Variable speed

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About inverter technology for AC, heating and ventilation

A quantum leap in energy efficiency
The HVAC market is rapidly changing. Inverter technology is the most energy efficient technology and is currently revolutionizing the way air conditioning systems are being designed and built to meet climate control needs around the world. The first to embrace variable-speed technology gains a competitive advantage.

Market needs: Avoid system oversizing, reduce electricity bill and inrush current, increased comfort and process are some key drivers for variable capacity.

Modulating technologies: There are several ways to modulate the cooling capacity in refrigeration or air conditioning and heating systems.

Bypass method

Working principle: Inverter compressors operate at different speeds generating a modulated refrigerant mass oil flow and cooling output.

Benefits: Better power supply quality, system diagnostics, humidity control, energy savings, accurate temperature control, less noise, process safety, greater comfort

Challenges: Compressor and drive selection, designer skills makes the system overall reliability and efficiency.

Variable speed video screen grab of rooftop units on a commercial building

Danfoss variable speed technology

By using the Danfoss inverter compressor solutions, significant energy savings can be achieved compared to traditional on-off compressors or mechanically-modulated technologies.

Better power supply quality, system diagnostics, humidity control, energy savings, accurate temperature control, less noise, process safety, greater comfort.

Inverter compressor technology offers new opportunities for air conditioning systems, first of all in terms of energy efficient buildings, reduced energy consumption and lower running costs. 

Additionally, the technology improves power factor leading to better power supply quality as well as better system diagnostics and open communication protocols which facilitate servicing. 

Continuous adaptation to cooling demand provides higher energy savings and accurate temperature control.

The compressor matches power input with cooling capacity. The unit copes effectively with changes in temperature and runs at partial load which can represent over 30% reduction in the energy bill every year compared to a fixed speed or mechanically modulated compressor. The stable temperature control secures the processes in an optimum way and provides greater comfort. All in all, this means energy efficiency in buildings.

The capacity modulation attenuates power demand peaks which also contribute to increasing the grid reliability and quality of power supplied.

The soft start control leads to a near zero inrush current and improves EER:

  • Direct-on-line (DOL) compressor start draws 5-6 times the nominal current until the compressor running speed is attained. Traditional fixed-speed compressor in a system can make 8-12 start/stop cycles and every start will draw high current from the supply, which leads to high power consumption and stress on supply, and on compressor mechanical parts.
  • Most inverter scroll compressors are soft starting which avoid torque surges. It prevents high mechanical stress on the machine, and generates lower service costs and decreased wear. Low in-rush current also helps in saving fixed costs charged by utilities (peak current calculation) and reduces mains and power backup loads.


Additional benefits are of inverter compressors are:

  • Better humidity control makes the unit suitable even for spa installations thanks to adjust the capacity of the unit and thus better control of the evaporating temperature.
  • Less noise than with conventional on-off systems during partial load operation.
    On top of the standard benefits for all inverter compressors and technology, Danfoss Commercial Compressors has prequalified inverter compressors and drives designed to work together.


This adds to the long list of benefits to manufacturers, consulting engineers and end-users in terms of:

  • Reduced number of components in the system
  • Higher reliability and continuous running
  • Easier to implement technology
  • Reduced applied costs
  • Faster time to market.


Figure 1

Energy consumption for cooling systems using different compressor configurations. Average consumption index based on simulations for 10-30TR compressors used in applications with low pressure ratio (rooftop); Index 100 = Danfoss inverter scroll VZH energy consumption.

Figure 2

Typical load profile in a building.
Only a few percentages of operations are at full load in a building. HVAC systems are designed for peak conditions (right side of the graph). These conditions are not those conditions in which the equipment will operate most of the time. This graph represents typical annual climate data and hours of run time (% of year) for each condition. 

Source: Danfoss HVAC tool

Benefits of Danfoss varible speed technology

Compressor and drive selection, designer skills makes the system overall reliability and efficiency.

Importance of the inverter drive: The compressor and drive need to be qualified to work together and for dedicated applications. The drive modulates the compressor speed and prevents it from operating out of the compressor operating limits. The inverter frequency drives need to use algorithms developed specifically for heating, ventilation and air conditioning (HVAC) or for refrigeration. They ensure that the system will run within the application constraints. The drive can also manage other devices such as oil injection valves or multiple compressors. As the compressor rotational speed changes, the amount of refrigerant — and oil — flowing through the compressor increases or decreases. The drive ensures that the compressor is optimally lubricated at all compressor speeds.

