Why is there an inverter on an air conditioner?

[kwired]

Thank you very much for your clear explanation. I Had no idea the torque issue was the central factor, and that AC-> DC->AC was a lot morre efficient was to gain speed control via frequency manipulation for high load applications, like an A/C compressor. Does all this back and forth result in much wastage?

The drive puts off some heat. If you were running the motor at supply side frequency then likely you are less efficient than if running the motor directly off the supply voltage because of drive losses.

 

[winnie]

Exactly. The drive itself is not perfectly efficient. A 5 Hp drive might be 96% efficient or more; smaller drives are less efficient. Drive efficiency includes the input rectifier, the DC link, the output inverter stage, and the control electronics consumption. Because of the imperfect waveform the motor is also slightly less efficient.

With many loads the gain in over all performance and efficiency that comes from speed control vastly exceeds the additional losses in the motor and motor speed control.

Back to the minisplit example: a traditional 'across the line' air conditioner or heat pump requires a compressor sized for the maximum expected load, operating at full speed all the time. The actual load on the compressor will change depending on conditions; but its speed only changes slightly with different mechanical load. Most of the time there is excessive refrigerant flow relative to the actual needs of the system; the compressor has to do the work of moving the refrigerant at high speed, and on top of this end up having to turn the system on and off to modulate the cooling/heating.

With variable speed, during periods of low demand the compressor operates at low speed, moving less refrigerant against lower back pressures, and doing much less work. The variable speed also provides for much better control of cooling and better dehumidification; and the improved dehumidification means that I'm comfortable with a higher room temperature; again improving effective efficiency.

-Jonathan

 

[ggunn]

We keep our thermostat at 78 degrees F and I feel chilled in most air conditioned public spaces.

 

[tortuga]

The main benefit is the ability to change the frequency of the AC output, which you cannot do with a transformer.

Well technically a ferroresonant transformer can change AC frequency, they are not used in motor controls that I know of though.
I have seen them used to change 60hz to 25hz.

 

[don_resqcapt19]

Well technically a ferroresonant transformer can change AC frequency, they are not used in motor controls that I know of though.
I have seen them used to change 60hz to 25hz.

I thought those only maintained a constant secondary voltage with fairly wide changes in the primary voltage.
How do they change frequency?

 

[tortuga]

I thought those only maintained a constant secondary voltage with fairly wide changes in the primary voltage.
How do they change frequency?

I just remember them from old phone systems
they had units like this to generate the 'ring' voltage:

https://www.seriss.com/people/erco/tmp/tellabs8101-and-circuit-description.jpg

 

[winnie]

I've not done a detailed analysis, but I _think_ that ringing intentionally uses saturation effects to act as a frequency divider. There are two primary coils, and they alternate which one is saturated and just passing current without generating additional flux in the core. The secondary coils are alternate polarity coupled with a tuned capacitor. Net result you get exactly 1/2 the input frequency on the output.

In general transformers (or other linear circuits) cannot change frequency, but with clever tricks which depend upon non-linear effects you can get harmonic multiples or submultiples of the input frequency.

But yes, the most common use for ferro resonant transformers was voltage regulation.

Another clever circuit was the 'magnetic amplifier', where a small DC current would adjust the saturation of a transformer to control a large AC current.

 

[LarryFine]

Another clever circuit was the 'magnetic amplifier', where a small DC current would adjust the saturation of a transformer to control a large AC current.

And vice versa, with an audio magnetic amp, where the AC input modulates the DC through a transformer.

 

[garbo]

Exactly. The drive itself is not perfectly efficient. A 5 Hp drive might be 96% efficient or more; smaller drives are less efficient. Drive efficiency includes the input rectifier, the DC link, the output inverter stage, and the control electronics consumption. Because of the imperfect waveform the motor is also slightly less efficient.

With many loads the gain in over all performance and efficiency that comes from speed control vastly exceeds the additional losses in the motor and motor speed control.

Back to the minisplit example: a traditional 'across the line' air conditioner or heat pump requires a compressor sized for the maximum expected load, operating at full speed all the time. The actual load on the compressor will change depending on conditions; but its speed only changes slightly with different mechanical load. Most of the time there is excessive refrigerant flow relative to the actual needs of the system; the compressor has to do the work of moving the refrigerant at high speed, and on top of this end up having to turn the system on and off to modulate the cooling/heating.

With variable speed, during periods of low demand the compressor operates at low speed, moving less refrigerant against lower back pressures, and doing much less work. The variable speed also provides for much better control of cooling and better dehumidification; and the improved dehumidification means that I'm comfortable with a higher room temperature; again improving effective efficiency.

-Jonathan

Just wish all drive manufacturers would use the same wire table for current and higher temperatures inside of drives especially models with build in bypass that we must abide by. While PM'ing such drives the wires on the the contactors are always 10 to 20 degrees F hotter then room air. Often wonder if they spent a few more dollars on larger guage wire that it would pay for itself in wasted heat. Believe modern VFD's consume approximately 3% of full load power.

 

[caribconsult]

To summarize, regarding the need for inverter air conditioners, the use of a rectifier to make DC out of the inbound, then inverting that DC to generate AC at different Hz levels, varying the requency, will control the compressor moter speed the more efficient way. Reminds me of the older hysterisis motors on turntables that would not run at correct RPM if the input juice varied, but still used AC but low torq for a turntable. Compressors need heftier power. Now it's more efficient and offers better control to us the full AC->DC->AC variable hz method. Who discovered this?

