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Old 07-26-13, 04:55 AM   #1
WyrTwister
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Default DC Inverter mini split

My mini split is the traditional technology , bottom of the line 13 seer el cheap o model . The condenser cycles , off and on , with cooling load / demand .
I read of the newer technology with DC Inverter drive for the compressor . I read of 380 VDC powering the motor ?
Now , we have wired quite a few 3 phase AC VFD's for various application . Largely HVAC .
I watched a youtube video by a guy " Down Under " that seemed to imply the mini split inverter drive compressors were 3 phase ?
I am aware that the VFD's I am familiar with , convert the AC to DC . The " slices & dices " the DC to produce " synthetic AC of the frequency , voltage and amperage to run the motor in question .
In theory , would not matter if the incoming AC is single phase or three phase .
So , am I getting close to the truth of the matter ? Are the inverter drive mini split compressors actually three phase AC ? Never heard of three phase DC , do not personally think it exists . Certainly not by the definition of three phase I learned . MANY years ago . Would think it would make Tesla spin in his grave .
Wonder if he would spin CW or CCW ? :-)
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Old 07-26-13, 09:00 AM   #2
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Originally Posted by WyrTwister View Post
...I watched a youtube video by a guy " Down Under " that seemed to imply the mini split inverter drive compressors were 3 phase ?
It would be helpful to us to have the link to that video your are talking about.

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...In theory , would not matter if the incoming AC is single phase or three phase .
Most of the 'off the shelf' VFDs want to see three phase coming in, but there are models that can work with 1 PH in and they can synthesize variable frequency 3 PH out. There are even models that can do both... but the max power out with 1 PH coming in is lower than max power out with 3 PH coming in.

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Are the inverter drive mini split compressors actually three phase AC ?
As far as I know, this is the case. One of our EcoRenovators (randen) bought a three phase compressor and hooked it up to a VFD and it ran successfully. There was a premature compressor failure, but as far as I know, it was not related to his use of a VFD. As I recall, the compressor he bought was not new, so there may have been a 'pre-existing condition' that caused the failure.

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Never heard of three phase DC , do not personally think it exists .
Personally, I think that "3 PH DC" is just marketing talk.

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Old 07-26-13, 03:40 PM   #3
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Default DC brushless motors?

Brushless DC electric motor - Wikipedia, the free encyclopedia

Heating and ventilations

There is a trend in the HVAC and refrigeration industries to use brushless motors instead of various types of AC motors. The most significant reason to switch to a brushless motor is the dramatic reduction in power required to operate them versus a typical AC motor.[8] While shaded-pole and permanent split capacitor motors once dominated as the fan motor of choice, many fans are now run using a brushless motor.[when?] Some fans use brushless motors also in order to increase overall system efficiency.[9]

In addition to the brushless motor's higher efficiency, certain HVAC systems (especially those featuring variable-speed and/or load modulation) use brushless motors because the built-in microprocessor allows for programmability, better control over airflow, and serial communication.
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Old 07-26-13, 07:13 PM   #4
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Brushless DC electric motor - Wikipedia, the free encyclopedia

Heating and ventilations

There is a trend in the HVAC and refrigeration industries to use brushless motors instead of various types of AC motors. The most significant reason to switch to a brushless motor is the dramatic reduction in power required to operate them versus a typical AC motor.[8] While shaded-pole and permanent split capacitor motors once dominated as the fan motor of choice, many fans are now run using a brushless motor.[when?] Some fans use brushless motors also in order to increase overall system efficiency.[9]

In addition to the brushless motor's higher efficiency, certain HVAC systems (especially those featuring variable-speed and/or load modulation) use brushless motors because the built-in microprocessor allows for programmability, better control over airflow, and serial communication.
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Old 07-26-13, 07:17 PM   #5
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AC_Hacker;30832

It would be helpful to us to have the link to that video your are talking about.



