VARIABLE FREQUENCY DRIVE (VFD)

Variable frequency drive (VFD) usage has increased dramatically in HVAC applications. The VFDs are now commonly applied to air handlers, pumps, chillers and tower fans. A better understanding of VFDs will lead to improved application and selection of both equipment and HVAC systems. This paper is intended to provide a basic understanding of common VFD terms, VFD operation, and VFD benefits. In addition this paper will discuss some basic application guidelines regarding harmonic distortion with respect to industry standards

Understanding the basic principles behind VFD operation requires understanding the three basic  sections of the VFD: the rectifier, dc bus, and inverter. The voltage on an alternating current (ac) power supply rises and falls in the pattern of a sine wave

When the voltage is positive, current flows in one direction; when the voltage is negative, the current flows in the opposite direction. This type of power system enables large amounts of energy to be efficiently transmitted over great distances. The rectifier in a VFD is used to convert incoming ac power into direct current (dc) power. One rectifier will allow power to pass through only when the  voltage is positive. A second rectifier will allow power to pass through only when the voltage is negative. Two rectifiers are required for each phase of power. Since most large power supplies are three phase, there will be a minimum of 6 rectifiers used Appropriately, the term “6 pulse” is used to describe a drive with 6 rectifiers. A VFD may have multiple rectifier sections, with 6 rectifiers per section, enabling a VFD to be “12 pulse,” “18 pulse,” or “24 pulse.” The benefit of “multipulse” VFDs will be described later in the harmonics section. Rectifiers may utilize diodes, silicon controlled rectifiers 

microprocessor to control when the power may begin to flow, making this type of rectifier useful for solid-state starters as well. Transistors include a gate circuit that enables a microprocessor to open or close at any time, making the transistor the most close at any time, making the transistor the most close at any time, making the transistor the most close at any time, making the transistor the most close at any time, making the transistor the most useful device of the three. A VFD using transistors in the rectifier section is said to have an “active front end.” After the power flows through the rectifiers it is stored on a dc bus. The dc bus contains capacitors to accept power from the rectifier, store it, and later deliver that power through the inverter section. The dc bus may also contain inductors, dc links, chokes, or similar items that add inductance, thereby smoothing the incoming power supply to the dc bus. smoothing the incoming power supply to the dc bus. The final section of the VFD is referred to as an “inverter.” 

The inverter contains transistors that deliver power to the motor. The “Insulated Gate Bipolar Transistor” (IGBT) is a common choice in modern VFDs. The IGBT can switch on and off several thousand times per second and precisely control the power delivered to the motor. The IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine wave at the desired frequency to the motor. Motor speed (rpm) is dependent upon frequency. Varying the frequency output of the VFD controls

motor speed: Speed (rpm) = frequency (hertz) x 120 / no. of poles