Understanding the basic design and operation principles behind VFDs will simplify selection, operation, and maintenance
Although much has been published on variable-frequency drives (VFDs), people continue to be confused by them. The intent of this article is to provide a basic overview of these devices by evaluating key features as well as their impact on a building’s electrical system.
A VFD is used extensively in modern facilities to save energy on mechanical systems, such as motors, pumps, etc. Selected to match motor curves to ensure speed and loads are matched, VFDs can help save motor energy by allowing for variable flow of air, water, etc., based on the demands and needs of a particular site. This is accomplished by converting the fixed frequency of incoming alternating current (AC) voltage to direct current (DC) — and then reconverting it back to AC voltage by varying the frequency at which the insulated gate bipolar transistors (IGBTs) are gated on and off.
A VFD operates by converting the input sinusoidal AC voltage to DC voltage and then back to AC voltage. This conversion occurs by using either silicon-controlled rectifiers (SCRs) or IGBTs. The DC voltage is switched using IGBTs to create an AC output voltage (called the inverter). The IGBT can switch on and off to create an AC voltage waveform that delivers power to the motor. The IGBTs create an AC waveform by using pulse width-modulated (PWM) switching (click here to see Fig. 1). The frequency at which the switching occurs, which varies from manufacturer to manufacturer, is called “carrier frequency.”
A typical 6-pulse VFD has six diodes as a front-end bridge rectifier that converts AC to DC. VFDs can also have 12 diodes — two sets per phase (2 × 2 × 3 = 12 pulse) — or 18 diodes — three sets per phase (3 × 2 × 3 = 18 pulse) — and so on (click here to see Table). One set of diodes is supplied by a Delta-Y transformer to create a phase shift on the AC side between the two rectifiers to reduce harmonics reflected back to the source.
A 6-pulse VFD develops the output DC voltage by taking each phase of the AC source and installing one set of diodes to gate on and off (click here to see Fig. 2). A 6-pulse VFD is most commonly used in the building system. Typical current total harmonic distortion (THD) back to the source can be as high as 35% at the input terminals of the VFD. You can install an inline inductor to reduce the reflected harmonics back to the point of coupling, as defined by IEEE 519. The inductor reduces the current distortion — and thus the voltage distortion — at the source. The input line inductors are typically 3% to 5% impedance. Base the selection of the inductor on harmonic evaluation of the electrical system at the building, impact of voltage drops across the inductor, and impact of power factor to the building electrical system.
The impact of harmonics should take into account available fault current (i.e., the stiffness of the electrical system). See IEEE Standard 519 for more details. However, if there are significant VFDs and other harmonic-producing devices in a building’s electrical system, such as electronic ballasts, uninterruptible power supplies, electronic switching devices, etc., think about using 12- or 18-pulse VFDs to reduce the harmonics reflected back to the source.
A 12-pulse VFD has phase shift transformers ahead of 6-pulse VFD to cancel the harmonics reflected back to the source. The phase shift transformers can be tuned to reduce harmonic distortion to less than 10% at the input terminals of the VFD (click here to see Fig. 3). The windings of the transformers are offset to cancel the largest harmonics from the VFD. By reducing the harmonics at the input terminals, the intent is to reduce voltage distortion at the source because the current THD at each piece of equipment is reduced. Conduct a harmonic evaluation to validate the findings. Disadvantages in providing a 12-pulse VFD are the cost (i.e., it can be up to 50% more) and the physical footprint required. A 6-pulse VFD for a typical 25-hp motor can be installed on the wall next to the motor. A 12-pulse VFD will be a free-standing “switchboard” type unit.
An 18-pulse VFD provides low harmonic distortion through phased cancellation of primary harmonics (5th and 7th) and the higher order harmonics that could cause resonance on capacitive and inductive loads (such as filters, transformer, etc.). Because cost can be prohibitive for most building applications, only consider this approach for facilities that have significant harmonics on the electrical system. The THD at the input terminals can be less than 5%; therefore, lower total voltage harmonic distortion will be realized at the source (i.e., point of common coupling), depending on the circuit impedance (click here to see Fig. 4).
Before selecting a VFD, evaluate the current THD. A 6-, 12-, 18- or even 24-pulse VFD should be evaluated to determine the best solution. Harmonics can cause faulty meter readings, motor bearing failure (due to electrical currents), blown fusing on capacitors, and/or telephone communication interference. A possible effect of harmonics is excitement of a system resonance that can significantly raise the voltage and cause system failures.
