Inverter (air conditioning)
Encyclopedia
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. A 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 pulse-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 microcontroller
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.
Traditional air conditioners use a compressor that is either working at maximum capacity or switched off periodically, to regulate the temperature of the room. A thermostat
Thermostat
A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...
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
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
to convert the incoming AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
current to DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
and then uses pulse-width modulation
Pulse-width modulation
Pulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
of the DC current within an inverter
Inverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
to produce AC current of a desired frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. The AC current is used to drive a brushless motor or an induction motor
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
. As the speed of an induction motor
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
is proportional to the frequency of the AC current, the compressors runs at different speeds. A microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
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
Payback period
Payback period in capital budgeting refers to the period of time required for the return on an investment to "repay" the sum of the original investment. For example, a $1000 investment which returned $500 per year would have a two year payback period. The time value of money is not taken into account...
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.