Constraints of Vehicle Power Invertors 1

[RRiver]

Researching this leads to finding a lot of information with differing opinions and arguing. My neighbor has a refrigerator that belongs to his daughter. She's a doctor that spends 6 months of the year as a volunteer in Columbia and the refrigerator is filled with supplies for her work in Columbia. During a recent power outage my neighbor ran the refrigerator from the power invertor in his Mercedes SUV. He asked me if that was OK and I didn't know. He did it so he obviously he can but should he? Recent events has brought this back up. Online there is an unlimited number of opinions from even electrical engineers about invertors and the different power waves used in invertors concerning what can and can't be done or maybe should or shouldn't be done. Manufacturer information is either ambiguous or even equivocal about invertor use.

Can anyone discuss manufacturer's guiding principles with invertor use decisions or anything else about invertor use. The only guidance I'm finding to have any reliability is "you get what you pay for" and invertors are available at drastically different price points while listing the same specs and OEM invertor replacement is not inexpensive.

Just FYI, the neighbor is currently waiting for virus restrictions to end so a natural gas Generac generator can get installed.

 

[BrianPetersen]

Are you talking about the kind that you plug into the 12 volt power outlet (the old "cigarette lighter")? Or a heavy-duty hard-wired type?

The 12 volt plug in type is limited to the amp rating of that outlet ... which is not much. The circuit probably has a 10 or 15 amp fuse. Even if it is a 15 amp fuse, that's only a 180 watt circuit (a little more if you consider the full 14 volts of the operating charging system - 12-and-change if it is on battery power). That isn't going to operate much of a refrigerator. It isn't going to operate much of anything ...

I've used a few of those, and the vehicle that it was plugged into never had an issue with them.

I suppose there is a theoretical possibility of electrical noise being produced from the inverter's internal DC-chopping circuits, but one would hope that (A) the device itself has some input filtering, and (B) the vehicle has that circuit fed from a circuit that is protected in some way from electrical noise, and (C) the vehicle's more sensitive electronic circuits have their own protection from electrical noise. On a vehicle with an old school wiring harness, that power would be coming from the battery through various switches and fuses, with the battery itself acting like a great big buffer. On a later one, it is probably coming from the body control module, which is a somewhat-expensive little gizmocontraption of its own.

Bear in mind that vehicle electronics themselves need electrical-noise protection from electrical noise that the vehicle itself generates. Spark plugs are noisy little things. And if your vehicle has a bulb-failure warning system (as most newer vehicles do) ... that system works by sending a PWM (pulse width modulated, i.e. chopped) signal out to the bulb to check for its response, even when the bulb is nominally off.

And on that note (literally) ... It seems that no one gives a crap about AM radio reception any more. If I'm a little bit distant from the transmitter of 680 News in Toronto, my late model vehicle superimposes two separate audible AM interference signals. One of them correllates to engine RPM ... probably the chopped DC signals to the spark plugs, and probably to the fuel injectors, and perhaps the rectifier in the alternator (which is itself a pretty good source of electrical noise). The other one correllates to whether a turn signal is in the midst of a flash cycle while the brakes are not being applied. That's probably the chopped PWM DC from the bulb-failure warning system causing that AM interference.

If your vehicle is properly designed to deal with all of its own electrical noise plus external interference like lightning strikes and the like ... I don't think you have to worry too much about something plugged into a fused DC circuit that's fed by the battery.

 

[BrianPetersen]

I should add that there is an unbelievable amount of PWM (i.e. electrical noise ...) in modern vehicles.

It's used for bulb-failure detection as mentioned above.

On some vehicles, the brake/tail bulb achieves the reduced-brightness mode via varying the PWM duty cycle from a lighting controller. It's a single filament bulb, bright enough to operate as the brake lamp, supplied with reduced power via PWM in "taillight" mode.

If it has LED lights (tail, turn, daytime-running), those are probably fed PWM.

The radiator cooling fan probably achieves a variable-speed function by being fed PWM. This allows the fan to make less audible noise (because it can be operated continuously at the lowest practical speed instead of just switching between off and full speed like the old school relay-fed fan motors would). Less audible noise, but surely more electrical noise.

And somehow with all this going on, all these various control modules communicate with each other via CANbus mostly without interference ... not always!

