Power Quality Glossary
This glossary defines power quality terms related to harmonics, rectification methods, voltage behavior, fault currents, and protective ratings in industrial electrical systems and VFD applications.
Part of the VFDS.com glossary. View the full glossary index.
Related Glossaries:
- Variable Frequency Drive Glossary - how VFDs interact with the electrical system
- Electric Motor Glossary - motor electrical characteristics, losses, and performance terms
- General Electrical Glossary - voltage, current, resistance, and electrical terms
Term Index:
Control Power Transformer (CPT)
Insulated Gate Bipolar Transistor (IGBT)
Intelligent Power Module (IPM)
Definitions:
6-Pulse Rectifier
A 6-pulse rectifier is the most common AC-to-DC conversion method used in variable frequency drives. It consists of a six-diode bridge that converts three-phase AC input power into DC bus voltage, producing six current pulses per AC cycle.
Because of this switching pattern, 6-pulse rectifiers generate relatively high levels of current harmonics on the power system, primarily the 5th and 7th harmonics. These harmonics can increase total harmonic distortion (THD), cause voltage distortion, and contribute to heating in transformers and conductors.
6-pulse rectifiers are widely used because they are simple, reliable, and cost-effective. They are typically acceptable in systems where harmonic distortion limits are not strict or where mitigation devices such as line reactors or filters are used.
Related terms: Power Factor
12-Pulse Rectification
12-pulse rectification is an AC-to-DC conversion method that uses two 6-pulse rectifiers fed by phase-shifted power sources, typically created using a transformer with multiple secondary windings.
By phase-shifting the input currents, certain harmonic components cancel each other, significantly reducing overall harmonic distortion compared to a 6-pulse rectifier. This results in lower current THD and improved power quality on the supply side.
12-pulse rectification is commonly used in medium- to high-power VFD applications where harmonic limits are tighter, but where the cost and complexity of higher-pulse solutions are not justified. The tradeoff is increased system size, transformer complexity, and cost.
Related terms: Line Reactor, Power Factor
18-Pulse Rectification
18-pulse rectification further improves power quality by using three phase-shifted rectifier bridges, producing eighteen current pulses per AC cycle. This configuration dramatically reduces input current harmonics and total harmonic distortion.
By increasing the number of pulses, lower-order harmonics are largely eliminated, resulting in near-sinusoidal input current under steady operating conditions. This reduces voltage distortion, minimizes transformer heating, and improves overall system efficiency.
18-pulse rectification is typically used in applications with strict harmonic requirements, sensitive electrical systems, or large installed drive power. While it provides excellent power quality performance, it comes with higher cost, increased transformer complexity, and larger physical footprint compared to 6- and 12-pulse designs.
Related terms: Line Reactor, Power Factor
Active Harmonic Filter
An active harmonic filter is a power electronic device that injects corrective currents into an electrical system to cancel harmonic distortion. It dynamically improves power quality by reducing total harmonic distortion and correcting power factor.
Related terms: Harmonic Filter, Passive Harmonic Filter
Amps Interrupting Current
Amps interrupting current is the maximum fault current a protective device can safely interrupt without damage. It is a critical safety rating for fuses and circuit breakers in power systems.
CE Compliant
CE compliant indicates that equipment meets European Union safety, electromagnetic compatibility, and environmental requirements. It is a regulatory conformity mark, not a performance rating.
Control Power Transformer (CPT)
A control power transformer provides low-voltage power for control circuits, relays, and electronics within electrical panels or motor control systems. It isolates and steps down system voltage for control components.
Related terms: Control Board, Two Contactor Bypass, Three Contactor Bypass
DC Bus
The DC bus is the intermediate section of a variable frequency drive that stores and stabilizes direct current (DC) power between the rectifier and inverter stages. It acts as an energy buffer, allowing the VFD to decouple incoming AC power from the controlled AC output supplied to the motor.
In a typical VFD, incoming AC power is first converted to DC by the rectifier. This DC power is then filtered and stored on the DC bus using capacitors, and sometimes inductors such as a link choke. The inverter draws from the DC bus to create a controlled output waveform using pulse-width modulation (PWM). A stable DC bus voltage is essential for consistent motor torque, speed control, and drive reliability.
