Variable Frequency Drive Glossary
This glossary defines key variable frequency drive (VFD) terms related to drive architecture, control methods, braking, fault behavior, and motor speed and torque regulation in industrial applications.
Part of the VFDS.com glossary. View the full glossary index.
Related Glossaries:
- Electric Motor Glossary - motor types, torque, speed, thermal behavior, and construction terms
- Power Quality Glossary - harmonics, voltage, current, grounding, and electrical system terms
- General Electrical Glossary - voltage, current, resistance, and electrical terms
Term Index:
Acceleration/Deceleration Time
Insulated Gate Bipolar Transistor (IGBT)
Medium Voltage Variable Frequency Drive
Open Loop and Closed Loop Control
Variable Frequency Drive (VFD)
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: Line Reactor, Power Factor, Transformer
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, Transformer
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, Transformer
Acceleration/Deceleration Time
Acceleration/deceleration time is the VFD setting that controls how long the motor takes to ramp from one speed to another, including startup and stopping. It is typically set as a time value (seconds) for a change from 0 Hz to a target frequency, and the VFD follows that ramp to limit how quickly motor speed changes.
This setting directly affects current draw, torque demand, and system stress. Short acceleration times require higher torque and can drive current high enough to trigger an overcurrent fault, especially with high-inertia loads or loads that start under heavy demand. Short deceleration times can cause the motor to generate energy back into the drive (regeneration), raising DC bus voltage and leading to an overvoltage fault if the energy cannot be absorbed.
Acceleration/deceleration time should be selected based on load inertia, required process response, and the system’s ability to handle braking energy. For fast stops or high-inertia loads, longer decel ramps or braking methods such as a braking resistor or other braking strategy may be required to prevent nuisance trips and reduce mechanical wear.
Related terms: Dynamic Braking, Coast to stop
Active Front End (AFE)
An active front end is a VFD input stage that uses controlled power electronics instead of diodes to convert AC power to DC. It allows bidirectional power flow, enabling regeneration back to the grid and reducing input harmonic distortion.
Auto-Tuning
Auto-tuning is a process in which the VFD tests an attached and unloaded motor to determine the best tuning parameters.
Braking Resistor
A braking resistor is an external component connected to a VFD to dissipate excess energy generated during motor deceleration or overhauling load conditions. It converts electrical energy into heat to prevent DC bus overvoltage.
Braking resistors are commonly used in applications requiring fast stops, frequent deceleration, or controlled braking of high-inertia loads.
Related terms: Overvoltage Fault, Regeneration, Dynamic Braking, Load Inertia, Acceleration/Deceleration Time
Bypass
A bypass is a switching arrangement that allows a motor to run across-the-line without the VFD in the power path. It is used for redundancy, maintenance, or emergency operation and is commonly implemented with two- or three-contactor bypass schemes.
Related terms: Two Contactor Bypass, Three Contactor Bypass, Electronic Back Bypass
Capacitor
A capacitor is an electrical component that stores energy in an electric field between two conductive plates. Capacitors are commonly used for filtering, energy storage, and power factor correction in electrical systems.
Carrier Frequency
Carrier frequency is the switching frequency at which a VFD’s power devices turn on and off to generate pulse-width modulation (PWM). It is typically expressed in kilohertz (kHz) and defines how many switching cycles occur per second.
Carrier frequency directly affects the quality of the VFD output waveform. Higher carrier frequencies produce smoother current, reduce torque ripple, and lower audible motor noise, especially at low speeds. This often results in quieter motor operation and improved speed smoothness.
However, increasing carrier frequency also increases switching losses in the VFD’s power devices, leading to higher heat generation and reduced overall efficiency. It can also increase electromagnetic interference and stress motor insulation, particularly with long motor cable lengths.
Lower carrier frequencies reduce switching losses and heat but can cause increased acoustic noise, current ripple, and torque pulsations. Selecting an appropriate carrier frequency requires balancing motor noise, efficiency, thermal limits, and application requirements.