OEM manufacturer skills in inverter system integration: not all OEMs are yet prepared to adopt inverter technology due to following reasons:

  1. Competence level and experience engineering OEM in implementing inverter technology. The strong experience in mechanical must be complemented with electrical engineering skills as well as controller development and programming.
  2. Inverter compressors continuously keep changing speed to match the load, which makes the oil management of the system more complex compared to traditional technology. Oil management mastering is a key issue – when not using an oil-free compressor type - where compressor manufacturer experience makes the difference
  3. The application support for OEMs by compressor and inverter drive manufacturer is critical in system integration.


Misconceptions about variable speed technology

Inverter technology reacts too slowly to load changes: some inverter scrolls ramp up/down speed range is between <0.1s; 3600s>Full load inverter losses 5-10% at full speed the inverter car lose 3% of efficiency. As full load is not the dominant profile, the loss will be minimal in the overall unit consumption.

Oil circulation increases at high frequency operation: oil circulation rate electronically controlled by some inverter scrolls can be less than 3% at full speed. Oil management is more complex with inverter compressor systems. Some inverter scrolls control oil circulation at both low and high speed requiring minimal design accommodations.

Inverters can't be used where EMC (Electro Magnetic Compatibility) is a concern: some inverter solutions offer 2 levels of EMC filtration which meet the most stringent European standards related to electromagnetic interference.

Avoid systems oversizing, reduce electricity bill and inrush current, increased comfort and process are some key drivers for variable capacity.

Many refrigeration and air conditioning systems require reliable processes which are more efficient, compact, environmental friendly, easy to install and to maintain. The cooling requirements vary over a wide range during the day and over the year due to ambient conditions, occupancy and use, lighting etc.

  • In comfort cooling, there may also be the need for a stable and accurate temperature and humidity control in areas such as hospitals, IT & telecoms, process cooling.
  • In applications such as schools, restaurants and office buildings, it is important that the cooling system is able to adapt to wide daily shifts in load.
  • In process applications such as fermentation, growing tunnels and industrial processes, accurate temperature settings are required to secure production quality.


Three different market trends are converging to create new opportunities for efficient, sustainable solutions: 

  • Energy efficiency
  • Intelligent systems
  • Environmental impact


Energy efficiency is no longer an option. Energy regulations are strengthening. Building energy codes and standards are developed globally and energy incentives are getting more attention. Another global issue is the energy security. How to make sure that we will not lack of energy one day?

HVAC and refrigeration systems are usually designed for peak demand which represent only a small percentage of the actual operation. Such oversizing leads to efficiency losses and extra costs for oversized equipment. Energy efficient capacity modulation is a way to match cooling capacity to cooling demand thereby matching these application requirements.

There are several ways to modulate the cooling capacity in refrigeration or air conditioning and heating systems. The most famous common in air conditioning are: on-off cycling, hot gas bypass, manifold configurations of multiple compressors, mechanical modulation (also called digital) and inverter technology. Each has advantages and drawbacks.

  • On-off cycling: results in switching off the fixed-speed compressor under light load conditions and could lead to short cycling and the reduction in compressor lifetime. Efficiency of the unit is reduced by pressure cycling and transient losses. The turndown capacity is 100% or 0%.
  • Hot gas bypass: involves bypassing a quantity of gas from discharge to the suction side. The compressor will keep operating at the same speed but thanks to the bypass, the refrigerant mass flow circulating with the system is reduced and thus the cooling capacity. This naturally causes the compressor to run uselessly during the periods where the bypass is operating. The turndown capacity varies between 0 and 100%.
  • Manifold configurations: several compressors can be installed in the system to provide the peak cooling capacity. Each compressor can run or not in order to stage the cooling capacity of the unit. The turndown capacity is either 0/33/66 or 100% for an even trio configuration and either 0/50 or 100% for a tandem.
  • Mechanically modulated compressor: this internal mechanical capacity modulation is based on periodic compression process with a control valve, the 2 scroll set move apart stopping the compression for a given time period. This method varies refrigerant flow by changing the average time of compression, but not the actual speed of the motor. Despite an excellent turndown ratio – from 10 to 100% of the cooling capacity, mechanically modulated scrolls have high energy consumption as the motor continuously runs.
  • Inverter compressor: uses a variable frequency drive – also known as inverter drive– to slow down or speed up the motor that rotates the compressor. This method varies refrigerant flow by actually changing the speed of the compressor. The turndown ratio depends on the system configuration and manufacturer. It modulates from 10% (depending on the compressor model) up to 100% at full capacity with a single inverter.


Air-conditioning is responsible for approximately 20% of the total annual electricity consumption in a country like the USA. The introduction of inverter drive technology in air-conditioning and heat pump systems presents an opportunity for significant energy savings due to efficient part-load operation, and also helps improve power quality of the grid. 