 

[IRstuff]

A compact refrigerator is relatively bullet-proof

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[RRiver]

Brian,

Thank you once again. With this I'm referring to the hard wired type of inverter. I also have to admit I didn't even consider noise from the inverter back in to the vehicle's electrical system.

To highlight some of the main concerns discussed or argued about inverter use it begins with the need for a true or real sine wave inverter being required to operate a full size home appliance like a refrigerator. One point of contention is if a PWM simulated sine wave inverter is actually "true" or "real?" To be honest, most of the arguments made one way or the other are beyond my understanding but it seems the issue isn't as contentious as it once was. Circuit advancements and inverter quality has mitigated a lot of opinions to "does it matter?" Some of what is still contentious are issues like the voltage reliability of an alternator running at engine idle speeds for any long period of time and how this effects an invertor. Another concern is with the variability and sometimes erratic nature of the inductive circuit or load of a large appliance like a refrigerator. The effect on the inverter from larger inductive loads is contentious. A final issue to highlight is the length and size of the extension cord used to run a home appliance from an vehicle inverter in the driveway. In the example of my neighbor running the refrigerator the extension cord was a minimum of 50'.

The circuit noise you discuss only makes sense but from my research it's not discussed much in information available. It's all about the inverter and the load on the inverter. An example of other contentious information is the reliability of modified sine wave converter with a load like a home appliance. There are a lot of different opinions. Another example, the extent and use of digital and analog devices\components in the inverter. The benefits and drawbacks of each are argued when it comes to the "better choice."

Another argument although it doesn't apply here, is the use of inverters that have both AC and DC inputs. I would best describe these inverters as back up power supplies. Not only do these inverters operate off the DC power of a vehicle but they also run off of AC generators and are used to filter the out going AC power.

Finally, many of the available inverters also operate on Can Bus controlled designs. CAN Bus circuits are darn near in almost everything now.

IRstuff, granted. The small refrigerators are bullet proof now. When looking at inverter features there are many that will state they are fine with appliances but many also state, appliances in the vehicle. A common use with these is the semi truck sleeper cab which until now I never realized can even come with washers and dryers. Go figure.

 

[BrianPetersen]

No idea about what the output AC looks like. I've only ever used those plug-in inverters to charge a laptop or some such thing, and the first thing the power supply for a laptop or most other gadgets does with the incoming AC is rectify it to DC and filter it anyhow. I've never had anything care.

I have a gasoline powered Yamaha EF2000i inverter generator that I use to power stuff on camping trips. Yamaha claims that the output AC from the inverter has a more accurate waveform than that of a conventional similar-size non-inverter generator. Honda makes the same claim about theirs. I've never had anything complain about the power supplied by the Yamaha, either ... aside from popping the circuit breaker because we plugged in too much stuff at the same time. Doesn't like any other big load plugged in along with the coffee maker. No idea how their inverter works but it is probably well filtered after the chopper turns the internal DC bus voltage into AC.

 

[RRiver]

Brian, I too have a Yamaha EF2000i(). I wanted the early model where 2 generators could be ganged but they were no longer available. I think it's a great generator though. My travel trailer is 30 amp and the Yamaha ran it with no problems. I quit coffee 20 years ago so I never had that problem. I did pop the breaker a few times from running the AC and microwave at the same time. That's because I forgot to turn the AC off first. Not a shortcoming with the generator. I don't think the inverters with a AC input are geared towards inverter generators. I also have an inverter Mig\Tig welder and it's always worked great and uses much less power than the older welders. Welding aluminum though requires a spool gun so it has it's limits.

I'm not sure my inverter question will have a firm answer. At least not for a while. I work with a museum's engineering data archive and one of the things we do is maintain a helpful glossary of archaic words\terms and words\terms with meanings that have changed over time. When using or researching information the researcher has to read within the context of the era being studied or they can get the intended meaning very wrong. Updating the glossary can get contentious with some heated opinions. One term that's getting a lot of attention now is "bad ground." The explosion of electronic circuits in vehicles has changed bad ground to usually mean now a bad connection to ground. Primarily because of the effects of "path of least resistance" can have on other circuits and resulting peculiar effects. A recent example I'm aware of is with power windows that didn't work unless the interior, overhead light was turned on at its dashboard switch. This is very different from what a bad ground was for the first 70 to 80 years of the automobile. A lot of younger engineers and educators don't view ground in terms of a ubiquitous point of an entire "electrical system" for current return. For the most part now everything is circuit centric rather than system centric. How to deal with this one gets contentious but often fun.

