The variable frequency drive (VFD) is arguably one of the most notable energy management tools ever introduced into Heating, Ventilation and Air-Conditioning (HVAC) systems. For the past 30 years or so, a whole host of variable load applications have witnessed VFDs installed on fan and pump motors. The reason behind this trend is energy savings of 50% or more over conventional constant speed drive solutions. In an era where energy costs have become a major concern for businesses and building owners alike, energy-efficient HVAC solutions are in high demand.
Put simply, a VFD is a device used to control the rotation speed of an AC electric motor by adjusting the frequency of the electrical power supplied to the motor. In the early days of VFDs, motor power was a limiting factor in HVAC applications, while noise was another issue. Today, however, these obstacles to adoption have largely disappeared.
Modern VFDs can be installed in practically any HVAC application while, with regard to noise, the latest drives function at frequencies above the audible spectrum, thus avoiding the previous issue of vibrating motor laminations, which in turn would transmit, annoyingly, to the motor mounts.
Overcoming issues such as these have released VFDs for widespread adoption in HVAC systems, promoting energy savings and reducing carbon emissions the world over. Simply by matching system capacity to the actual load across the year, it is possible to achieve major reductions in motor energy consumption.
Most VFDs used in HVAC applications are inverters using sine-coded pulse width modulation (PWM) technology. This type of VFD functions by converting the incoming AC power to DC through the use of a diode bridge rectifier. The VFD then passes the filtered, smoothed voltage on to the inverting section of the device, before controlling the voltage and frequency sent to the motor via bipolar transistors.
Those thinking that energy saving is the only major benefit of VFDs should think again. There are many more advantages to be had, such as reduced wear and tear on motors. VFDs apply very low frequencies and voltages to motors, ramping up both parameters steadily to normal operating levels and therefore extending motor life expectancy. Conversely, a motor without a VFD draws high current at start-up, creating high temperatures and elevated stress in the windings. As a consequence, premature failures are more commonplace.
The accuracy of control is a further benefit of VFD technology in HVAC applications, which is a significant factor in multi-story buildings, for example. Such structures tend to rely on booster pumps to maintain domestic water supply pressures on all floors. This is generally sufficient within a certain range but does not compare with the capabilities of the latest VFDs, which deliver far more precise control over a much greater spectrum of flow rates.
The precision factor also comes into play on variable volume fan systems used commonly to distribute conditioned air in HVAC units. A system of variable inlet vanes in the fan system and variable air volume boxes is a typical control configuration, but this is not as efficient or precise as a VFD-based system. Similarly, a VFD deployed in a variable refrigerant flow system can be used to control the operating speed of the compressor to match the load, reducing energy requirements under part-load conditions.
The most commonly used motors in HVAC applications are induction motors; three-phase for commercial/industrial premises and single-phase for certain smaller installations. These are fundamental to the core operating function of HVAC systems. Electric motors power blowers to move air; drive compressors to compress refrigerant, and power pumps to move water for chilled water and hot water applications.
It is possible to apply VFDs to both three- and single-phase motors, typically at a suitable percentage of their rated speed (refer to VFD manufacturer recommendations) to prevent any potential for the motor overheating due to inadequate airflow. Operational factors should also be considered, such as indoor air quality requirements and air distribution requirements.
There are few, if any, real downsides to using VFDs in HVAC applications. Some might argue that using PWM to control motor speed creates EMI (electromagnetic interference) due to the rapid rise and fall in signal periods. Any kind of interference is clearly undesirable, but simple measures can be employed to counter its effects.
For example, minimising the cable lengths between the VFD and motor will provide a significant benefit. Long cables offer greater potential for reflected voltage. Here, using the lowest VFD carrier frequency is advisable as lower frequencies help maximise the potential length of cable between the VFD and motor.
Armoured cable is another obvious tactic in the battle against interference as this will shield system components from high-frequency electric fields. Copper or aluminum are the preferred shielding materials; steel is less effective. Further strategies include using separate metal conduits for input power, output power, control wires and communication wires.