Mini-split two heads and Transformer

[robertglines1]

start up spike. Make sure you buy inverter  type. My 12,000 pulls 8 amp max @120 V. the 9,000 pull 6 amp max @120 V

[bevans6]

There is no such thing as 120V half-phase power, but I understand what you mean.  The transformer will take the 120V single phase AC in and double it to 240V single phase AC.  Exactly the same, only twice as much voltage.  The transformer will have a center tap, that becomes neutral and the two live legs are each 120V single phase with respect to the center tapped neutral.  This is exactly what the main power transformer on the pole outside your house does (except it is usually taking something like 7KV single phase and transforming it down to 240V center tapped single phase) and the power is, for all intents and purposes, identical to house power.

Start up current is simply a matter is selecting the appropriate size transformer.  If you need 20 amps at 240V to start the thing, buy a 5,000 watt transformer.

Brian

[eagle19952]

so...how many evaporator units/heads will one compressor unit run...?

[Oonrahnjay]

There is no such thing as 120V half-phase power, but I understand what you mean.  The transformer will take the 120V single phase AC in and double it to 240V single phase AC.  Exactly the same, only twice as much voltage.  ... 

     Thanks, Brian, that's very helpful and it's getting to the heart of the lack of understanding that I have.  But there is a detail I'm not clear on ad I'd appreciate a little clarification.

      1)  For "ordinary household current", I think that I have seen info that N. American 120V, as shown on a waveform graph, is starting at 0 voltage on the waveform, rising in the sine wave to 120V above the line and then falling back to the 0V point; then a gap of the same time (set by the Hz of the power) at 0V, after which the process repeats itself.  For 240V, the waveform begins at 0V and rises to 120V then g back to 0V; from here, the waveform goes mirror-image "down" to 120V (on the "other leg") and then back up to 0V.  This 240V power is a full "positive" sinewave and a complementary "negative" sinewave as a single cycle -- "positive" and "negative" meaning of course the layout on a graphic readout, not pos and neg in an electrical sense.

      Am I understanding this right so far?  If so, it would seem to me that an electrical load that was designed to operate on 240V single phase would want that full sine wave "up and down". 

      And that is the point of my lack of understanding here.  If 120V is as I've described it above on a graphic readout and 240V is also as I've described it, does the 120V - 240V transformer change the waveform as well as the total voltage?  Or when you say "exactly the same, only twice as much voltage", will the 240V output begin a 0V, go up to 240V and then back to 0V -- and if it does, does that matter?

      Sorry for lack of understanding here but I'm just trying to figure out how the system we're talking about here would work and what that will translate to on my bus.  Thanks for the help.  BH

[HB of CJ]

I asked once and got a bunch of answers on how to post pictures.  Sosss ... I will be the fall guy and politely ask again: ...

Are you disabling BOTH of your factory roof escape hatches to install your split A/C units?  Respectfully.  HB

PS:  And if sossss .... WHY?

[ArtGill]

Both 120vac and 240 vac are full wave 60 cps just the wave height for 240 is twice as high as the 120.  Remember that a pure 240 vac device does not need a neutral, just two hot legs.  The transformer will have hot leg and a neutral on one coil and output 240 on two wires on the other coil.  The number of wraps of each coil will have a ratio of 2:1, with the 120 vac input will have the more wraps than the 240 vac output.  These coils are wrapped around a steel core.

TBoone, I'm in the process of making the installation.  I have checked the current draw on an actual unit and found the current draw ramped up slowly.  The compressor and fan speeds are variable and therefore the current draw is variable controlled by cooling demand.

I have seen up to four inside units supported by one out side.

Yes, I am using the locations of the roof escape hatches for my units, but the hatches have been long gone.

I love it when others post their project pictures and I wanted my project to be a benefit to other, but I will post how my project turns out when I finish it.

Art

[Oonrahnjay]

Both 120vac and 240 vac are full wave 60 cps just the wave height for 240 is twice as high as the 120. ...

     Thank you, Art.  I misunderstood items that I had previously read on this.  This information is very helpful to me and it makes me understand how the plain transformer can work for this system.

I wanted my project to be a benefit to other,
Art   

     Very much so.  This discussion has pretty much solved most of my air con design problems and issues.

[tbfisher]

TBoone, I'm in the process of making the installation.  I have checked the current draw on an actual unit and found the current draw ramped up slowly.  The compressor and fan speeds are variable and therefore the current draw is variable controlled by cooling demand.

   Thanks Art. Sounds like the 110V to 220V conversion will be no problem. Now I just need to figure out a good place for the condenser unit in my coach. I am really looking forward to having a very efficient and QUIET system since the noise level is 80dB+ in my coach with the AC running. I bet that coach manufacturers and converters are seriously looking at this also.

TBoone Fisher
05 Monaco Dynasty Diamond IV
Granbury, TX

[luvrbus]

  Thanks Art. Sounds like the 110V to 220V conversion will be no problem. Now I just need to figure out a good place for the condenser unit in my coach. I am really looking forward to having a very efficient and QUIET system since the noise level is 80dB+ in my coach with the AC running. I bet that coach manufacturers and converters are seriously looking at this also.