Power factor is an important measure of how hard the power utility must work to deliver electricity. 

Increasing the power factor of installations can directly impact the quality of power supply. For electric utilities, this equates to fewer losses and improved reliability of service. For building- and homeowners, this means more efficient equipment with the possibility of local utility, state and/or federal incentive programs. 

Power factor is the combination of the displacement power factor (displacement of active and reactive power) and the distortion power factor (distortion of the electrical power due to harmonics).

Power factor close to 1 means:

  • No interference with other installed equipment,
  • No interference with the mains, 
  • Reduced losses and higher efficiency. 


The use of inverter drives allows a significant enhancement in power factor, since the displacement power factor is close to 1. The distortion power factor however negatively affects transformers, cabling, fuses and circuit breakers due to harmonics. 

The Danfoss inverter technology has a low distortion and high power factor (0.98) due to correction from a DC choke whilst other inverter frequency drives or mechanically modulated systems can have a power factor as low as 0.60. 

Other benefits of the prequalified drive correction in Danfoss inverter solutions: 

  • Boosts the immediate circuit voltage whenever necessary which improves operating capabilities at low supply voltages. 
  • Increases voltage within the AC inductor, reducing current and stress throughout the entire chain of power components while minimizing costs. 
  • The higher DC-link voltage reduces motor current and losses. 
  • The Active Power Factor Correction (PFC) can be switched on and off as desired for best system efficiency.

An inverter compressor is designed to operate with a drive which steplessly adjusts the speed of the compressor motor to deliver the cooling need.

It can either be a scroll, rotary or reciprocating, semi-hermetic, open compressors, including screw, centrifugal and axial compressors. This type of compressor uses a special drive to control the compressor motor speed (measured in round per second RPS). Inverter compressors can operate at different speeds: the inverter compressor is specifically designed to run at different motor speeds thereby generating a modulated refrigerant mass oil flow and cooling output.

The variable frequency drive indicates the cooling requirement to the compressor motor. The motor continuously adapts speed to produce more or less cooling capacity to match the exact cooling need. The combination of the two devices – compressor and drive – gives a continuous cooling modulation. The variable speed principle requires a very sturdy compressor for full speed running and a special compressor lubrication system for systems using oil. Excellent oil management is a critical requirement to secure compressor lifetime. The oil management system guarantees good lubrication of the scroll set at low speed and prevents excess oil from being injected into the circuit when operating at full speed to maintain a perfect Oil Circulation Ratio.


  • Danfoss inverter scroll compressor type VZH
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    Danfoss inverter scroll compressors for commercial air conditioning use variable-speed technology to deliver the highest possible cooling capacity and energy efficiency.

  • The Danfoss VTZ reciprocating inverter VTZ compressor
    Reciprocating inverter - VTZ

    Danfoss reciprocating inverter compressors avoid oversized and short cycling systems. The compressor automatically adapts to the current load. By combining an efficient Danfoss Maneurop® reciprocating compressor with a Danfoss Drive, we provide an innovative and intelligent package that utilizes variable-speed technology to ensure superior efficiency across the entire operating range for precision cooling. 

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    Danfoss is the leading manufacturer of oil-free compressors and is the pioneer of the Danfoss Turbocor® compressor - the world’s first oil-free magnetic bearing compressor for the HVAC industry.

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    Danfoss Turbocor® model TG oil free compressors are available in four different models ranging from 40 tons to 150 tons / 140 kW to 540kW utilizing ultra-low GWP HFO1234ze.

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Case studies

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    The new Alsik hotel in Southern Denmark sets a higher standard in energy-efficiency and is built to reach 76% CO₂-neutrality. A main contributor is the 900-kW chillers by Clint (a company of G.I. Industrial Holding) which features award-winning Danfoss Turbocor oil-free compressors.

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    Danfoss conducted a study to see how long term operation affected performance on Danfoss Turbocor® compressors. The results showed that oil-free, magnetic bearing compressors maintained consistent performance even after 10+ years of operation.

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  • VLT® Micro Drives help Vermont small businesses boost efficiency for rooftop HVAC

    VFDs are used for a wide range of motors, not just RTU fan motors. Danfoss was able to pre-program the VLT® Micro Drive specifically for RTU applications and offer it to contractors. Because the programming work is done upfront, the contractor simply mounts the drive inside the RTU, wires it up, and enters the nameplate installation. Most mechanical contractors can do the job in about half a day.

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  • VLT® HVAC Drive joins exclusive company of 1000 Efficient Solutions globally
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