The DC bus plays a critical role during transient conditions. During acceleration, it supplies energy to the inverter. During deceleration or overhauling load conditions, the motor can regenerate energy back into the DC bus, causing bus voltage to rise. If this excess energy is not managed, it can lead to an overvoltage fault. Methods such as braking resistors, regenerative drives, or longer deceleration times are used to control DC bus voltage.
DC bus behavior is closely tied to power quality and protection. Inrush current occurs when DC bus capacitors initially charge at power-up, which is why VFDs use pre-charge circuits and sometimes line reactors. DC bus voltage level, ripple, and stability directly affect inverter switching performance, heat generation, and component life.
Proper DC bus design and management are essential for safe, efficient VFD operation. It influences fault performance, braking capability, harmonic behavior, and overall system reliability across both low-voltage and medium voltage drive applications.
Displacement Power Factor
Displacement power factor is the cosine of the phase angle between the fundamental voltage and current waveforms in an AC system. It accounts only for phase shift caused by inductive or capacitive loads and does not include the effects of harmonic distortion.
Distortion Power Factor
Distortion power factor accounts for the reduction in effective power factor caused by harmonic current distortion in an electrical system. It reflects losses introduced by nonlinear loads such as rectifiers and variable frequency drives and is a key component of true power factor.
Gate Drive Board
The gate drive board is a PCB containing the circuitry necessary for operating (gating) the output transistors of the VFD. The gate drive board can also monitor main circuit temperature, current, and voltage. Typically, smaller VFDs have no separate gate drive, but combine gating with logic power supplies to form a power board.
Ground
Ground is a reference point in an electrical system connected to earth or a common conductive path. Proper grounding is essential for safety, noise reduction, and reliable operation of power electronic equipment.
Ground Fault
A ground fault occurs when electrical current unintentionally flows to ground through an abnormal path. Ground faults can cause equipment damage, safety hazards, and nuisance tripping if not properly managed.
Harmonics
Harmonics are unwanted frequency components in an electrical system that are integer multiples of the fundamental power frequency. In power systems with nonlinear loads such as variable frequency drives, harmonics distort current and voltage waveforms, increasing losses, heating equipment, and degrading overall power quality.
Harmonic Distortion
Harmonic distortion is the measurable deviation of a voltage or current waveform from a pure sine wave caused by harmonic frequency components. It is commonly expressed as total harmonic distortion (THD) and used to evaluate power quality, equipment stress, and compliance with electrical standards.
Harmonic Filter
A harmonic filter is a device used to reduce harmonic distortion in electrical systems by attenuating specific harmonic frequencies generated by nonlinear loads. Harmonic filters improve power quality, reduce total harmonic distortion, protect upstream equipment, and help systems comply with harmonic standards and utility requirements.
Related terms: Active Harmonic Filter, Passive Harmonic Filter, Hybrid Harmonic Filter
Heat Sink
A passive thermal management component used to dissipate heat away from critical motor components, such as the stator or controller. Heat sinks are critical for preventing overheating, improving motor lifespan, and maintaining efficiency.
Related terms: Thermal Management
Hybrid Harmonic Filter
A hybrid harmonic filter combines passive filtering components with active harmonic compensation to reduce harmonic distortion and improve power quality.
Related terms: Harmonic Filter
Inductance
Inductance is the property of an electrical conductor or coil that resists changes in current by storing energy in a magnetic field. When current flowing through an inductive component changes, the magnetic field around the conductor expands or collapses, generating a voltage that opposes the change in current according to electromagnetic principles. Inductance is measured in Henries (H).
In AC power systems and motor drives, inductance influences current flow, voltage behavior, and harmonic performance. Inductive components such as reactors, transformers, and motor windings help smooth current, limit inrush current, reduce harmonic distortion, and affect power factor and transient response.
Inductive Load
An inductive load stores energy in a magnetic field and resists changes in current. Motors, transformers, and reactors are common inductive loads that affect power factor and transient behavior.
Inrush Current
Inrush current is the high, short-duration current drawn by electrical equipment when it is first energized. This current can be several times higher than normal operating current and typically occurs for a few milliseconds to several cycles.
In AC power systems, inrush current commonly occurs when motors start, transformers are energized, or capacitive devices are connected to the supply. The magnitude of inrush current is influenced by system voltage, source impedance, equipment design, and the point on the voltage waveform at which energization occurs.
