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Old 20-07-2013, 08:37   #16
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Funny how AC is ridiculed with examples of inefficient AC fridge vs efficient DC fridge. All of those DC fridges use a Danfoss compressor that runs on (3-phase) AC. The Danfoss controllers that we all loath have to convert our 12V DC back into AC, with all the losses mentioned on the other side. Sooooo.. my take is that an AC Danfoss compressor is more efficient than a DC version.

Like already stated before: commercial ships all use AC.
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Old 20-07-2013, 08:52   #17
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Re: Moving towards a AC boat.

The problem with AC fridges is not the compressor its the rest of the fridge.

Consumers, not surprisingly, with ready access to lots of power prefer the convenience of a large interior volume and front opening over thick insulation and top opening.
Recently there has been AC fridges developed with very efficient insulation (including vacuume panels) that make them far more efficient than their predecessors.

Large ships run generators full time and are far less concerned with electrical effieciency than recreational boats. Their needs are closer to the domestic consumer.

The goal of many long distance cruisers is to do without a generator and the requirements are very different to large ships.
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Old 20-07-2013, 17:40   #18
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Re: Moving towards a AC boat.

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Originally Posted by s/v Jedi View Post
Like already stated before: commercial ships all use AC.
A commercial ship is a multi-unit apartment structure built on top of an oversized generator. A recreational boat - not so much, usually.
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Old 20-07-2013, 20:08   #19
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Originally Posted by Lake-Effect View Post

A commercial ship is a multi-unit apartment structure built on top of an oversized generator. A recreational boat - not so much, usually.
Really? The ships I have been on were all of high end sophisticated design which was 100% based on efficiency in order to optimize availability and thus profit during the operational life of the ship. There is nothing like oversized gensets nor appartment structures built on top of them.
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Old 20-07-2013, 20:38   #20
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Re: Moving towards a AC boat.

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Really? The ships I have been on were all of high end sophisticated design which was 100% based on efficiency in order to optimize availability and thus profit during the operational life of the ship. There is nothing like oversized gensets nor appartment structures built on top of them.
Not looking for a fight, really. But even on the most sophisticated, energy-efficient commercial ship, there's an abundance of available and otherwise wasted energy from the ship's main propulsion systems available for electrical systems and heating, not to mention the requirements for redundant generation capabilities for operational safety.

I can't see a direct line between the power technology and capabilities required on a modern commercial ship, and our own requirements aboard a small pleasure-craft. Vastly different requirements, and orders of magnitude difference in power availability and uses.
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Old 20-07-2013, 20:43   #21
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Re: Moving towards a AC boat.

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It'll be nice if we could have 3 ph inverter if a circuit needed it for high start torque, maybe multispeed (Pole changing), slow for hauling an anchor in fast for letting out, no brushes to worry about.
Modern 3-phase squirrel-cage induction motors are a beautiful thing. They have been so well developed that bearing life is now often the limiting factor of the motor's service interval.

These days, 3-phase power is pretty easy to get too. In the old days, you had to use a ronk box or similar rotary phase converter to get 3-phase from a single phase primary. Those big heavy klunky things are now only used in the more gruesome applications because static inverters have become small, lite, cheap & readily available. I put one on a 1.5hp Bridgeport milling machine about 10 years ago & it's still running like a champ today. Static phase inverters can be found here: Enco - Guaranteed Lowest Prices on Machinery, Tools and Shop Supplies

If you want to get a little fancier, you can move up to a variable frequency inverter, that gives you variable voltage & variable frequency 3-phase output so that you can have variable speed & variable torque control of a 3-phase motor. You can program either speed or torque as the limiting factor.

At speeds below the nameplate RPM of the motor, they operate in the constant torque range & can give you full motor torque as the speed decreases. Above nameplate RPM, they operate in constant horsepower range & available torque drops off as speed increases. At 2 times nameplate RPM, you have roughly half of normal torque available. You can program lower torques if you want to. Speed control is infinite in theory, but in practice, you normally run them from about 10% of nameplate RPM to about 300%. If you run for long periods of time at low speeds with a fan cooled motor, you may need to provide additional cooling to the motor.

The vast majority of elevators built today use these to control the speed of the car going up & down. The S-curve acceleration & deceleration is why modern elevators are able to give you that smooth comfortable ride with no sudden jerks. They are pretty reliable little units. Depending on the individual model, they can be run off of single phase, 3-phase or high voltage DC primary power & still give variable 3-phase output. All this stuff is pretty efficient these days too. Losses below 5% are not uncommon.

One of the nice things about a variable frequency drive inverter is that it lets you start a motor gently, so that you don't get the sudden inrush currents that would normally be present if you just throw the switch on a motor the old fashion way. A 2 or 3 second starting curve will often cut your surge current to about 1/4 of what it was before. A simplified VFD that only has this one function would be called a soft starter. These devices are very good at reducing the beating that your electrical system takes from surge currents.