 

[RRiver]

Brian,

You peaked my interest with your noise\interference comments so I looked in to it. From Wilfred Voss, an expert on the automotive CAN Bus system. His words, noise and interference isn't an issue. It's one of the benefits of using a voltage differential between CAN_H and CAN_L. Any noise or EMI will effect both wires the same but the differential voltage will remain constant. I didn't expect that. He acknowledges the noise just as you have but it just doesn't matter. The actual signal status is based on the voltage differential.

 

[3DDave]

"piqued"

 

[RRiver]

Thank you Dave. Not the first time I've made this mistake.

 

[itsmoked]

Whatever your refer runs on is fine. Inverters come in three flavors. PWM, Modified Sine Wave, and Sine-wave.


A PWM inverter simply makes full voltage square waves that add up to the power that is delivered on each half cycle of a normal power sine wave.

PWM are really bad for anything other than resistive loads. That's loads like incandescent light bulbs or heaters. The waveform supplies about what would be supplied energy-wise from a wall outlet sine wave. You can see that something that's simply being heated wouldn't probably care much about it. Motors and a lot of electronic loads would NOT be happy with this simplistic waveform.

Next up are Modified Sine Wave inverters. These are more sophisticated PWM inverters. Here, they use several PWM waveforms added together to more closely approximate a sine wave. Issues crop up when cheaper ones only use, maybe, two different waveforms making the approximation really pathetic. Others may use up to six or seven. This can cause confusion in the market because the higher number ones will run almost anything and the cheaper ones just about nothing but resistive loads. The cheap ones paint modified sine wave inverters with a less than stellar reputation.

Good modified sine wave inverters are reliable and can run almost anything. Things that they don't run well are devices with power factor correction that are trying to match their current draw up closely with the delivered voltage sinewave. They get confused by the odd sine wave they're trying to match. Also, devices that watch the sine wave for timing. Sometimes VCRs freak-out, occasionally other electronic devices don't cope well either. Some electronic devices with rectifier front-ends have trouble because the provided PWM shapes make them pull all their power during a brief instant, others don't care, it depends on their power supply design.

Motors do okay with reasonable modified sine wave forms. Motors are heavily inductive which actually filters the modified wave form into more of a sine wave. If the refrigerator starts every time if will be fine on a modified sine wave inverter.

The major problem I've seen with refers on inverters is that often the inverter is not large enough to start the refer compressor motor reliably. You hook up, say, a 600W inverter to run your 200W refer and the refer kicks on, you nod and walk off. Since motors can draw 7 times their running current on starting maybe the next time or the time after that (possibly hours later) the inverter can't quite start the refer and trips it's protection circuit breaker or even a fuse. Due to the nature of refrigerator operation it's easy to not realize the refrigerator is not running because it has no power verses just being in its off cycle.

A modified sine wave running into a motor will not provide the same efficiency so the motor will run a little warmer. With a refer this doesn't matter in normal operation because a refer's motor is bathed in cool refrigerant and the motor heat is dumped off the condenser coils along with the box heat.

Sine wave inverters do frequently provide better sine waves than your utility because utility sine waves are often distorted due to various poor loads they're saddled with.

Inverter generators are fine for anything they're sized correctly to run.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Comcokid]

One issue of DC/AC inverters frequently overlooked is input ripple current. If this is dealt with and how well this is dealt with can determine just how reliable an inverter is over time.

All inverters first step the DC input to a high voltage DC=, in a DC/DC stage. Then the high voltage DC is turned into an AC in a DC/AC stage. 115 VAC RMS is 115*SQRT2 at the peak, and this peak AC is produced at 2x the output frequency. So, unless there is energy storage between the DC/DC and the DC/AC stage you will find a large change in the DC input current at 2x the output frequency. There are pure sine-wave inverters that have a lot of input ripple current.

To reduce the ripple current the energy storage is performed with electrolytic capacitors or film capacitors. But electrolytic capacitors have limitations on current ripple themselves as they will self-heat and the lifetime of the electrolytic is severely reduced due to the internal temperature rise. Film capacitors have little in current ripple limitations. But the energy density is far less than that of electrolytic so you need a lot more.

