12 volt AC unit

[posplayr]

I decided to do a little deeper dive into the scroll-type compressors this morning compared to more conventional automotive AC units.


This is an animation describing the principle of a scroll-type compressor. This type along with variable frequency control comprises the primary technology improvements of the newer DC AirConbetter than.

For instance, an AC motor is synchronous meaning it runs at a speed directly linked to the AC voltage sinusoidal waveforms. For 120VAC this is fixed at nominally 60 Hz. With modern power electronics (similar to a DC to AC inverter) a variable VAC frequency can be produced and so the 12/24VDC AirCon compressor motors are still synchronous, but the frequency can be adjusted for how high of a compressor output is required. The higher the frequency the higher the pressure output (to maximum efficiency; see chart below).

https://www.youtube.com/watch?v=yNgqI4XPUZc

This video disassembles is a typical automotive compressor that uses a swash plate or wobble plate (a tilted rotating plate) to move pistons in and out for compression.

https://www.youtube.com/watch?v=urFVLKlpf1I

Worth mentioning there is what is termed a Variable Displacement Compressor (VDC). This wobble plate compressor provides a way of altering the angle of the wobble plate. The higher the angle the more displacement the pistons and the more volume/pressure is generated. Apparently, this type can be operated without a clutch as a flat swashplate is equivalent to having the clutch disengaged effectively stopping any compressor operation.

The main disadvantage of the scroll-type compressor vs a wobble plate type is that there is lower volumetric efficiency at lower RPMs (e.g. 500-1500). However, the scroll has several advantages:[Reference Reichelt, J. 1986],

• Smaller size, lighter weight
• high-speed operation
• smaller torque pulsation, quiet operation
• higher durability against slugging
• longer life (reduction in the number of parts)

The attached figure gives you an idea of volumetric efficiency (higher is better) by comparing a wobble/swash plate (WSP) to scroll-type compressors. The WSP tends to be more efficient at a lower RPM where it achieves a peak Coefficient of Performance (COP) (typically 1.5). The Scroll-type archives a higher COP (e.g. 1.7) at a higher RPM.

This is the primary reason WSP has been used in automotive applications where the compressor is belt driven off the engine. However, for a DC voltage with a DC-to-AC inverter application, the frequency can be electronically adjusted to optimize compressor operation.

So in a very real sense, the 12/24V DC AirCon units have the best of both worlds, quite a reliable operation of a scroll-type compressor that can be operated at peak efficiency. In contrast, the 120VAC compressors must be optimized for a fixed frequency and short of shutting the compressor down the consumption will be relatively constant varying only by fan power.

Reference Reichelt, J., "Refrigerating Compressors for Air Conditioning of Passenger Cars" (1986). International Compressor Engineering
Conference. Paper 551. https://docs.lib.purdue.edu/icec/551
See page 543 bottom of page
https://docs.lib.purdue.edu/cgi/view...0&context=icec

[b. rock]

Sort of right. Scrolls can still be variable speed with AC, usually an ECM in the small sizes or VFDs in the big sizes (yes I'm aware that an ECM is DC, but it is supplied AC). Digital scroll is another one to look up for reading material. Each manufacturer slices it just a little differently, but the end result is largely the same.

Piston/recip compressors are popular because they're dirt simple, cheap to build, and can reasonably modulate with mechanical methods.

COP is usually used for the whole system, not just the compressor. Compressor efficiency plays a part, but so does condenser condition, the condenser fan, and of course - outside air temp.

I don't go mudding, but even just driving down a dirt road with melting snow or after a big rain will completely coat the underside of whatever I'm driving. The cleaner a condenser is, the more effective and efficient it is. But a sealed box would work just fine.

[Wowbagger]

Originally Posted by posplayr

So in a very real sense, the 12/24V DC AirCon units have the best of both worlds, quite a reliable operation of a scroll-type compressor that can be operated at peak efficiency. In contrast, the 120VAC compressors must be optimized for a fixed frequency and short of shutting the compressor down the consumption will be relatively constant varying only by fan power.

Not quite true anymore. Many home AC units are running variable frequency drives on the compressors, exactly for the benefits being discussed.

[posplayr]

Originally Posted by Wowbagger

Not quite true anymore. Many home AC units are running variable frequency drives on the compressors, exactly for the benefits being discussed.

Thanks, but then don't they suffer an efficiency loss due to a double conversion loss (12VAC ->DC -> Vactiable frequency AC)?

Are any of the Rooftop RV AirCons variable speed?

I have three 5K BTU class window airconditioners and all seem to be fixed frequency because even in ECO mods the power draw remains virtually the same and MAX.

[posplayr]

Originally Posted by b. rock

Sort of right. Scrolls can still be variable speed with AC

Perhaps I did not properly edit my post or assumed context from my previous posts. The latest Chinese rooftop AirsCons are scroll type with variable drive frequency. This is what I previously referred to as the
"best of both words" (from the 1980's Automotive AC analysis comparing wobble plate with scroll type)

[Wowbagger]

Originally Posted by posplayr

Thanks, but then don't they suffer an efficiency loss due to a double conversion loss (12VAC ->DC -> Vactiable frequency AC)?MAX.