Carrier frequency is a key tuning parameter in VFD setup and has a significant impact on drive performance, motor life, and system power quality.
Related terms: Insulated Gate Bipolar Transistor (IGBT), Reflected Wave, Insulation Class, Thermal Management
Catch a Spinning Load
Catch a spinning load is a drive function that detects a motor already rotating and synchronizes the VFD output to the motor speed before applying torque. This prevents large current surges and mechanical stress when restarting a rotating load.
Related terms: Flying Start
Coast to stop
Coast to stop is a motor stopping method where power is removed and the motor slows naturally due to friction and load inertia. No braking torque is applied by the drive.
Related terms: Acceleration/Deceleration Time
Commissioning
Commissioning is the process of verifying that a variable frequency drive system is installed, configured, and operating according to design requirements. It includes parameter setup, functional testing, safety checks, and validation of performance under actual load conditions. Commissioning ensures the drive, motor, and connected equipment operate reliably, safely, and as intended before full operation.
Related terms: Motor Nameplate Parameters, Vector Control, Acceleration/Deceleration Time, Overcurrent Fault, Overvoltage Fault
Common Busing
Common busing is a method for connecting the DC bus sections of separate VFDs, or operating multiple independent inverter sections from a common DC source. The advantage of this method is that motor operation sequencing can be used to balance motoring and regenerating so that little or no dynamic braking is necessary.
Constant and Variable Torque
Constant Torque: A torque requirement that does not naturally change with speed, and sometimes requires intermittent overload.
Variable Torque: A torque requirement that naturally increases with speed (i.e fan or pump), and Also known as earth, ground is the does not require intermittent overload.
Control Board
The control board is a printed circuit board (PCB) that is the main interface component used to connect external equipment and operator interface components to and from the VFD. Acting as the VFD’s brain, the PCB accepts real-world commands such as “Run” or “ Speed Up” and executes the target function. The control PCB generally interfaces to the VFD’s main circuit via the gate drive board.
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.
DC Injection Braking
DC injection braking is a VFD stopping method in which direct current is applied to the motor windings after output frequency is reduced. The injected DC creates a stationary magnetic field that produces braking torque as the rotor moves through it.
This method provides controlled stopping at low and zero speed without external hardware. Because braking energy is dissipated as heat in the motor windings, DC injection braking is best suited for short-duration or infrequent stops and must be limited to prevent overheating.
Related terms: Duty Cycle, Dynamic Braking, Braking Resistor, Thermal Management
Derating
Derating is the practice of reducing a motor or drive’s rated output to account for operating conditions that increase thermal or electrical stress. Factors such as high ambient temperature, altitude, switching frequency, duty cycle, and cooling limitations require derating to maintain reliability and prevent overheating or premature failure.
Diode
A diode is a semiconductor device that allows electrical current to flow in only one direction. Diodes are commonly used in rectifiers to convert AC power to DC in power electronic equipment.
dV/dt
dV/dt represents the rate of change of voltage over time, typically expressed in volts per microsecond (V/µs). In variable frequency drive (VFD) systems, dV/dt describes how quickly the inverter output voltage transitions during the switching of power devices such as IGBTs. Because modern drives switch very rapidly, these voltage transitions can occur in extremely short time intervals.
High dV/dt levels are associated with the pulse-width modulation (PWM) output waveform produced by VFD inverters. Rapid voltage changes can place additional electrical stress on motor insulation, especially when long motor cables are used. This can also contribute to phenomena such as reflected wave voltage, which may increase peak voltage at the motor terminals.
To reduce the effects of high dV/dt, systems may use mitigation devices such as load reactors, dV/dt filters, or sine wave filters. Managing dV/dt helps improve motor reliability, reduce insulation stress, and limit electromagnetic interference in VFD-driven motor systems.
