Compressors – What are the key differences?
- Fixed Speed Compressor – A fixed speed compressor has an on/off motor which drives the reciprocating or scroll compressor. It is either on or off giving 100% capacity or no capacity. The control system can turn the compressor on when there is a requirement for the operation causing the refrigerant to flow through the system. Although there is more advanced and efficient technology, fixed speed compressors remain popular due to the lower capital cost and simplicity of operation as a tried and tested solution.
- Inverter compressor – An Inverter compressor uses a variable frequency drive, also known as inverter drive, to slow down or speed up the motor that rotates the compressor. changing the speed of the compressor varies the refrigerant flow which can be a very effective way to control the temperature.
The invertor drive converts incoming AC current to DC and then through a modulation in an electrical inverter produces current of desired frequency. A microcontroller can sample each ambient air temperature and adjust accordingly the speed of the compressor.
Inverter air conditioning units have increased efficiency and extended compressor life due to the sharp fluctuations in the load being eliminated. This makes the inverter units quieter, with lower operating cost.
- Digital Scroll Compressors – A Digital Scroll compressor incorporates a pulse width modulated (PWM) signal that opens and closes a solenoid valve. The compressor is switched between the loaded and unloaded state with a cycle time of typically 20 seconds. Duration of the scroll loaded state within this cycle time determines the capacity.
Digital Scroll modulation is based on axial compliance that allows the fixed scroll to move vertically, by a very small amount, to ensure that the scrolls are always radially loaded with optimal force. This holds the two scrolls together, at all operating conditions, ensuring high efficiency. When a Digital Scroll is operating in normal “Loaded” mode the compressor behaves just like a standard scroll compressor providing 100% capacity. Controlling the mechanical axial load is achieved when the solenoid valve is energized. The movement of discharge flow to suction lifts the fixed scroll, separating from the orbiting scroll. The compressor motor continues to run at normal speed but with no compression taking place. Full lubrication is always maintained as the motor still operates 100% rpm in the unloaded state.
The time period for a complete On/Off sequence is called the cycle time, and this can be between 10 and 30 seconds.