TBoone Fisher
05 Monaco Dynasty Diamond IV
Granbury, TX

Coach converters will never go that route they are not into efficiency and roof tops are coming with the inverter technology Coleman plans on marketing one next year(made in China). 21,000 BTU will keep a bus cool when parked but not on the road.I have 65000 BTU of cooling ( 5 Penguins) foam spray and double pane windows plus my dash air of 44000 BTU but I live in AZ too and I like cold air    

[Oonrahnjay]

    OK, I found a diagram on a Samlex website (not that I'm impressed by their inverters, but that's another story).   Is this what we're looking at?


   And I've been trying to visualize how the two legs of the 240V are arranged.  I've slightly altered the above diagram and used a NEMA 4-pin image for clarity, although I'm going to guess that most everyone will "hardwire"; I also would guess that the 240V air con circuit will not include 120V sockets, they'd be in a separate circuit but this is just for simplicity.  Is this correct?


  Still trying to get a good picture in my head.   Thanks,  BH

[bevans6]

Electricity is fun stuff.  Believe it or not, I learned this stuff in grade 11 shop class.  We had a shop class just for electricity.  I bet they don't now.  I later took electronics in college, and worked with this stuff my whole life.

AC voltage, your typical 120VAC from the wall plug, has a hot lead and a neutral, with the neutral referenced to ground.  The voltage on the hot lead starts at zero (with reference to neutral), rises to a peak voltage of 170 volts, goes back to zero, goes to minus 170 volts and back to zero, so the actual voltage swing is 340 volts and the average voltage is zero.  That is one cycle, and it does it 60 times a second.  Why do we call it 120VAC instead of 170 volts or the even more logical 340 volts?  Because electricity grids started out with DC voltage, and the 120 VAC as we call it is the equivalent of 120 DC volts.  The 120 AC volts is sometimes marked "RMS volts" on a meter, and happens to be 0.707 (the sin of 45 degrees) times the peak voltage of 170 volts (rounded off the decimal points).

240 volts is two 120 volt legs that happen to be exactly opposite added together.  When the first 120v leg's waveform goes positive, the second goes negative. At 1/240th of a second they are each at their peak of 170 volts  with respect to neutral, for a total of 340 volts, and at 1/120th of a second they are both at zero volts with respect to neutral, and they do that again for a complete cycle at 1/60th of a second.  If you take one of the legs and call it the reference, then what you would see is a normal sine wave shape that goes from zero to 340 volts and back again, so 340V times 0.707 is our familiar 240VAC.

RMS means "the root of the mean of the squares" and if I ever actually understood the math of why the sin of 45 degrees happens to be the AC equivalent of DC power, I sure as heck don't remember now.  I expect I will google it and try to remember sometime today...  It involves calculus, and I don't like calculus...

Brian

[tbfisher]

Coach converters will never go that route they are not into efficiency and roof tops are coming with the inverter technology Coleman plans on marketing one next year(made in China). 21,000 BTU will keep a bus cool when parked but not on the road.I have 65000 BTU of cooling ( 5 Penguins) foam spray and double pane windows plus my dash air of 44000 BTU but I live in AZ too and I like cold air    

  I can see that there wouldn't be much concern about efficiency but IMHO, noise is a bigger issue. I doubt that they could ever make a roof top unit that is as quiet as a mini-split cassette would be. I have a neighbor who has a very nice mult-islide Newell. I was sitting in it for the first time when the AC came on. Without me saying anything about the obvious noise level, he promptly commented that the AC was very noisy. I think he said it had 4 Coleman units. IF I paid that much for a coach and it were that noisy inside, I would be VERY pissed off to say the least.

TBoone Fisher
Granbury, TX

[luvrbus]

  I can see that there wouldn't be much concern about efficiency but IMHO, noise is a bigger issue. I doubt that they could ever make a roof top unit that is as quiet as a mini-split cassette would be. I have a neighbor who has a very nice mult-islide Newell. I was sitting in it for the first time when the AC came on. Without me saying anything about the obvious noise level, he promptly commented that the AC was very noisy. I think he said it had 4 Coleman units. IF I paid that much for a coach and it were that noisy inside, I would be VERY pissed off to say the least.

TBoone Fisher
Granbury, TX

Yep but those 4 roof tops on a Newell have a total of 60,000 BTU cooling not 20,000 BTU and all are 110v no assembly required

[tbfisher]

Yep but those 4 roof tops on a Newell have a total of 60,000 BTU cooling not 20,000 BTU and all are 110v no assembly required

   Do you think that since they are sitting in direct sun, ducted through a very inefficient system and subject to all the abuse that roof ACs take that they are actually delivering 60K BTU ?? Also what effect do you think that driving them down the road in a 60++mph typical AZ or TX hot air blast might have on them?

I doubt that roof ACs ever get close to effectively delivering their rated output;-)

TBoone
Granbury, TX

[brmax]

I'm looking at these for a positive, and heres actually an old reason take a look see for yourselves when ya have to look at the news on the tv again.
These other countries I see many but not all have a lacking electrical infrastructure compared to ours. When I see the housing, apartment flats and other buildings in the screen shot that's panning, Its pretty clear they have these type systems it looks like to me. So as a basic they must have a decent service factor as most of the screen shots are desert and tropical, with marginal electric. I think with that they could be tough enough. Now that I'm looking at these some more and could use some laymen information in there use going down the road with required venting, protection, drafting issues, Just some things I might consider planning toward.
Thanks for the topic and
good day
Floyd