Related terms: Reflected Wave
Dynamic Braking
Dynamic braking is a method used to slow or stop an electric motor by converting the motor’s rotational energy into electrical energy and dissipating it as heat. In variable frequency drive (VFD) systems, this occurs when the motor acts as a generator during deceleration or when driven by an overhauling load. The generated energy flows back into the drive’s DC bus.
To prevent the DC bus voltage from rising to a fault level, the excess energy is diverted to a braking resistor through a braking transistor or braking chopper circuit. The resistor converts the electrical energy into heat, allowing the motor to decelerate more quickly than it would through normal ramp-down or coast-to-stop methods.
Dynamic braking is commonly used in applications with high inertia or frequent stopping requirements, such as cranes, conveyors, elevators, and machine tools. It improves stopping performance and reduces deceleration time, but it does not return energy to the power system like regenerative braking.
Related terms: Acceleration/Deceleration Time, Braking Resistor
Electronic Back Bypass
Electronic back bypass is a control method that transfers a motor from variable frequency drive operation to across-the-line power by electronically synchronizing the motor to the incoming line voltage before the bypass contactor closes. The VFD adjusts its output frequency, voltage, and phase so the motor matches the utility power waveform. This synchronization minimizes torque shock, inrush current, and mechanical stress during the transfer to bypass operation.
Related terms: Bypass
Ethernet
This term defines the hardware and message transport specifications of networking devices, otherwise known as nodes. It is comprised of the first two layers of the OSI model for the Physical Layer (e.g. cable, RJ45 connector) and Data Link Layer (i.e. defining how a message is moved from one device to another).
Fieldbus
Fieldbus is a term used for a communication network that can be used for automation and is defined by the physical means used to transmit messages along with how a message is structured. These networks are serial in nature. A physical implementation of a fieldbus network can be comprised of RS-485 or Ethernet. Many VFDs have built-in fieldbus connections though some fieldbus protocols require option boards.
Flying Start
Flying start is a VFD function that allows the drive to synchronize with a motor that is already rotating before applying full control. The VFD detects motor speed and phase to avoid torque shock.
Flying start is useful in applications where motors may coast or be driven by the load, such as fans or conveyors, and prevents mechanical stress and nuisance faults during restart.
Related terms: Slip, Load Inertia, Overcurrent Fault, Acceleration/Deceleration Time, Catch a spinning load
Frequency
Frequency is the number of complete AC voltage or current cycles that occur each second, measured in Hertz (Hz). In power systems, standard supply frequency is typically 50 Hz or 60 Hz, depending on the region. In AC motors, frequency directly influences rotational speed because the speed of the stator’s rotating magnetic field is proportional to the supply frequency. Variable frequency drives control motor speed by adjusting the output frequency supplied to the motor.
Frequency Resolution
Frequency resolution is the smallest increment of output frequency change that a variable frequency drive can produce. It defines how precisely the VFD can adjust motor speed.
Higher frequency resolution allows smoother speed control, finer tuning, and improved stability, especially at low speeds or in applications requiring precise positioning or process control. Limited resolution can result in noticeable speed steps, hunting, or uneven motor operation.
Frequency resolution is determined by the VFD’s internal control algorithms and digital processing capability. While high resolution is not critical for all applications, it becomes important in systems where small speed changes have a significant impact on performance or product quality.
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.
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.
HMI
An HMI or human-machine interface is a device or software system that allows operators to monitor and control equipment through graphical displays and input controls.
Installation
Installation is the process of physically mounting, wiring, and integrating a variable frequency drive, motor, and associated components into an electrical system. It includes mechanical mounting, power and control wiring, grounding, and ensuring compliance with electrical codes and manufacturer requirements. Proper installation is critical to safe operation, reliable performance, and successful startup and commissioning of the system.