I looked on you tube . The guy is Aussi50 , but did not find exactly the video I remembered ? :-(



Most of the 'off the shelf' VFDs want to see three phase coming in, but there are models that can work with 1 PH in and they can synthesize variable frequency 3 PH out. There are even models that can do both... but the max power out with 1 PH coming in is lower than max power out with 3 PH coming in.


Yep , I know . They could be powered from the DC from wind or photo voltaic DC .



As far as I know, this is the case. One of our EcoRenovators (randen) bought a three phase compressor and hooked it up to a VFD and it ran successfully. There was a premature compressor failure, but as far as I know, it was not related to his use of a VFD. As I recall, the compressor he bought was not new, so there may have been a 'pre-existing condition' that caused the failure.



Personally, I think that "3 PH DC" is just marketing talk.


I kind of got that idea .


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Old 07-27-13, 10:10 PM   #6
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Here's some basic info on BLDC motors..

Brushless DC Motors & Control - How it Works (Part 1 of 2) - YouTube

Brushless DC Motors & Control - How it Works (Part 2 of 2) - YouTube


I think the name "Inverter" (converting DC to AC) is a bit confusing,
since the DC really isn't so much converted into AC, but into DC pluses.
Sorta like PWM pulses where Pulse-Width and the frequency of those pluses,
set the RPMs of the motor.

Grid--->230VAC--->DC--->(Transistor switches) making DC pulses to feed the Brushless DC motor..

There are mini-split designs that use only DC, (from PV etc) no AC is needed.
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Old 07-27-13, 10:57 PM   #7
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Inverter drives, DSP drives, brushless DC drives, FOC drives, etc. are basically the same thing. There's a lot to learn in that field. (I have studied that stuff extensively...) In summary, inverter drive is the generic term that covers everything. Brushless DC is generally reserved for when the motor is a synchronous motor. There are analog inverters and DSP inverters, with the analog type generally being either V/Hz or resolver based and the DSP type usually FOC based.

As an example, brushless fans are very common due to their reliability, efficiency, and ability to rotate faster than the 3000RPM or 3600RPM a line connected induction motor can do. (The last bit is very important for small fans since a 3600RPM 40mm fan is quite disappointing indeed...) Most brushless fans use cheap resolver based square wave inverters, but in high performance applications, more advanced inverter technology is common. Cindy Wu sensorless FOC drive, used in variable speed Delta fans, is based around a 32 bit DSP. In that case, the inverter is not just an AC voltage source, but it actually reads back the currents and back EMF coming off the motor, sends them through some Cindy Wu black magic going on inside the chip, and uses that to synthesize the PWM signals that run the actual inverter.

Here's two traces off the Delta fan in my PC:

Top trace is the phase to ground voltage, bottom trace is the neutral to ground voltage. You'll note that the phase to neutral voltage is close to a sine wave, and the current would be even closer to an ideal sine wave due to the inductive nature of the motor. (Could not measure due to the way such fans are built. In fact, I was lucky to be able to scope the neutral and one of the phases...) The fan is running at full speed, so you can clearly see "overmodulation" going on on the peaks of the phase waveform.

For those interested, here's someone else's teardown of some fans:
Pictures of A Dead Delta Fan and It's Internal Parts █ NO 56K-TONS OF PICS █ - Overclockers Forums
You'll note that the Deltas and Nidecs don't have resolvers, but the Sanyo does. That's because while Delta and Nidec both use DSP drives (at least on the variable speed fans), but Sanyo is more or less stuck in the analog age. That's problematic from a performance standpoint since at higher speeds, the drive waveform needs to be advanced (analogous to timing advance on an engine), which DSP drives inherently do but is considerably more difficult to do in analog. (Intel fans are made by Sanyo and there's an endless list of complaints of poor performance!) You could design the resolver to be physically advanced, but that causes an efficiency loss at low speeds and might complicate starting. It appears that some newer Sanyo fans have a little circuit in the ASIC that mixes in part of the next phase coming off the resolver at high speed to effectively advance it and thereby close the performance gap, but it just doesn't have the beauty of a proper DSP drive.