I can go into nauseating detail on these things if anybody wants me to, right down to board level construction & various generations of technology from the 1980's to present. I have pretty deep roots in this technology.
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Old 20-07-2013, 20:48   #22
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Re: Moving towards a AC boat.

In the past boats were built with 120 volt DC systems and also 6 volt DC systems. 32 volt DC systems are still around and I'm seeing more and more 24 volt systems. There is talk in the industry of going to 48 volt DC systems.

I have two brand new never been used 6 volt bilge pumps in my office.

Some of the 32 volt boats have had a 12 or 24 volt system added because it's easier to find equipment. They kept the 32 volt for starting the engines. Of course these are pretty big boats so they also have 110 volt and 220 volt AC power on board.
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Old 20-07-2013, 20:56   #23
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Re: Moving towards a AC boat.

Generators output A/C, so if you run a generator for all your power, A/C makes more sense. For a battery-based system, you want to run off its DC.
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Old 21-07-2013, 08:55   #24
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Re: Moving towards a AC boat.

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Generators output A/C, so if you run a generator for all your power, A/C makes more sense. For a battery-based system, you want to run off its DC.
Correction - most of the generators sold to consumers are AC output, because the #1 requirement is for remote, emergency or standby power to power consumer devices. On Land. Marine use is a tiny fraction of the generator market.

Many of the newer small generators are inverter generators, which means that they aren't required to spin at some multiple of 60 Hz - this improves efficiency. It also means that an onboard inverter takes care of producing the final AC output.

For recreational marine application, where the boat already has a heavy-duty inverter, I think that the ideal generator would supply a solid 12v to charge batteries and drive the existing inverter if necessary.

In all of this thread, we've identified areas where alternating current could be used and why, but the case has not yet been made that terrestrial AC standards would be more useful than the current 12v DC standard on our boats.
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Old 21-07-2013, 10:00   #25
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For a boat 12 vdc makes sense. LED lighting, dc supplies for electronics, very efficient dc fans and solar panels make 12v an east choice.

Let us not forget that DC is a lot less likely to grab hold of you and cause the 120vac shuffle. One frayed/rubbed through wire and not only do you have a chance to blow a fuse but when you reach into that compartment running your new antenna wire and touch it by mistake you get to have a good check of your cardiac health.
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Old 21-07-2013, 14:18   #26
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Re: Moving towards an AC boat

we'll see 24vdc boats before we see 120vac boats. I don't know why everything is still being made in 12v.
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Old 21-07-2013, 15:03   #27
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Re: Moving towards a AC boat.

The natural output of a generator is AC. You can convert it to DC pretty easily, however AC is what is produced from a rotating magnetic field.

24 doubles the voltage, halves the current, so it seems a worthwhile improvement. The key really is equipment that uses 24vdc.

Quote:
Originally Posted by Lake-Effect View Post
Correction - most of the generators sold to consumers are AC output, because the #1 requirement is for remote, emergency or standby power to power consumer devices. On Land. Marine use is a tiny fraction of the generator market.

Many of the newer small generators are inverter generators, which means that they aren't required to spin at some multiple of 60 Hz - this improves efficiency. It also means that an onboard inverter takes care of producing the final AC output.

For recreational marine application, where the boat already has a heavy-duty inverter, I think that the ideal generator would supply a solid 12v to charge batteries and drive the existing inverter if necessary.

In all of this thread, we've identified areas where alternating current could be used and why, but the case has not yet been made that terrestrial AC standards would be more useful than the current 12v DC standard on our boats.
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Old 21-07-2013, 15:21   #28
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Originally Posted by nimblemotors View Post
The natural output of a generator is AC. You can convert it to DC pretty easily, however AC is what is produced from a rotating magnetic field.

24 doubles the voltage, halves the current, so it seems a worthwhile improvement. The key really is equipment that uses 24vdc.
What do you call a "generator"? All the popular Honda generators do generate DC, which is why they can be variable RPM. They have an internal inverter to create AC output because people want AC.
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Old 21-07-2013, 15:42   #29
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What do you call a "generator"? All the popular Honda generators do generate DC, which is why they can be variable RPM. They have an internal inverter to create AC output because people want AC.
Not quite exact. Honda gens produce high voltage multiphase AC that is then fed to an "inverter". The voltage and freq switching then takes place. And out pops household current.
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Old 21-07-2013, 15:56   #30
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Not quite exact. Honda gens produce high voltage multiphase AC that is then fed to an "inverter". The voltage and freq switching then takes place. And out pops household current.
Sure. But only if from now on we call our DC alternators also AC generator instead
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