Related terms: Ground, Ground Fault, Voltage, Current, Inrush Current, Pre-Charge Circuit, Control Power Transformer (CPT)
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
LCI Drive
An LCI drive, or load commutated inverter drive, is a type of medium voltage variable frequency drive commonly used to control large synchronous motors. It uses thyristors and relies on the motor’s back EMF to commutate, or switch, the inverter current from one device to the next. LCI drives are typically applied in high-power applications such as compressors, pumps, fans, and process systems where efficiency and proven performance at large motor sizes are important.
Related terms: Medium Voltage Variable Frequency Drive, Synchronous Motor, Back EMF (Electromotive Force), Inverter
Related resources: LCI Drive Replacement
Medium Voltage Variable Frequency Drive
A medium voltage variable frequency drive (MV VFD) is a motor control system designed to operate at voltages typically above 1 kV, most commonly 2.3 kV, 3.3 kV, 4.16 kV, 6.6 kV, 11 kV, and 13.8 kV. MV VFDs are used to control the speed and torque of large electric motors while reducing current, improving efficiency, and enabling controlled starting and stopping in high-power applications.
MV VFDs are applied where motor power levels are too large for low-voltage drives to be practical. Operating at higher voltage allows the same power to be delivered at lower current, which reduces conductor size, cable losses, and thermal stress on electrical equipment. Typical applications include large pumps, compressors, fans, mills, conveyors, and critical process equipment in industries such as oil and gas, mining, water and wastewater, power generation, and heavy manufacturing.
Unlike low-voltage VFDs, medium voltage VFDs use specialized power topologies and switching devices to handle higher voltages safely and efficiently. Common MV VFD designs include multilevel converters such as neutral-point clamped, cascaded H-bridge, and flying capacitor topologies. These designs produce stepped output waveforms that closely approximate a sine wave, reducing harmonic distortion, voltage stress on motor insulation, and common-mode voltage.
MV VFDs typically incorporate input isolation transformers, advanced protection systems, and robust cooling methods such as air-to-air or air-to-water heat exchangers. Switching frequencies are generally lower than in low-voltage drives to limit device losses, making waveform quality and topology selection critical to motor compatibility and power quality performance.
When properly applied, medium voltage VFDs provide precise speed control, reduced mechanical stress, improved process control, and significant energy savings for large motor systems. Correct selection requires careful consideration of motor insulation class, cable length, harmonic limits, fault levels, and system grounding to ensure long-term reliability and safe operation.
Related terms: Variable Frequency Drive (VFD), Three-Level Output, Reflected Wave
Motor Nameplate Parameters
Motor nameplate parameters are the rated electrical and mechanical values provided by the motor manufacturer that define how the motor is designed to operate safely and efficiently. These parameters are physically listed on the motor nameplate and are used by variable frequency drives to configure control, protection, and performance calculations.
Common motor nameplate parameters include rated voltage, rated current, rated frequency, rated speed, horsepower or kilowatt rating, power factor, efficiency, service factor, and insulation class. Together, these values describe the motor’s intended operating limits and thermal capability.
In VFD applications, accurate entry of motor nameplate parameters is critical. The drive uses this information to calculate torque, estimate slip, apply proper current limits, and enable advanced control methods such as vector control or sensorless vector control. Incorrect or missing nameplate data can result in poor torque performance, nuisance faults, overheating, or inadequate motor protection.
Motor nameplate parameters do not describe how the motor will perform under all conditions, but they provide the baseline needed for safe operation. When combined with correct drive setup, load characteristics, and duty cycle, they help ensure reliable motor control and long service life.
Open Loop and Closed Loop Control
Closed-Loop Control: Uses feedback from sensors to adjust the motor’s operation continuously, ensuring it meets the desired performance criteria (e.g., maintaining speed or position).
Open-Loop Control: Operates without feedback, relying solely on input commands. It’s simpler and cheaper but less accurate, as it does not correct for disturbances or changes in load.
Output Filter
An output filter is a device installed between a variable frequency drive and a motor to smooth the drive’s PWM output waveform. Output filters reduce voltage rise time, reflected wave effects, insulation stress, acoustic noise, and electromagnetic interference, improving motor reliability especially in long-cable or sensitive applications.