Well made true sine-wave inverters will either use a larger number of electrolytic for the high voltage DC to keep the ripple current spread over many capacitors, or will use film capacitors. Either way the cost and size is much higher, and such inverters end up in commercial or industrial products.

Even a high-end consumer inverter will usually have a compromise between the input ripple and the energy storage and the expected use lifetime of the inverter to keep cost and size low.

 

[itsmoked]

Comm; Don't forget that depends on whether or not it's a low frequency or high frequency method inverter. Low freq ones take what's offered ans simply drive a transformer so possibly no capacitors are required. Only a strong back.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Comcokid]

Itsmoked. I think you misunderstood what I was pointing out. In my post the PWM frequency is not involved. The issue is independent of the PWM frequency used by either the primary or secondary stage. I'm strictly talking about the frequency of the inverter output be it 50Hz, 60Hz or 400Hz.

I have seen 60Hz inverters, with pure sine-wave output, that did incidentally operate the PWM of the conversion stages at around 25kHz, yet did have a massive pk-pk input current waveform at 120Hz on the input DC. Because, unless you store energy in-between stages, the instantaneous power output to the load at 60Hz determines the instantaneous DC power input to the inverter. You see it in the input DC current waveform.

 

[itsmoked]

Hmm. If it's a single stage inverter. Lets say 24Vdc in and the method is to directly feed the primary of a transformer with 60Hz of 24V to be stepped up to 120Vac you're saying there will still be a lot of ripple current because of the output sine wave's energy distribution? Do I have that right?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dgallup]

If you are feeding the inverter from battery and or solar panel charge controller, what problem does inverter input ripple current pose?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Comcokid]

I did a quick search and came up with the document below. Disclaimer: I design power inverters for aircraft (28VDC in, 50, 60, 400Hz out, up to 1500VA), but I am not familiar with this company or the products they sell. Somewhat an advertisement for this company. Nevertheless, it covers the AC ripple on the DC side of the inverter at a basic level.

https://www.victronenergy.com/live/_media/ve.bus:4._ripple_in_a_ac_battery_system.pdf

More issues arise when the vehicle system is getting old or has been modified and the resistance of the DC components increases (wires, breakers, fuses, change-over switches, connections, battery terminals, etc. And battery resistance goes up as a battery ages. Technical term for the sum of all these resistances is 'source impedance'. Alternators do not put out pure DC either, but a rectified DC smoothed/filtered by the battery, and you can begin to get a beat-frequency mix between the alternator output related to engine speed and the fixed frequency of the ripple created by the inverter.

 

[itsmoked]

Ah, Victron-the-over-priced. (LOL) I'll check that out.

Oooo, a ripple beat frequency, that sounds ugly.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Why is ripple bad? Instantaneous voltage dips?
Beat frequency ripple? You won't get a beat frequency off the alternator unless the alternator is charging.
Alternator ripple in Volts? Not much?
When the alternator is charging the battery terminal voltage rises, even at the minimum point of the ripple.
Bottom line:
Even if the alternator charging results in a ripple beat frequency, the minimum voltage to the inverter will be greater than with the alternator not charging.
So what are the results of a beat frequency?
Interesting.
Looks messy on a scope.
Still better performance than with the alternator not running.
Have I missed anything?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Comcokid]

Waross If you're running an inverter with any load, the alternator is always going near full-tilt as the current draw is BIG! Also, ever heard of 'alternator whine'? If you're a ham radio operator who's ever used a transceiver in a car or truck you might have. If you ever have carefully listened to the opening of the Pink Floyd song "Wish You Were Here", you've heard alternator whine. Pink Floyd uses it to set the mood in the opening of the music. A song is playing on a bus radio, and someone on the bus begins accompanying the song on a guitar. You hear the rising and falling (in frequency) alternator whine in the radio audio to plant the idea of boredom and loneliness of riding a charter buss between concert events while on a tour. The alternator whine varying slightly as the bus rides up and down gentle slopes. Perfect setup for Wish You Were Here.

Now, add a lot of 60 Hz and harmonics that are fixed-frequency into that frequency varying alternator whine. And as it varies with the engine rpm setting up base or vibration-like beat notes that move around with engine RPM. Now put it on an RV where you might actually be trying to listen to music while driving, or running the engine to provide power while parked.

Other than Pink Floyd, I can't think of anything positive about this kind of noise. Of course, many people today want amps that will rattle the body panels on their vehicles, so maybe all of the added noise for them is desirable.