Excessive inrush current can cause voltage dips, nuisance tripping of protective devices, contactor wear, and stress on upstream electrical components. In systems with multiple loads or limited source capacity, inrush events can negatively affect overall power quality.
Variable frequency drives reduce motor inrush current compared to across-the-line starting by gradually ramping voltage and frequency during startup. However, inrush current can still occur on the line side of a VFD when charging DC bus capacitors, particularly if pre-charge circuits are absent or improperly sized.
Insulated Gate Bipolar Transistor (IGBT)
An Insulated Gate Bipolar Transistor, or IGBT, is a high-power semiconductor switching device used in variable frequency drives and other power electronic equipment to control voltage and current.
IGBTs combine the voltage-controlled gate of a MOSFET with the high current and voltage capability of a bipolar transistor, making them suitable for medium- and high-power switching applications.
In VFDs, IGBTs are used in the inverter stage to switch DC bus voltage using pulse-width modulation (PWM). The switching frequency of the IGBT determines how often this voltage is turned on and off, which directly affects output waveform quality, harmonic content, switching losses, heat generation, acoustic noise, and overall drive efficiency. Higher switching frequencies improve waveform quality but increase losses and thermal stress, while lower frequencies reduce losses but increase harmonic distortion.
Related terms: Pulse-Width Modulation (PWM), Carrier Frequency, Inverter, Three-Level Output
Intelligent Power Module (IPM)
An Intelligent Power Module integrates power switching devices, gate drivers, and protection circuits into a single package. IPMs simplify VFD design and improve reliability by providing built-in fault detection and thermal protection.
Inverter
The inverter is the output stage of a variable frequency drive that converts DC bus power into a controlled AC output for the motor. It generates the required voltage, frequency, and phase to control motor speed and torque.
In a VFD, the inverter uses high-speed switching devices such as insulated gate bipolar transistors (IGBTs) and pulse-width modulation (PWM) to create an effective AC waveform from DC power. The switching pattern directly affects motor performance, influencing torque smoothness, acoustic noise, heating, insulation stress, and electromagnetic interference.
The inverter also manages energy flow during dynamic conditions such as acceleration, deceleration, and regeneration. Its design, switching frequency, and topology are critical to overall drive efficiency, thermal performance, and motor reliability, especially in high-power and medium voltage applications.
Related terms: Carrier Frequency, Sine Wave, Reflected Wave, Three-Level Output
Isolation Transformer
An isolation transformer is a transformer that electrically separates the primary and secondary circuits while transferring power between them. It provides voltage transformation and isolates equipment from the supply to improve safety, reduce electrical noise, and limit the effects of ground faults and transients.
Related terms: Ground, Ground Fault, Surge Protective Device
K-Factor Transformer
A K-factor transformer is designed to handle nonlinear loads that produce harmonic currents, such as variable frequency drives and power electronics. It is built with enhanced thermal capacity to withstand additional heating caused by harmonic distortion.
Related terms: Harmonics, Harmonic Distortion, Inrush Current
Kilovolt-Amperes (kVA)
Kilovolt-amperes (kVA) is a measure of apparent power in an AC electrical system. It represents the combination of real power and reactive power and is used to size electrical equipment.
Related terms: Power Factor
Leakage Current
Leakage current is unintended current that flows through insulation or parasitic paths. Excessive leakage current can create safety issues and interfere with protective devices.
Leakage Inductance
Leakage inductance is the portion of inductance that does not contribute to useful energy transfer. It influences voltage spikes, transient behavior, and electromagnetic interference.
Let-Through Current
Let-through current is the amount of current that passes through a protective device during a fault before it interrupts the circuit. Lower let-through current reduces stress on downstream equipment.
Line Reactor
A line reactor is an inductive device installed on the input side of a variable frequency drive, between the power source and the drive. Its primary purpose is to limit current distortion, reduce harmonic content, and protect the VFD from line-side disturbances.
By adding impedance to the supply, a line reactor smooths input current, reduces peak current during DC bus charging, and helps mitigate voltage transients caused by utility switching or upstream equipment. Line reactors are commonly used with 6-pulse rectifier drives to improve power quality and reduce stress on rectifier components.
