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In most cases, a ventilation systems is requested to operate in varying conditions. As a result, fans that operate at a constant speed are often over-performing which results in energy loses. Variable frequency drives, or VFD, are devices, often found in commercial facilities, that are used to control the rotational speed of fan’s motors. Besides fan speed control, VFD are also being used in air conditioning compressors and for the control of water pumps.

VFDs can be paired with numerous types of motors, for example single phase AC motors, IE2, IE3 and IE4 induction motors, AC permanent magnet motors, brushless DC motors and synchronous reluctance motors. They can offer very precise and efficient motor control with relatively easy configurations. Modern VFDs may communicate via a range of communication protocols, eg. Modbus RTU. Depending on the application VFDs may offer different protection ratings. Models with IP20 enclosures are designed for indoor mounting, preferably in a cool, dry and enclosed environment such as an electrical cabinet. IP66 enclosures are designed for harsh environments or direct machine mounting.

Controlling the rotational speed of a fan motor can offer significant benefits, most importantly the reduction of energy consumptions. In addition, adjusting the speed to the exact requirement of the conditioned space results in lower noise levels and reduced stress on system’s components. The use of a VFD can also offer softer starts and stops of moving components which can significantly reduce the wear of mechanical components and increases the durability of an HVAC system as a whole.

Operating principles

Variable frequency drives convert alternating current to direct current and then back to alternating current with variable frequency and voltage. The rotational speed of the fan motor changes by varying the frequency of the output voltage, typically through the use of pulse-width modulation (PWM). This results in an almost perfect sinusoidal motor voltage and consequently a more quite motor control.

The VFD is installed in the power supply of a motor and it generally consists of three parts: The rectifier, the DC bus and the Inverter. The three phases of a power supply connect to the rectifier which allows electricity to move in a single direction and blocks the flow in opposite direction. This is achieved through the use of a number of diodes which are connected in parallel. As a result, electricity exits the rectifier as a rough direct current output, which then enters the DC bus. In general, the aim of the DC bus is to smooth out the output of the rectifier. This is achieved with the use of capacitors and inductors that act like a filter that produces a constant direct current output, which then enters the final stage of a VFD, the Inverter. The aim of the inverter is to produce a form of alternating current out of the direct current input it receives from the DC bus. This Inverter uses Insulated-Gate Bipolar Transistors (IGBT) which act as a electronic switches that open and close in order to control the electricity flow. Through this flow path and flow duration control the inverter manages to produce alternate current as its output. The output voltage is controlled by how long the switches remain closed whereas the frequency is controlled by the timing of the switches.

Inspection and maintenance of VFD

It is very important to frequently inspect any VFD installed in your HVAC system, in order to ensure that there is no dust build up its air vents. Insufficient cooling can result in reduced performance whereas the presence of moisture absorbing dust particles inside the VFD can cause it to fail. One of the most common maintenance measures is to spray compressed air which is free of moisture through the VFD’s air vents. Furthermore, it is important to check whether there is condensation in the place where the VFD is installed as high moisture spaces can cause circuit board corrosion.

Inspection of electrical connections is also of great importance. Mechanical vibrations and fluctuations in the temperature of the VFD can cause some of the electrical connections to degrade. Faulty electrical connections may cause damage to power components, voltage faults, failure of fuses and protective components.

There are several benefits of regular and proper VFD maintenance. First of all, maintenance increases the longevity of the VFD by correcting faults before turning into major component failures thus leading to smaller downtimes. Additionally, preventive maintenance leads to better performance and more reliable operational capacity of the system, eliminating unexpected repair or replacement costs.

FI - Frequency inverter
FI-E - Frequency inverter