In modern world, Using the latest advancements in Inverter technology, air conditioners are quieter to run and more economical to operate than conventional air conditioning system. So, Let’s learn about the DC Inverter based air conditioning system and the operating principles of these new air conditioning system.
There are basically four parts of cooling or refrigeration system and all these air conditioners essentially go through the systematic cycle of compression, condensation, expansion, and evaporation in a simply closed circuit. The compression phase contains a compressor device which makes use of an electric motor to compress the refrigerant
coolant. Today hermetically sealed reciprocating compressor is widely used for the refrigeration and air conditioning applications. This type of compressor can use a drive to control the compressor motor speed to modulate cooling capacity.
Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, micro drive, and popularly here in this application called as an Inverter. Since Inverter is a device which converts Direct Current (DC) input into Alternating Current (AC) output, it is referred to as DC Inverter (to remove any ambiguity because a DC-DC converter is commonly confused with DC-AC Inverter). A DC Inverter Air Conditioner has a compressor which is VFD or Inverter controlled (as described above) and the whole package is commonly called as Inverter Air Conditioner or DC Inverter Air Conditioner.
So, the basic operating principle of an Inverter is quite straightforward as it is comparable to the technology used in automobiles, where you need to press the throttle harder to go faster. Compressor speed (as an analogy say the engine running and its pistons creating the torque) affects the performance output (speed of the car). The Frequency (throttle controlled by VFD) is directly related to the motor’s RPM’s and hence cooling or heating performance (car’s speed). In other words, the faster the frequency, the faster the RPM go. If an application does not require an electric motor to run at full speed, the VFD can be used to ramp down the frequency and voltage to meet the requirements of the electric motor’s load. As the application’s motor speed requirements change, the VFD can simply turn up or down the motor speed to meet the speed requirement.
In a more convenient and technical way, an Air Conditioner described as using “Inverter technology” or “Inverter equipped” has the ability to continuously control its thermal transfer rate by modifying the compressor’s speed in response to the demand for cooling . Variable Refrigerant Flow (VRF) phenomena is typically adopted with an Inverter equipped Air Conditioner which adds a VFD and Control Circuitry alongside the compressor in order to support variable motor speed and thus variable refrigerant flow helps regulate the conditioned-space temperature.
By contrast, traditional air conditioners regulate the temperature by utilizing a compressor that is periodically either working at maximum capacity or switched off entirely. Fixed speed compressors start or stop automatically in order to sustain the desired temperature, but an inverter unit regulates the capacity of the compressor instead. Frequent on/off switching of the compressor means high initial torques for the compressor to start running and hence more surge of currents during transient times, hence increase in electricity bills.
So much so, the thermodynamic cycle is explained with this image:
Note that the four-part heat exchange cycle is divided at the center into a high side and a low side. The same refrigerant is used to move the heat from one area, to cool this area, and to expel this heat in another area. Gases give off heat when changed from gas to liquid.
- The refrigerant comes into the compressor as a low-pressure gas, it is compressed and then moves out of the compressor as a high-pressure gas.
- The gas then flows to the condenser. Here the gas condenses to a liquid, and gives off its heat to the outside air.
- The liquid then moves to the expansion valve under high pressure. This valve restricts the flow of the fluid, and lowers its pressure as it leaves the expansion valve.
- The low-pressure liquid then moves to the evaporator, where heat from the inside air is absorbed and changes it from a liquid to a gas.
- As a hot low-pressure gas, the refrigerant moves to the compressor where the entire cycle is repeated.
A micro controller can sample each ambient air temperature and adjust accordingly the speed of the compressor. The alternating current is used to drive an induction motor since the speed of an induction motor is proportional to the frequency of the AC, the compressor can now run at different speeds with the use of VFD.
The inverter air conditioning units have increased efficiency in contraction to traditional air conditioners, extended life of their parts and the sharp fluctuations in the load are eliminated. This makes the Inverter AC units quieter, with lower operating cost and with less broke downs. The Inverter AC units might be more expensive than the constant speed air conditioners, but this is balanced by lower energy bills. The payback time is approximately two years depending on the usage. The additional electronics and system hardware adds cost to the equipment installation but can result in substantial savings in operating costs. Eliminating stop-start cycles increases efficiency.
Generally, VFDs allows you to match the speed of the motor-driven equipment to the load requirement. Electric motor systems are responsible for more than 65% of the power consumption in industry today. Optimizing motor control systems by installing or upgrading to VFDs can reduce energy consumption in your facility by as much as 70%. Additionally, the utilization of VFDs improves product quality, and reduces production costs. Combining energy efficiency tax incentives, and utility rebates, returns on investment for VFD installations can be as little as 6 months.
By operating at varying speeds, VRF units work only at the needed rate allowing for substantial energy savings at partial-load conditions. Heat recovery VRF technology allows individual indoor units to heat or cool as required, while the compressor load benefits from the internal heat recovery. Energy savings of the fixed speed compressor in a standard air conditioner runs at 100% capacity when it is started, but an inverter unit starts at a low-level and then progressively enhances its capacity, depending on the requirement to heat up the room or cool it down. DC inverter air conditioners make use of a variable frequency drive to regulate the motor’s speed, thereby controlling the speed of compressor too. Frequency acts as the input variable to control the compressor speed. The current ambient air
temperature is sampled by using a micro controller and the compressor’s speed is adjusted appropriately.
Sources: Wikipedia.com, Bijlihazir.pk, ElectronicsHub.com etc