Related terms: Inrush Current, Pre-Charge Circuit, Rectifier/Converter
Line-to-Line Resistance
Line-to-line resistance is the electrical resistance measured between two phases of a three-phase system. It is used to evaluate wiring integrity and balance.
Link Choke
A link choke is an inductor installed in the DC bus of a VFD, between the rectifier and inverter stages. Its function is to smooth DC current, reduce ripple, and improve harmonic performance by stabilizing DC bus voltage.
By reducing current ripple in the DC link, a link choke lowers stress on power electronic components and can improve overall drive efficiency and reliability. Link chokes are often used as an alternative or supplement to line reactors in harmonic mitigation strategies.
Related terms: Rectifier/Converter
Load Reactor
A load reactor is an inductive device installed on the output side of a VFD, between the drive and the motor. It is used to reduce voltage spikes, limit current transients, and protect motor insulation, particularly in installations with long motor lead lengths.
Load reactors help mitigate reflected wave effects by slowing the voltage rise time of the VFD output. They also reduce electromagnetic interference and can improve motor reliability in inverter-driven systems.
Related terms: Reflected Wave, Sine Wave, Carrier Frequency, Insulation Class
Matrix Converter
A Matrix converter is an AC-to-AC VFD that doesn’t have a rectifierconverter or a DC bus section to convert AC to DC to AC, as in most commercially available drives. Nine bi-directional switches are controlled depending on target to that point. When a VFD chassis output voltage and frequency. Benefits include four-quadrant operation in a small footprint, low input current harmonic distortion, and lower common mode voltage and common mode current. A Matrix drive’s output voltage is, however, limited to approximately 90% of input voltage.
Overload Protection
Overload protection prevents equipment damage by limiting current when operating conditions exceed rated values. It protects motors and drives from overheating and long-term stress.
Passive Harmonic Filter
A passive harmonic filter uses inductors, capacitors, and resistors to reduce harmonic distortion in electrical systems. These filters target specific harmonic frequencies generated by nonlinear loads.
Related terms: Harmonic Filter
Peak Current
Peak current is the maximum instantaneous current a system or component experiences. It is a critical parameter for sizing power electronic devices and protection components.
Phase-Shifting Transformer
A phase-shifting transformer is used to create a controlled phase displacement between input and output voltages. It is commonly used in multi-pulse rectifier systems to reduce harmonic distortion by causing harmonic currents to cancel each other.
Related terms: 12-Pulse Rectification, 18-Pulse Rectification
Power Factor
Power factor is the ratio of real power, measured in watts or kilowatts, to apparent power, measured in volt-amperes or kilovolt-amperes, in an AC electrical system. It indicates how effectively electrical power is being converted into useful work.
A power factor of 1.0 represents ideal utilization, where all supplied power performs useful work. Lower power factor values indicate the presence of reactive power, harmonic distortion, or both, which increase current draw without increasing useful output.
In systems with inductive loads such as motors and transformers, power factor is reduced due to phase displacement between voltage and current. Power electronic equipment, including VFDs, can also reduce power factor through current distortion caused by rectification and switching.
Power factor is often described as either displacement power factor, which accounts only for phase shift, or true power factor, which includes both phase displacement and harmonic distortion. Utilities may impose penalties or limits based on true power factor because low power factor increases losses, reduces system capacity, and stresses electrical infrastructure.
Quiescent Current
Quiescent current is the small amount of current drawn by electronic circuits when they are energized but not actively switching or driving a load.
Rectifier/Converter
One of the three primary sections of a VFD’s main power circuit, and first in terms of power flow. Incoming AC line voltage is rectified to DC voltage in the converter section, which consists of diodes, silicon-controlled rectifiers (SCRs), or insulated gate bipolar transistors (IGBTs) connected in a full-wave bridge configuration.
Related terms: 6-Pulse Rectifier, 12-Pulse Rectification, 18-Pulse Rectification, DC Bus, Variable Frequency Drive (VFD)
Reflected Wave
Reflected wave is a voltage phenomenon that occurs when a variable frequency drive output is transmitted through a motor cable whose impedance does not match the impedance of the motor. This mismatch causes part of the voltage waveform to reflect back toward the drive, resulting in elevated peak voltages at the motor terminals.
