The Inverter, or Variable Frequency Drive (VFD), is an electronic power component that, by varying the frequency of the motor power supply, modulates the compressor rotational speed (RPM) and in such a way controls the compressor capacity.
In Air Conditioning applications chillers and heat pumps typically spend most of their operating time at part loads. That means the compressor of a chiller encounters many hours of off-design operation. It is important, then, that it can perform as efficiently and reliably as possible at these part load conditions.
Inverter technology can ensure the best performance regardless of the cooling demand. In fact, thanks to the Variable Frequency Drive (VFD) technology, chillers match the varying load requirements, in any circumstance. Inverter technology continuously modulates the speed of the compressor’s motor, which is the most efficient way to perform the capacity control of the compressor. In this way, Variable Frequency Drive provides chillers with outstanding part load or seasonal efficiencies, also allowing great energy saving opportunities.
Another benefit of using an Inverter is that VFD usually generates a lower starting current than typical starters. Because of that, the inrush current does not exceed the full load operating current. This feature can help to reduce electrical installation costs and meets the requirements on maximum possible inrush current when provided by local institutions.
Chiller and heat pumps with Inverter-driven compressors can be very efficient, which leads to consistent savings in the annual energy bills. This is an important aspect, fully aligned with the goals of the most recent European regulations on the performance of HVAC equipment as the EU Ecodesign Directive. Also, Inverter technology is becoming more and more important to reach the targets set by the EU Energy Performance Buildings Directive (EU EPBD).
The use of VFD in cooling or heating equipment is positively regarded when dealing with a Green Building project under the protocols BREEAM (Building Research Establishment Environmental Assessment Method) or LEED (Leadership in Energy and Environmental Design).
Daikin has a history of leadership in the field of Inverter technology, which is available for scroll, centrifugal and screw compressors. In fact, Daikin have been pairing up its Single Screw compressors with air-cooled Inverter technology since 2004. In 2013 Daikin introduced the refrigerant-cooled Inverter – which is also available for centrifugal compressors since 2019.
Daikin refrigerant-cooled Inverter is produced in-house in Italy, by the Power Electronic factory in Vicenza. It differentiates from other VFD types for being directly integrated in the compressor’s body. This feature allows the electronics of the Inverter to be cooled by the refrigerant in the chiller’s circuit.
Inverters are usually air-cooled, where the cooling vector is the surrounding air. But the capacity of the air to cool the VFD depends on temperature and density. Conditions such as altitude and ambient temperature must be considered when using an air-cooled Inverter. In fact, the power and the current that the Inverter can manage is subject to derating according temperature and altitude. For this reason, based on the ambient condition, some restrictions must be considered for air-cooled VFD.
Using glycol is another way to cool VFD. In this case the heat from the Inverter is removed by a glycol mixture moving into a loop between the electrical panel (where the VFD components are) and rejected into the ambient through a heat exchanger. However, the effectiveness of this system is influenced by the ambient conditions as well, as it is not possible to have the glycol coolant below the air temperature. Also, the glycol mixture is pushed through the cooling loop by a dedicated pump which is a sensitive component requiring frequent maintenance.
Daikin refrigerant cooled VFD differentiates from other Inverter types for being refrigerant-cooled, and for not being affected by environment conditions such as ambient temperature, altitude or air quality (presence of pollutant, dust or sand).
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