The carrier frequency and circuit impedance is a key feature in selection of a VFD to be aware of in regard to switching speeds and resonance. The manufacturer’s representation should ensure that the resonance frequency is skipped during startup of the VFD. The resonance frequency is a function of the VFD carrier frequency and impedance of the circuit; therefore, it’s specific to the site conditions.
The resonance in an electrical system occurs when there are harmonics on the electrical system — and where the inductive/capacitive loads are tuned to specific frequency to allow for oscillation due to impedance of the circuit. The effects of resonance can be explained briefly by the basic formula, V = I × Z. When current and impedance are excited to high levels, the voltage increases causing heating on the electrical system.
In addition to the reflected harmonics back to the source and to the motor, VFD selection should include voltage rating and system lockout. Keep in mind the nominal voltage of a VFD with upper and lower limits. Also, evaluate the impact of overvoltage on a VFD. Some VFDs tend to lock out with overvoltage condition after select restarts. You should also ensure that single-phase protection includes both time and magnitude components.
How does the motor nominal rating (typically 460V) correlate with nominal voltage of VFD (typically 480V)? The system utilization voltage should be such that it does not go beyond the high voltage rating of the motor (causing increased current and prematurely burning the motor). At the same time, it should not be too low where a sag on the system voltage, combined with building voltage drop, does not shut down the VFD.
When sizing the input breakers for a VFD, think about when the VFD is in bypass mode. Is the bypass starter specified as across the line or reduced voltage? Although VFDs are important in reducing the inrush current to generators, consideration should be given when a VFD is operated in bypass while on generator.
VFDs have losses, just like other electronic devices that transform voltage. Depending on type (6-, 12- or 18-pulse) and manufacture of VFD, the losses can be 4% to 10%. These losses should be taken into consideration in sizing electrical systems and requirements for ventilation.
In addition to matching the VFD to the motor, consider the environment as well. What are the derating factors for altitude and temperature? This will vary from manufacturer to manufacturer. In addition, evaluate the VFD for the seismic rating based on International Building Code and client requirements. A VFD may be designed to survive a seismic event, operate after a seismic event, or operate during and after a seismic event.
VFDs have been known to affect the bearing of the motor and cause premature failures. The ratio of V/T can create parasitic capacitance between the motor stator and the rotor, which induces a voltage on the rotor shaft. If this voltage, referred to as “common mode voltage,” builds up to a sufficient level, it can discharge to ground through the bearings. High carrier frequency can also contribute to bearing failure. The higher carrier frequency can cause increased current discharge pulses, but this also means the VFD will run more quietly.
Ensure carrier frequency is less than 6 kHz with adjustable frequency. Ensure the motor is as close as possible to the VFD to minimize conductor length. If the conductor is longer than recommended by the manufacturer, consider providing an inductive or RCL filter (click here to see Fig. 5).
Reduced harmonics should be considered for soft loads (such as at the end of an electric utility feed in a rural area or when powering loads with on-site generators) to reduce the voltage THD at the point of common coupling (PCC). High voltage distortion can cause malfunction of electronic devices.
Ensure the VFD output conductors are sized per manufacturer recommendations, and consider adding output line reactors. The line reactors should be sized to match the distance requirements of a VFD that is not located near the motor load.
Sizing of the electrical infrastructure should take into account the VFD losses, voltage tolerance, and footprint requirements. The electrical system should be sized for both when the motor is operated in VFD mode and when in bypass (across the line) mode.
In addition to cost, consider many other factors when selecting a VFD. The carrier frequency is critical to reduce impact on the motor bearings and motor life. Install the output and input inductors to reduce harmonic distortion at the motor and at the PCC for most applications. If the site has high harmonics on the electrical system, a 12-pulse VFD would typically be used. To minimize resonance of higher order harmonics, an 18-pulse VFD may be required.
During site startup and acceptance testing, pay special attention to voltage set points relative to utility source. The resonance frequency should be skipped during startup and on-site generation equipment should be tested with the VFD in normal operation mode and in bypass mode. The bottom line is you should always select VFDs based on careful evaluation of how they will interface with the electrical system of the building in addition to the pairing with mechanical motor selection.
Battish is a principal charged with leading RTKL’s mission-critical design and engineering capabilities in Baltimore. He can be reached at email@example.com.