Overcurrent Fault
An overcurrent fault occurs when motor or drive current exceeds the VFD’s rated limit. This can result from excessive load, rapid acceleration, mechanical binding, or short-circuit conditions.
Overcurrent protection prevents damage to power components and motors. Persistent overcurrent faults indicate improper sizing, incorrect parameter settings, or mechanical issues.
Related terms: Rated Current, Load Inertia, Acceleration/Deceleration Time, Locked Rotor Torque, Overload Protection
Overload Fault
An overload fault occurs when a motor or drive draws excessive current for an extended period, indicating that the load exceeds the rated capacity of the equipment.
Overload Rating
Overload rating specifies how much current or torque equipment can safely handle above its continuous rated value for a limited period of time. It is typically expressed as a percentage of the rated value along with the allowable duration, such as 150 percent for 60 seconds. The higher the overload level, the shorter the allowable time before protective limits are reached. Overload ratings are used to accommodate temporary conditions such as motor starting, acceleration, or short-term load increases without damaging the equipment.
Related terms: Insulation Class, Thermal Management
Overtemperature Fault
An overtemperature fault occurs when internal temperature exceeds safe operating limits in a motor or drive. This protection prevents damage to insulation, semiconductors, and other components.
Overvoltage Fault
An overvoltage fault occurs when the DC bus voltage inside a variable frequency drive rises above the drive’s safe operating limit. This condition triggers a protective shutdown to prevent damage to internal power components.
The most common cause of an overvoltage fault is regenerative energy generated when a motor decelerates too quickly or is driven by an overhauling load. During deceleration, the motor acts as a generator and returns energy to the DC bus. If this energy is not dissipated or absorbed, DC bus voltage increases rapidly.
Overvoltage faults can also be caused by high incoming line voltage, sudden load changes, or inadequate power system impedance. In systems with frequent starts and stops or high inertia loads, overvoltage faults are especially common if deceleration times are set too aggressively.
Mitigating overvoltage faults typically involves increasing deceleration time, adding a braking resistor, enabling regenerative braking, or adjusting drive parameters to better manage returned energy. Proper drive sizing and braking strategy selection are essential to prevent recurring overvoltage conditions.
Related terms: Regeneration, Dynamic Braking, Acceleration/Deceleration Time, DC Injection Braking
Phase Imbalance
Phase imbalance occurs when the voltage or current levels in a three-phase system are unequal. This condition can cause overheating, vibration, and reduced motor efficiency.
Phase Loss
Phase loss occurs when one phase of a three-phase power supply becomes disconnected or unavailable. Motors operating under phase loss can quickly overheat and fail.
PID
The Proportional, Integral, and Derivative (PID) control algorithm is widely used throughout industrial control. When a process loop is created by adding feedback (from a variable such as airflow, pressure, or level) and sent to the VFD, regulation of the process is possible via PID loop control. The VFD’s PID algorithm uses mathematical properties to determine reaction to changes between the system set point and its actual state as measured by feedback.
Pre-Charge Circuit
When line power is first applied to a VFD, the DC bus capacitors are in an uncharged state and behave much like a short circuit.The large inrush of current caused by this shorted state can damage the capacitors and other VFD main same ground.circuit components. A pre-charge circuit limits the inrush current while the capacitors begin to charge. Once the capacitors charge to the target voltage, a contactor bypasses the pre-charge circuit.
Related terms: Line Reactor
Protocol
The term protocol is defined as a set of rules defining how messaging is created for use on fieldbus networks. The rules will detail how communication between devices (such as a VFD) is to be constructed and delivered. Differing protocols generally cannot be mixed in a single network without use of a gateway device. Protocol is generally the software specification portion of “fieldbus protocol.” Examples of protocols are EtherNetIP, DeviceNet, PROFIBUS, PROFINET, Modbus RTU, and Modbus TCP
Pulse-Width Modulation (PWM)
Pulse-width modulation (PWM) is a switching technique used by variable frequency drives to control motor voltage and frequency by rapidly turning power devices on and off. By varying the width and timing of these pulses, the VFD creates an effective AC output with controlled amplitude and frequency from a DC power source.