First of all, I was talking about home units, being fed 240VAC.


But, to speak to 12VDC units - you HAVE to make some form of AC at some level, or else the motor won't spin.



Old school DC motors make the AC by commuting the DC with the brushes and the contacts on the rotor - the rotor spins, the DC gets switched around on the rotor, the magnetic field rotates (relative to the rotor; relative to the stator the field is just twisted by 180 degrees divided by the number of poles on the motor).


More modern motors, like the ones in your typical computer case fan, commute the DC around fixed coils in the stator using electronics to detect where the rotor is and to steer the current. Such switching is very efficient: modern transistors have ON resistances that are in many cases lower than the resistances of the wires to the silicon.


In a high torque application like a compressor, you pretty much HAVE to bump the voltage up from 12V, to get enough current pushed through enough windings to have enough magnetic field to move the load. At 12V, to have enough windings to do the job, you have too much resistance from the copper, and you cannot push enough current to get the job done. Again, modern switching power supplies are very efficient at getting that done - better than 95% in many cases. The trivial losses to making higher voltage DC are more than made up for in avoiding resistance losses.

[posplayr]

Reading some of the comments in this thread, it seems as if my posts are drawing objections to things that are out of context to the OP's original purpose/questions/discussion. See Steve's dilemma as stated below:

Originally Posted by Steve Hunt

Yes the other device is a Renogy 50A DC to DC charger with MPPT. I don't have any solar but wanted to go ahead and be able to add it in the future. I suppose I may change my mind but at this point it's pretty distant on my projects list. Trying to make a decision on AC or DC air cond has made it to the top of the list. My biggest issue right now is any experience to confidently make the decision on the Air conditioning. It's got to happen though, it's impossible to sleep in the Southeast when ambient is a low of 78 and 90% humidity. Thanks for your input, very helpful.

The context of my own analysis is to determine the relative benefits of a 120VAC Rooftop AC vs a 12/24VDC RV rooftop AC. 120VAC means the device is powered from a single-phase 120VAC source either a generator, shore power, or conceivably an inverter (?). 12/24VDC means that the device runs off of a battery bank.

Rooftop AC units that have been around for decades but they are power hogs require typically in excess of 1000+ watts. The traditional RV is usually set up for either shore power or 120VAC generators to support rooftop AirCon.

There is a relatively new class of 12/24VDC AirCons that have been on the market, and the prices coming out of China have been steadily dropping as compared to the traditional 120VAC units.

With respect to Steve Hunt's original question and point of decision, what are the relative benefits of 12/24VDC rooftop AirCons relative to the 120VAC units? And of particular interest, when we consider the latest 12/24VDC AirCons from China (scroll type with variable frequency control) is there any basis for concluding (beyond the stated performance) that these units should be quieter, have lower power consumption, and have higher efficiency?

As I previously posted, is there any reason to believe that there are fundamental technical differences between a rooftop 120VAC unit and a typical 5-8K BTU household window unit? For example, are these 120VAC units all synchronous swashplate compressors? And what benefits are to be expected by the newer scroll-type compressors.

Originally Posted by posplayr

On the compressor look for a "scroll type variable frequency". I have never tested one but the variable frequency implies a solid-state inverter that can drive at higher or lower frequencies where power output is proportional to frequency.

In contrast, from my testing with a typical house/window AC unit (5000 BTU), the power (using a Kilowatt meter) pretty much stays the same regardless of fan speed. I presume that power consumption is dominated by the compressor that is constan6t/synchronized to the 60 hz AC.

[posplayr]

Originally Posted by Wowbagger

First of all, I was talking about home units, being fed 240VAC.

I'm glad you clarified, as it seemed (to me at least) as if your comments were likely out of context.

Originally Posted by Wowbagger

But, to speak to 12VDC units - you HAVE to make some form of AC at some level, or else the motor won't spin.

Having taken an undergraduate class in rotating machinery in what is now about 45 years ago, I know what you are talking about but there is simply no 120VAC generated in a brushed DC motor. Current reversal yes but no sinusoidal voltage/current characteristic of 120VAC.

Originally Posted by Wowbagger

More modern motors, like the ones in your typical computer case fan, commute the DC around fixed coils in the stator using electronics to detect where the rotor is and to steer the current. Such switching is very efficient: modern transistors have ON resistances that are in many cases lower than the resistances of the wires to the silicon.

You are correct in bringing up Brushless DC motors because these are probably closer to a variable frequency control for the scroll-type compressors. However high-speed,variable-speed BDC motors used in robotics are even closer to what is being described as "variable frequency".

Yes MOSFET/IGBT switching electronics are very efficient because these types of electronics devices typically achieve 100 amps capacity using milli-ohm levels of saturated ON resistance. However, in power conversion for example (from 12VDC to 120VAC) there is more going on and the efficiencies are closer to 85-90% (at best) than 95 % . That means losses are 2-3 times worse than you claim.

Getting back to my question about double conversion loss. You stated that AC AirCons used variable frequency control. You have clarified that this is 240VAC whole residence rooftop AC rather than RV roof-top at 120VAC.