As for induction motor drives, the type that looks most promising for the A/C compressors we typically use in our projects is the Shannon Liu quadrature drive. Exploiting the fact that "single phase" induction motors (most of them, anyways) are actually 2 phase motors, it generates two sine waves 90 degrees out of phase in order to get control very similar to that achieved with a 3 phase motor. (On a side note, the smaller BLDC fans are pretty much always 2 phase due to the difficulties of making a 3 phase motor that small.) The windings are very different so don't expect it to run quite as smooth as a proper 3 phase motor. It is also next to impossible to find an off the shelf module that works that way, but DIYing one could be considered. Contrary to common belief, an induction motor running from a DSP inverter works with similar efficiency to a synchronous motor running from a DSP inverter. The fact that Tesla's using induction motors says a lot about it. For a fan load, where there is a fixed torque/speed curve, a well matched synchronous motor is more efficient, but for more variable loads like vehicle traction or A/C compressors, induction motors are actually more efficient at part load.
Induction Versus DC Brushless Motors | Blog | Tesla Motors
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Old 07-28-13, 01:07 AM   #8
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All this is very interesting and very enlightening .

I can see , if the need is to break the 3,600 rpm barrier , it would be useful .

If you have a DC source , to begin with , like PV or wind energy , it could be very useful .

But , I think I am missing something ? For the mini splits , at least ( if you do not scale it up for the bigger HVAC systems ) the input is 1 phase AC . Either 120 or 240 ( possibly 208 VAC ) . It is quite doable to build a 1 phase " traditional " VFD . And use a " traditional " 3 phase AC motor .

For a HVAC compressor , I do not see the need to break the 3,600 rpm barrier ?

So , what is the need to use the brushless DC , instead of the 3 phase AC motor ?

Less circuitry / cost in the electronics ? Greater energy efficiency in the whole system ?

As a side note , I am guessing both the electronics for the BLDC motor and a " traditional " VFD would require filtering / shielding to prevent unwanted " electrical noise " from being impressed back on the AC power source ?


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Old 07-28-13, 08:59 AM   #9
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Being able to run faster than 3600RPM allows the use of low pressure refrigerants that were previously impractical. My friend Brittany Benzaia has developed a centrifugal compressor with a switched reluctance motor that operates at over 100kRPMs, allowing the use of water as a refrigerant in air conditioning.

Synchronous motors are more common in inverter drives since they're easier to understand. But newer control algorithms have closed the gap somewhat. Another interesting part about the Shannon Liu quadrature drive is how it actually does some filtering and FFTs on the motor current to detect the actual rotor speed without a resolver, exploiting the pulsating torque characteristics of reciprocating and rotary compressors. I don't think that would work very well on a scroll...

Filtering is needed, but much less than you think. Take apart an old PC power supply and you'll see that the filter is quite small.
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Old 07-28-13, 09:57 AM   #10
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Being able to run faster than 3600RPM allows the use of low pressure refrigerants that were previously impractical. My friend Brittany Benzaia has developed a centrifugal compressor with a switched reluctance motor that operates at over 100kRPMs, allowing the use of water as a refrigerant in air conditioning.

Synchronous motors are more common in inverter drives since they're easier to understand. But newer control algorithms have closed the gap somewhat. Another interesting part about the Shannon Liu quadrature drive is how it actually does some filtering and FFTs on the motor current to detect the actual rotor speed without a resolver, exploiting the pulsating torque characteristics of reciprocating and rotary compressors. I don't think that would work very well on a scroll...

Filtering is needed, but much less than you think. Take apart an old PC power supply and you'll see that the filter is quite small.



Water as a refrigerant ? Bet that would chap the refrigerant / chemical companies ? They would need to figure out a way to get the EPA to ban it . Or , at least , heavily regulate it .

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