Reflected wave effects are most pronounced in VFD installations with long motor lead lengths, high DC bus voltage, or fast switching devices such as IGBTs operating at high carrier frequencies. Under these conditions, the combined incident and reflected voltage waves can produce peak voltages significantly higher than the VFD output voltage.
Excessive reflected wave voltage can stress motor insulation, accelerate insulation aging, and lead to premature motor failure, particularly in older motors not designed for inverter duty. It can also contribute to increased electromagnetic interference and nuisance faults.
Common mitigation methods include load reactors, dv/dt filters, sine wave filters, and proper cable selection. Managing reflected wave is an important part of ensuring motor reliability and maintaining acceptable power quality in VFD-driven systems.
Related terms: Pulse-Width Modulation (PWM), Sine Wave, Insulation Class
SCCR
SCCR, or Short Circuit Current Rating, is the maximum prospective fault current that an electrical device, assembly, or industrial control panel can safely withstand without causing a fire, explosion, or loss of enclosure integrity.
SCCR is expressed in kiloamperes (kA) and represents the available short-circuit current at the point where the equipment is installed. If the available fault current exceeds the SCCR, protective devices may fail to clear the fault safely, creating a serious safety hazard.
In VFD systems, SCCR is determined by the weakest component in the power path, such as contactors, fuses, circuit breakers, reactors, or the drive itself. Panel-level SCCR is not the same as the SCCR of individual components and must be evaluated as a complete system.
SCCR is a critical compliance requirement under standards such as UL 508A. Proper SCCR coordination ensures that protective devices limit let-through current during a fault and that the system can safely withstand worst-case short-circuit conditions.
Related terms: Amps Interrupting Current, Overcurrent Fault, Overvoltage Fault, Under Voltage Fault
Single-Phase
Single-phase power uses one alternating voltage waveform. It is common in residential and light commercial applications but limits available power capacity.
Related terms: Phase
Static VAR Generator
A static VAR generator is a power electronic device that dynamically supplies or absorbs reactive power to regulate voltage and improve power factor in real time. Unlike fixed capacitor banks, SVGs respond instantly to changing load conditions, making them effective for stabilizing voltage, reducing flicker, and improving power quality in systems with rapidly varying loads.
Three-Phase
Three-phase power uses three voltage waveforms offset by 120 degrees. It provides more efficient power delivery and is standard in industrial applications.
Related terms: Phase
Transformer
A transformer is an electrical device that transfers energy between circuits using electromagnetic induction to increase or decrease voltage levels. It consists of primary and secondary windings wound around a magnetic core and is used for power distribution, isolation, and voltage conversion in electrical systems. Transformers are commonly used in industrial systems to support motor drives, power distribution, and system reliability.
Related terms: Turns Ratio, Primary Winding, Secondary Winding, Core Saturation
Related resources: NexE Transformer Solutions
Transformer Inrush Current
Transformer inrush current is the high, short-duration current drawn when a transformer is first energized. It occurs as the magnetic core is magnetized and can be several times higher than normal operating current, potentially causing voltage dips or nuisance tripping.
Related terms: Inrush Current, Voltage Sag
UL Recognized
UL Recognized indicates that a component meets specific safety requirements for use within a larger system. It differs from full UL Listing, which applies to complete products.
Voltage
Voltage is the electrical potential difference that drives current through a circuit. Proper voltage levels are essential for equipment performance and longevity.
Voltage Constant
Voltage constant relates voltage to speed or flux in electrical machines and control systems. It is used in modeling and performance calculations.
Voltage Rating
Voltage rating is the maximum voltage a component or system can safely withstand. Exceeding this rating can cause insulation failure or catastrophic damage.
Voltage Regulation
Voltage regulation describes how well a transformer maintains its output voltage as load conditions change. Poor regulation results in larger voltage drops under load
Voltage Sag
Voltage sag is a short-duration reduction in RMS voltage, often caused by motor starting, faults, or large load changes. Sags can trigger VFD undervoltage trips and disrupt sensitive control equipment.
Wattage
Wattage is a measure of real power consumed or delivered by a system. It represents actual energy usage rather than apparent power.
Related terms: Power Factor
X/R Ratio
The X/R ratio is the ratio of reactance to resistance in an electrical system. It influences fault current magnitude, decay rate, and protective device performance.