Inverter compressor

An inverter compressor is a gas compressor that is operated with an inverter.
In the hermetic type, it can either be a scroll or reciprocating compressor. This type of compressor uses a drive to control the compressor motor speed to modulate cooling capacity. Capacity modulation is a way to match cooling capacity to cooling demand to application requirements.
Market needs 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. 
Different modulating technologies.
There are several ways to modulate the cooling capacity in refrigeration or air conditioning and heating systems. The most common in air conditioning are: on-off cycling, hot gas bypass, use or not of liquid injection, manifold configurations of multiple compressors, mechanical modulation (also called digital) and inverter technology. Each have advantages and drawbacks. 
Working principle.
The variable-frequency drive controls the speed of compressor motor. The compressor is specifically designed to run at different motor speeds to modulate cooling output. Variable speed operation requires an appropriate compressor for full speed operation and a special compressor lubrication system. Proper oil management is a critical requirement to ensure compressor lifetime. Proper oil management provides proper lubrication for scroll set at low speed and prevents excess oil from being injected into the circuit when operating at full speed.
Applications.
Variable speed technology can be implemented in HVACR, close control and process cooling applications and as diverse as packaged or split air-conditioning units, rooftops, chillers, precision cooling, VRF and condensing units.
Challenges in adopting inverter scroll in HVAC systems.
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 and bearings are optimally lubricated at all compressor speeds.