 

[garbo]

To summarize, regarding the need for inverter air conditioners, the use of a rectifier to make DC out of the inbound, then inverting that DC to generate AC at different Hz levels, varying the requency, will control the compressor moter speed the more efficient way. Reminds me of the older hysterisis motors on turntables that would not run at correct RPM if the input juice varied, but still used AC but low torq for a turntable. Compressors need heftier power. Now it's more efficient and offers better control to us the full AC->DC->AC variable hz method. Who discovered this?

The great versitale IGBT'S require a DC input so that's why for at least the last 25 years or more the changed over to using IGBT'S. They can have a switching frequency of at least 5,000 Hertz. Place that I retired from had over 500 VFD'S some over 15 years old and over 120,000 hours of running time and never had one of the 480 volt three phase ( input ) rectifiers go bad and luckily only a few IGBT'S go bad a year. We blow out these drives several times a year and larger ones cleaned fan filters when needed.

 

[ggunn]

The bottom line answer to the question is that converting AC power to DC and then back to variable frequency AC with an inverter enables the use of AC synchronous refrigerant pump motors where the speed is determined by AC frequency.

 

[Jraef]

Who discovered this?

Nikola Tesla “discovered” that efficiencies changed with applied frequency, although for him, there was no simple way to change the frequency without changing the generator. His first AC motors on pumps powered off of Niagara Falls were 25Hz because he experimented and found that to be the ideal frequency for motor efficiency. The pumps that were keeping New Orleans dry were still 25Hz up until hurricane Katrina (which was part of the problem getting them back on line after the generators were flooded). We later settled on 50 or 60Hz for lighting to avoid detectable flicker, so once that became the distribution standard, motor design standards were made to match.

Variable speed of AC motors was “invented” in 1921 at Westinghouse by Benjamin Lamme, the engineer who implemented Tesla’s generators at Niagara, using a motor-generator set. That technology was used extensively until solid state electronics came along. This later became what was known as a “Ward-Leonard Drive” system, very common for hoists and ship cranes. After WWII, there were so many Ward-Leonard drives taken off of Navy and Merchant Marine ships and given to industry for virtually no cost, that they remained a dominant type of variable speed control for decades.

The first solid state variable frequency drive was made in 1958 at GE using thyristors, a type of SCR, which were the dominant form of VFD for years because transistors were not yet reliable. These eventually morphed into what were called “Current Source Inverters” and were very bulletproof, but the thyristor firing was dependent on the motor circuit, so had to be “tuned” to each motor in the initial setup, then re-tuned if the motor changed. These were still being used right up into the 1990s when PWM drives made it unnecessary to tune the drive to the motor.

The first PWM transistor drives were invented in 1960 by Martti Harmoinen at Stromberg in Finland, but it was for large motors used in traction drives on streetcar systems. Stromberg later was merged with Asea and a Swiss company called Brown Boveri to become ABB.

The first commercially available small packaged PWM VFD as we know them today was developed and released by Yaskawa in 1974, but we didn’t start seeing widespread adoption of that technology until the late 1980s - early 1990s in the US. The big explosion in PWM VFD use happened when transistor technology changed radically with MOSFET type transistors that were smaller-cheaper-faster, slowing them to be miniaturized because they gave off less heat and thus able to become hyper efficient (by comparison). That really took off in the late 90s, early 2000s.

So the concept itself is as old as the AC motor, but the ease of implementation was only facilitated recently (comparatively) with the advancement of technology.

 

[garbo]

Years ago while working at a candy plant they had 5 HP Reeves drive ( for speed adjustment ) that once speed was set they never changed it . The movable pulley would freeze up taking two or three guys eight hours to change it. Was buried dead center of a 40' long machine. We purchased 5HP motors bolted to a gear box and installed drives on them. They seldom changed speeds but they were still running trouble free 6 or 8 years later. Drives with the ability to have a slow ramp up speed prolongs chains, sprockets etc. During the winter in they had me set the ramp up speed to 8 to 10 minutes on old HVAC units during cold weather to prevent one of the three freeze stats from tripping out. These units ran 24/7 and only Shutdown for maintenance or filter changes. Belts lasted a lot longer after we replaced old starters with drives that had at least a 20 second ramp up time.

 

[winnie]

Thanks for that fantastic history @Jraef .

 

[ptonsparky]

Thanks for that fantastic history @Jraef .

Good Golly Molly!
I hope it took some time for research to refresh his memory.

The smarts along with experience on this forum is beyond belief.

 

[Jraef]

Good Golly Molly!
I hope it took some time for research to refresh his memory.

The smarts along with experience on this forum is beyond belief.

Wrote a paper on this for a class once. Much of it was already told to me by the guy who first trained me on VFDs, but being verbal, I had to go backward from what I could remember of what he told me and look it up. That was in the days of AltaVista (for those who remember), ie Google didn’t exist yet, but I found all of that even back then.

 

[LarryFine]

That was in the days of AltaVista (for those who remember), ie Google didn’t exist yet, but I found all of that even back then.

Ask Jeeves.