I would suggest that by the time you convert 120VAC to DC in order to reconstruct a 120VAC power at the variable frequency, you would be doing something akin to double conversion in a UPS. Both of these conversions are going to 85-90% and the net losses are then 0.85^2-0.90%^2 or 64% to 81% efficient respectively.

For this reason, it would be much more efficient for a scroll-type compressor to run off of DC and have a much more efficient single conversion rather than double conversion using 120VAC.

To be clear I'm treating the Brushless DC driver circuitry as a conversion similar to your description of brushed DC-generating AC. However, this type of conversion is going to be more efficient (approaching 95%) than say generating a pure sinewave from 12VDC (85-90%).

Originally Posted by Wowbagger

In a high torque application like a compressor, you pretty much HAVE to bump the voltage up from 12V, to get enough current pushed through enough windings to have enough magnetic field to move the load. At 12V, to have enough windings to do the job, you have too much resistance from the copper, and you cannot push enough current to get the job done. Again, modern switching power supplies are very efficient at getting that done - better than 95% in many cases. The trivial losses to making higher voltage DC are more than made up for in avoiding resistance losses.

Again you are getting somewhat out of context talking about motors well outside of the 1 hp range which would be consistent with the RV Rooftop AirCons. For example, the scroll-type compressors are rated for a typical range of at 400-800 watts while the 120VAC units are typically 1000 watts and above (recall there are 750 watts per Hp).

If you look at the array of Brushless DC motors being used in the Ebike market, the main reason for increasing voltage is to increase the motor speed. This is due to the need to overcome motor back EMF so you can get current to flow in the motor when the back EMF is maxed out.

I'm just addressing your comments but left to wonder what the relevance is of any of the OP's original discussion?

[Wowbagger]

Originally Posted by posplayr

Having taken an undergraduate class in rotating machinery in what is now about 45 years ago, I know what you are talking about but there is simply no 120VAC generated in a brushed DC motor. Current reversal yes but no sinusoidal voltage/current characteristic of 120VAC.

At no point did I say that a standard brush motor generated 120 volts. I said it generated AC - as in, alternating current, which can exist at any voltage.

Originally Posted by posplayr

Getting back to my question about double conversion loss. You stated that AC AirCons used variable frequency control. You have clarified that this is 240VAC whole residence rooftop AC rather than RV roof-top at 120VAC.

Which I said in response to the statement that 12V systems had the edge over home units because of the use of VFD in 12V systems vs. normal synchronous motors in 240VAC systems (which, off topic, such motors are NOT synchronous, as they are not being supplied with 3 phase current. They are usually slotted rotor induction motors, not synchronous). Since 240VAC systems now also use VFD that is no longer an advantage that only 12V systems have, and the statement I was replying to is incorrect.

Originally Posted by posplayr

I would suggest that by the time you convert 120VAC to DC in order to reconstruct a 120VAC power at the variable frequency, you would be doing something akin to double conversion in a UPS. Both of these conversions are going to 85-90% and the net losses are then 0.85^2-0.90%^2 or 64% to 81% efficient respectively.

Not true. Making DC from AC is extremely simple and efficient, and no where near the 85% you quote. Even if you did the stupid thing, and used a full wave bridge rectifier, the efficiency would be higher than 90%. And if you did the smart thing, using switched MOSFETS or IGBJTs, you get well over 95% conversion to DC.
You are probably thinking of REGULATED DC, where the voltage of the DC is tightly controlled, and due to the need to control the voltage tightly you get a bit of a penalty on conversion.

Originally Posted by posplayr

For this reason, it would be much more efficient for a scroll-type compressor to run off of DC and have a much more efficient single conversion rather than double conversion using 120VAC.

And as stated above, you have to make a rotating magnetic field somehow - there WILL be a conversion to alternating current somewhere in the system. Alternating current does NOT require nor imply true sinusoidal waveforms.


But the simple fact is "you have what you have" - if you have 12VDC you use that, if you have 240VAC you use that, and if you have 300VDC from a solar panel array you can use that. But saying that "DC is more efficient than AC" is meaningless if you don't have a source of DC, and do have a source of AC.

[Steve Hunt]

Originally Posted by Steve Hunt

Just trying to uncover as much as possible...don't know what i don't know...I get tired of being disappointed by some functionality or lack of that is never told in spec sheets or general research but found out after you spend $thousands. I'm headed over to a distributor of it today, we'll see what I learn. Hopefully they have a functioning unit and a knowledgeable person. 22-28 db is highly unlikely, not to mention ambient db will be lucky to be 50ish

Well the trip to the local distributor was a bit disappointing (big surprise eh). They did not have a functioning display or a display at all and knew nothing of the type compressor being used. They order when needed. They have sold a dozen or so (some 12v and some 24v) with no negative feedback from the buyers. $3300 shipped for either voltage. He said made in Canada. It did appear that the guy I was talking to fielded all the inquiries and transactions of this product so I got all the info they had to offer. It seemed like the place was geared towards the semi-truck world.