In a VFD, PWM is generated in the inverter stage using high-speed switching devices such as Insulated Gate Bipolar Transistors (IGBTs). The motor does not receive a smooth sine wave directly. Instead, it responds to the average value of the rapidly switched voltage, which produces controlled torque and speed.
PWM characteristics directly affect motor performance and power quality. Higher switching frequencies improve output waveform quality, reduce audible motor noise, and improve low-speed smoothness. However, higher frequencies also increase switching losses, heat generation, and electromagnetic interference. Lower switching frequencies reduce losses but increase current ripple, acoustic noise, and torque ripple.
Proper selection of PWM settings balances efficiency, thermal limits, motor insulation stress, and system noise. PWM is a core function of all modern VFDs and plays a central role in drive performance, reliability, and compatibility with connected motors.
Related terms: Carrier Frequency, Reflected Wave
Reduced Voltage Soft Starter
An electrical device that gradually increases voltage to an AC motor during startup, providing a "soft start" by limiting high initial current (inrush current) and torque, which reduces mechanical stress on equipment.
Reference Source
The reference source specifies where the VFD’s speed command will come from. It is also known as the Frequency Reference, since most VFDs default to using frequency in hertz as the default speed command unit. Sometimes, but not always, the reference source will come from the same source as the run source.
Regeneration
A motor can become a generator and send power back to the main line whenever the rotor is rotating faster than the stator field. Under such a condition, the load is said to regenerate. This may occur whenever the VFD attempts of the current drawn by a VFD’s to decelerate the motor, or when the load overhauls the motor. In this state, the motor’s back electromagnetic field is greater than applied voltage, which causes than applied voltage, which causes increasing bus voltage and probable VFD fault. To avoid VFD faults during regeneration, some form of power dissipation is used, such as dynamic braking or line regeneration.
Related terms: Overvoltage Fault, Braking Resistor, DC Bus, Common Busing
Retrofit
Retrofit refers to the process of upgrading or modifying existing equipment by adding or replacing components, such as installing a variable frequency drive on a motor system that previously operated across the line. Retrofit projects improve efficiency, control, and reliability without requiring complete system replacement.
Related terms: Variable Frequency Drive (VFD), Induction Motor, Efficiency, Harmonics, K-Factor Transformer
Run Source
Every VFD needs to be configured as to where the command to start the motor will come from. Generally there are local (e.g. the keypad) and remote (e.g. a network command) sources of the start command. Choosing which source, local or remote, can usually be selected via the keypad itself. Warning: Generally only a single Run Source can be active at one time. All the other sources will be ignored.
Self-Synchronization
The ability of a synchronous motor to maintain synchronization without constant external adjustment.
Sensorless Vector Control
Sensorless vector control is a type of vector control in which the VFD estimates motor speed and rotor position without using a physical feedback device such as an encoder or resolver. Instead, the drive calculates motor behavior using measured voltage, current, and an internal motor model.
By estimating rotor position electronically, sensorless vector control provides many of the torque and speed regulation benefits of closed-loop control while reducing system cost, wiring complexity, and mechanical components. It offers significantly better low-speed torque and dynamic response than basic V/F mode.
Sensorless vector control performs best at moderate to high speeds where motor electrical characteristics are easier to model accurately. At very low speeds or zero speed, estimation accuracy decreases because back EMF is minimal, which limits torque precision and speed holding capability compared to encoder-based systems.
Sensorless vector control is widely used in industrial applications that require good torque performance and responsiveness without the added cost or maintenance of feedback devices. It is commonly applied in conveyors, pumps, fans, mixers, and general-purpose machinery.
