Inverter

An inverter is an electrical device that converts direct current (DC) to alternating current (AC) the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.

Solid-state inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.

The inverter tag found on some air conditioners signifies the ability of the unit to continuously regulate its thermal transfer flow by altering the speed of the compressor in response to cooling demand.

Traditional air conditioners use a compressor that is either working at maximum capacity or switched off periodically, to regulate the temperature of the room.  thermostat is used to measure the ambient air temperature and switch the compressor on when the ambient air temperature is too far from the desired temperature.

An alternate way to meet the varying cooling demand is to vary (“modulate”) the capacity of the compressor. An air conditioner compressor has two components, a mechanical part – the actual compressor and the electrical part – the motor to drive the compressor. Either of the components can be used to modulate the capacity. In digital scroll compressors  – the mechanical Scrolls of the compression mechanism is controlled to modulate and the electric motor runs at constant speed. The other method is to control the speed of the compressors by various means.

“Inverter” air conditioners use a variable-frequency drive to control the speed of the motor and thus the compressor. The variable-frequency drive uses a rectifier to convert the incoming AC current to DC and then uses plus-width modulation of the DC current within an inverter to produce AC current of a desired frequency. The AC current is used to drive a brushless motor or an induction motor.  As the speed of an induction motor is proportional to the frequency of the AC current, the compressors runs at different speeds. A micro-controller  can then sample the current ambient air temperature and adjust the speed of the compressor appropriately. All this electronics of course does add to the complexity and cost. Conversion energy losses from AC to DC and then back to AC can be as high as 4 – 6% for each conversion step.

Eliminating stop-start cycles increases efficiency, extends the life of components, and helps eliminate sharp fluctuations in the load the air conditioner places on the power supply. Ultimately this makes inverter air conditioners less prone to breakdowns, cheaper to run, and the outdoor compressor is generally quieter than a standard air conditioning unit’s compressor.

While at the beginning of the 1990s inverter air conditioners had some drawbacks, these have been mostly overcome – the conversion losses are lower and filters suppress most of the electromagnetic interference generated in inverters. Running at full load, a compressor delivers its best efficiency and outperform inverters. Inverter-based air conditioners have their strengths in environments where a partial load is common, as they are significantly more efficient than conventional air conditioners in those situations. For conventional households where each indoor unit is connected to a single dedicated outdoor unit, inverters are the preferred option, as partial loading is the common mode there. The higher initial expense is balanced by lower energy bills. In a typical setting the pay-back-time is about two years (depending upon the usage). For more modern installations where an outdoor unit is connected to multiple indoor units there are better options also available.

The inverter principle has been applied to a wide range of household appliances, including microwave ovens, washing machines and so on.


 

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