. I agree with everything you have so eloquently stated. I am also and electronics engineer (Retired) Design and layout of multi-megawatt grid-tie solar systems. I have personally worked with several of the engineers at Xantrex on large projects. (Good people). The so called AC on the battery leads is termed supply ripple or ripple current which with an inverter can be very large.

Just another note, when installing inverter wiring, it is wise to run both the negative and positive very close together, actually tape or heat shrink them together and if possible make both wires the same length. Do to the large ripple current there is an inductive component to the wiring, (this is what is being read on you ac amp meter by the way) keeping the wire close to each other will tend to cancel some of this component. also keep them away from large metal objects (say 6 - 10 inches ,not feet) or the wires can actually get noisy ... like Buzzz.

As to the roto zip tool I believe it is actually a DC motor with a rectifier front end, I think his inverter may be putting out only half wave or just low voltage altogether which would slow the tool.

[rwidman]

Originally Posted by irwin37

............. The so called AC on the battery leads is termed supply ripple or ripple current ..............

That's exactly what I was trying to say as well but I was getting nowhere. While I can't claim to be an "engineer" (well I could, after all this is the Internet), I was educated in electricity and electronics and that was my career for many years. I think transmitterdan is blowing smoke trying to impress people.

[transmitterdan]

The problem with the simplistic lay definition of AC and DC is that it's difficult to talk about RMS and the various frequency components of a DC signal. We don't have a way to talk about the frequency of a DC signal. Any waveform can be described by the amplitude and phase of its various frequency components. The DC term has zero frequency. All the other terms have a frequency and phase and are thus by definition AC. I apologize that some are confused and I agree it's not appropriate for this forum to delve into Fourier series and superposition theory. But the wire must obey these theories. Also, a clamp on ammeter functions on these theories and a good meter can measure the RMS AC current and the zero frequency DC current exactly as I understand the meaning of those terms. A signal does not have to go through 0 to have an AC frequency and phase. And a signal that passes through 0 can have a DC value. The simplistic lay definitions taught in high school science class of AC and DC are not sufficient to calculate the RMS or peak values of inverter current. And these are important in understanding why a 100A DC current is more than 100A RMS and a lot more than 100A peak. And there are posts in this thread arguing that wrong understanding which leads to wrong wire selection.

Xantrex could do a better job of explaining the wire selection in their manuals. The manual Sandero has is not clear enough for a DIY person IMO.

[transmitterdan]

Originally Posted by rwidman

That's exactly what I was trying to say as well but I was getting nowhere. While I can't claim to be an "engineer" (well I could, after all this is the Internet), I was educated in electricity and electronics and that was my career for many years. I think transmitterdan is blowing smoke trying to impress people.

I apologize for giving that impression and I can assure you I care little about what anyone thinks of me personally. But I do care that bad information and bad assumptions about electrical systems can kill people. A fire aboard a boat is life threatening. Sandero's experience was a near fire. His boat was saved by the low voltage shutdown of the inverter. But that doesn't always happen.

As people move to larger battery banks and larger inverters they simply have to understand wire sizing. We have reached the point where modified sine wave inverters often cost less than the wire and fuse needed to hook them up. And people simply don't want to believe that can be the case. But it is.

[Auspicious]

*sigh*

Originally Posted by transmitterdan

We don't have a way to talk about the frequency of a DC signal.

Because DC doesn't have a frequency. Yes, a signal may have DC and AC components. We can separate those and deal with them independently. DC and AC may themselves vary independently, as can AC signals of varying frequency - that's how things like BOPL work. We have lots of ways to talk about varying DC and AC. Not all are continuous functions.

Originally Posted by transmitterdan

Any waveform can be described by the amplitude and phase of its various frequency components.

Sure. Not everything we measure is a waveform.

Originally Posted by transmitterdan

I agree it's not appropriate for this forum to delve into Fourier series and superposition theory.

Why? I don't think anyone has said that isn't appropriate. Fire away. M. Fourier developed a wonderfully elegant mechanism for breaking complex signals into their components. Superposition just lets you bolt them back together again. It's wonderful stuff for audio and radio work. We can readily apply it to an inverter with a constant load. With varying loads things get more complex but if you want to play with discontinuous functions we can do that. Have at it.

Originally Posted by transmitterdan

Also, a clamp on ammeter functions on these theories and a good meter can measure the RMS AC current and the zero frequency DC current exactly as I understand the meaning of those terms.

That depends on the meter. We'll have to go back to the specs and ultimately to the circuit diagram.

Originally Posted by transmitterdan

And these are important in understanding why a 100A DC current is more than 100A RMS and a lot more than 100A peak.

Pardon? You're going to have to show me the math for pure 100A DC having an RMS current different from 100A. No superimposed AC...

You have at least two EEs and a marine engineer saying you are mistaken and yet you persist in describing our posts as based on "lay" information. Frankly I believe that any technical person who cannot responsibly describe an issue in terms any reasonably competent person can understand does not have sufficient grasp on the issues at hand.

[Auspicious]

Originally Posted by transmitterdan

As people move to larger battery banks and larger inverters they simply have to understand wire sizing. We have reached the point where modified sine wave inverters often cost less than the wire and fuse needed to hook them up. And people simply don't want to believe that can be the case. But it is.

On this we can agree. I have yet to find a manufacturer that doesn't provide reasonable wire sizing guidance. Sometimes that means big wires and people get cranky about the wire costs. Some people lose sight of the need to count the wire in both directions (battery to inverter and inverter to battery). We are talking about significant current and heat will increase as the square of the current. Big wire is good. Proper crimped connections are good.

[rwidman]

Originally Posted by transmitterdan

.......... But I do care that bad information and bad assumptions about electrical systems can kill people...........

But that's exactly what you're doing. Not the part about making sure the cables are capable of carrying the required current but the part about getting alternating current out of a battery or measuring alternating current on the conductors from the battery.

You are throwing technical terms around like you know something and you may in fact know something but you should know that pulsating DC is not "alternating current".