Related terms: Vector Control, V/F Mode, Motor Nameplate Parameters
Sine Wave
A sine wave is a smooth, continuous waveform that represents ideal alternating current or voltage, characterized by a single frequency and a consistent amplitude. Utility power delivered to facilities is intended to be a pure sine wave.
In motor applications, a true sine wave produces smooth torque, low electrical noise, and minimal stress on motor insulation. However, variable frequency drives do not output a pure sine wave. Instead, they use pulse-width modulation (PWM) to approximate a sine wave by rapidly switching voltage on and off.
The difference between a true sine wave and a PWM waveform affects motor heating, audible noise, torque ripple, and insulation stress. Output filters such as sine wave filters or dV/dt filters may be used when a closer approximation to a sine wave is required, particularly with long motor cables or sensitive motors.
Related terms: Carrier Frequency, Reflected Wave, Load Reactor, Insulation Class
Soft Start
Soft start is a motor starting method that gradually increases applied voltage to reduce inrush current and mechanical stress during startup. It allows smoother acceleration than across-the-line starting but does not provide speed control during normal operation. Soft starters are commonly used as a lower-cost alternative to VFDs when only controlled starting is required.
Startup
Startup is the process of initially energizing and bringing a variable frequency drive and motor system into operation. It includes powering the system, verifying basic functionality, and gradually running the motor under controlled conditions. Startup may be part of commissioning but focuses specifically on safely initiating operation and confirming proper response.
Related terms: Pre-Charge Circuit, Inrush Current, Acceleration/Deceleration Time, Run Source
Three Contactor Bypass
This is another setup that allows motor operation across the line or through the VFD. One contactor is installed between the incoming line and VFD input. Another (bypass) is installed between the incoming line and motor. A third is installed between the VFD output and motor. A three-contactor bypass allows the motor to be run directly from the incoming line, bypassing the VFD. This allows for VFD servicing while the motor is being run from the incoming line, and can also be used to run the motor at constant speed at a higher efficiency than with the VFD in circuit.
Related terms: Bypass
Three-Level Output
A three-level output is a modified output PWM pattern that uses extra IGBTs, a neutral point clap, and custom switching patters to achieve three possible output voltage levels (E/2, 0, -E/2 with E/2 being the DC bus mid-point voltage). A three-level output from a VFD will naturally lead to lower common-mode voltage and noise and dramatically reduce bearing currents that cause premature motor bearing failures.
Related terms: Pulse-Width Modulation (PWM), Carrier Frequency, Insulated Gate Bipolar Transistor (IGBT), Bearings, Ground
Two Contactor Bypass
A two contactor bypass is a VFD setup that allows motor operation across the line or through the VFD. One contactor is installed between the incoming line and motor, while the other is installed between the VFD output and motor. A two-contactor bypass allows the motor to be run directly from the incoming line, bypassing the VFD. The bypass can be used to run the motor at a constant speed directly from the incoming line in case of VFD failure.
Related terms: Bypass
Under Voltage Fault
An under voltage fault occurs when the incoming supply voltage or DC bus voltage drops below the VFD’s minimum operating threshold. This can be caused by weak power systems, voltage dips, or upstream disturbances.
Under voltage faults prevent unstable operation and protect internal components. Frequent occurrences may indicate inadequate supply capacity or wiring issues.
Related terms: Voltage Rating, Single-Phase, Three-Phase
V/F Mode
V/F mode, also called volts-per-hertz control, is a basic variable frequency drive control method that maintains a constant ratio between output voltage and frequency. This approach keeps motor magnetic flux approximately constant as speed changes.
V/F mode controls motor speed by adjusting frequency while proportionally scaling voltage. Because it does not actively model or control motor torque, V/F mode is considered a scalar control method. It is simple, stable, and compatible with most standard induction motors.
V/F mode performs well in applications with predictable, steady loads and minimal torque variation, such as pumps, fans, and blowers. It is less effective in applications requiring fast torque response, accurate low-speed control, or operation under rapidly changing load conditions.
Compared to vector control methods, V/F mode offers lower complexity and easier setup but reduced dynamic performance. It remains a common choice where simplicity, reliability, and cost are prioritized over precise torque control.
Related terms: Volts Per Hertz (V/Hz), Induction Motor, Magnetic Flux, Saturation, Torque-Speed Curve
Variable Frequency Drive (VFD)
A variable frequency drive, commonly abbreviated as VFD, is an electronic device used to control the speed, torque, and direction of an AC electric motor by varying the frequency and voltage of the power supplied to the motor.
A VFD operates by first converting incoming AC power into DC power using a rectifier, then filtering it through a DC bus, and finally converting it back into controlled AC power using an inverter. By adjusting output frequency and voltage, the VFD allows precise control of motor operation across a wide speed range.
Variable frequency drives are widely used to improve energy efficiency, reduce mechanical stress, and enhance process control. They enable soft starting and stopping, reduce inrush current, and allow motors to match load demand rather than operating at constant speed. This is especially beneficial in applications such as pumps, fans, conveyors, and compressors.
Modern VFDs support multiple control methods, including V/F mode, vector control, and sensorless vector control, allowing them to meet a wide range of performance requirements. They also incorporate protection functions such as overcurrent, overvoltage, and undervoltage fault detection to safeguard both the motor and the drive.
Proper selection and configuration of a VFD improves system efficiency, extends motor life, and increases operational flexibility. As a result, VFDs are a core component of modern motor-driven systems in industrial, commercial, and HVAC applications.
Related terms: Pulse-Width Modulation (PWM), Vector Control, Rectifier/Converter, Insulated Gate Bipolar Transistor (IGBT)
Vector Control
Vector control is an advanced variable frequency drive control method that independently controls motor torque and magnetic flux. It does this by mathematically modeling the motor and regulating current components in real time, rather than simply controlling voltage and frequency.
Unlike basic V/F mode, vector control treats the motor as a dynamic system. The VFD continuously calculates the motor’s operating state and adjusts output to maintain precise torque control, even as load conditions change. This results in faster response, better speed regulation, and improved performance at low speeds.
Vector control significantly improves torque production at low and zero speed compared to scalar control methods. It allows the motor to deliver consistent torque without excessive slip or current, making it well suited for applications with demanding load characteristics, frequent speed changes, or tight process control requirements.
Vector control can operate with or without a speed feedback device. When used without feedback, it relies on motor models and electrical measurements. When combined with an encoder or resolver, it becomes closed-loop vector control, offering even higher accuracy and dynamic performance.
While vector control increases system complexity and setup requirements, it provides substantial benefits in efficiency, torque control, and stability. It is commonly used in conveyors, extruders, hoists, machine tools, and other applications where precise motor control is critical.
Related terms: Sensorless Vector Control, V/F Mode, Motor Nameplate Parameters, Torque, Magnetic Flux
Volts Per Hertz (V/Hz)
Volts per hertz (V/Hz) describes the ratio of output voltage to output frequency applied to an AC motor by a variable frequency drive. Maintaining a proper V/Hz ratio keeps motor magnetic flux at an appropriate level as speed changes.
In an induction motor, magnetic flux is directly related to the applied voltage and frequency. If voltage is reduced too much relative to frequency, flux decreases and available torque drops. If voltage is too high for a given frequency, flux increases excessively, leading to magnetic saturation, higher current, and overheating. V/Hz control balances this relationship to produce stable motor operation.
V/Hz is the foundation of basic V/F mode control in VFDs. The drive increases voltage proportionally as frequency increases, following a predefined V/Hz curve. This method is simple, reliable, and effective for applications with steady loads and modest torque requirements.
While V/Hz control does not actively regulate torque, it provides predictable motor behavior and wide compatibility with standard induction motors. More advanced control methods such as vector control build on this concept by dynamically adjusting flux and torque independently.
Related terms: Torque, Torque-Speed Curve