Everything about Variable Frequency Drives (VFDs) – quick answers to your questions
Welcome to the Fluxcon FAQ. On this page, you will find clear and practical answers about the Variable Frequency Drive (VFD) — also known as a frequency inverter or frequency converter — and its application in electric motors.
Discover how a VFD controls motor speed and torque, reduces energy consumption (for example in pumps and fans), minimizes mechanical wear, and improves process stability in HVAC systems, water treatment, compressors, and conveyor systems.
Use this FAQ to quickly navigate through:
– fundamental principles
– installation and commissioning
– troubleshooting
– optimization
Prefer personal advice? Contact our specialists.
FAQ — Fundamentals of Variable Frequency Drives and electric motors
A Variable Frequency Drive (VFD) is a power electronic device that converts fixed mains voltage and frequency into a variable output voltage and frequency. This enables precise control of motor speed and torque.
Instead of running a motor continuously at 50/60 Hz, a VFD allows dynamic speed adjustment. This results in:
– significant energy savings (especially in fans and pumps, based on affinity laws)
– improved process control (stable pressure, flow, and speed)
– reduced mechanical wear due to soft start and stop
Technically, a VFD consists of three main components:
1. A rectifier (AC to DC conversion)
2. A DC bus (energy storage and smoothing)
3. An inverter (DC back to AC using PWM with variable frequency and voltage)
With vector control or field-oriented control (FOC), modern drives can regulate both speed and torque with high precision, either sensorless or with encoder feedback.
The most commonly used English term is Variable Frequency Drive (VFD).
Other widely used terms include:
– AC Drive
– Frequency Converter (FC)
– Variable Speed Drive (VSD)
– Adjustable Speed Drive (ASD)
Although these terms are often used interchangeably, they all refer to electronic systems that control the speed of an AC motor by varying frequency and voltage.
In industrial applications, VFD is the dominant term, especially for asynchronous and synchronous motors (IM, PMSM, SynRM).
A Variable Frequency Drive (VFD) is an electronic device used to control the speed of AC motors.
It works by converting incoming AC power into DC, stabilizing it in the DC bus, and then converting it back into AC with adjustable frequency and voltage using PWM (Pulse Width Modulation).
This allows the motor to deliver exactly the required speed and torque, improving both efficiency and process performance.
VFDs support different control methods:
– V/f control (simple and robust)
– Vector control / FOC (high precision and dynamic performance)
Depending on the application:
– Sensorless control is often sufficient
– Encoder feedback is recommended for high precision or low-speed operation
A Frequency Converter (FC) is another term for a Variable Frequency Drive.
It is commonly used in Europe and refers to a device that converts fixed grid frequency into variable frequency and voltage for AC motor control.
Functionally, an FC is identical to a VFD and consists of:
– a rectifier
– a DC bus
– an inverter using PWM
Applications include HVAC, water systems, material handling, compressors, lifting systems, and industrial processes.
ASD (Adjustable Speed Drive) and VSD (Variable Speed Drive) are broader terms for systems that control motor speed.
They include:
– AC drives (VFDs)
– DC drives
– mechanical speed control systems
In most modern industrial applications, ASD/VSD typically refers to a VFD.
The main benefit is demand-based operation instead of simple on/off control, resulting in:
– lower energy consumption
– improved process stability
– reduced wear and maintenance
The primary purpose of a Variable Frequency Drive is to control the speed and torque of an electric motor.
By adjusting frequency and voltage, a VFD allows systems to deliver exactly the required output (flow, pressure, speed), instead of operating at full capacity and using mechanical throttling.
Key benefits include:
– energy savings
– reduced mechanical stress
– lower noise levels
– improved product quality
Additional features often include:
– soft start and stop
– integrated PID control
– communication via fieldbus (Modbus, Profinet)
– advanced motor protection
A VFD operates on the principle that motor speed is proportional to the supply frequency.
The process consists of three steps:
1. AC is converted to DC via a rectifier
2. The DC voltage is stabilized in the DC bus
3. The inverter generates a new AC signal using PWM with adjustable frequency and voltage
By maintaining the correct V/f ratio or using vector control, the motor operates efficiently across the entire speed range.
Modern VFDs use microprocessors or DSPs to control switching devices (IGBTs or MOSFETs) at high frequency, creating a simulated sine wave output.
EMC and harmonic mitigation are critical in environments with sensitive IT power supplies. The use of line reactors, DC chokes or Active Front End (AFE) drives can significantly reduce total harmonic distortion (THDi). Redundancy (N+1) and rapid alarm handling help minimize the risk of thermal events. See Wikipedia: data center and the Fluxcon wiki.
Practical references: PID/commissioning FLC500 p.62–64, p.44–48; harmonics/filters p.139–140. HVAC/fan case: Fluxcon blog.
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In oil and gas, Variable Frequency Drives (VFDs) and electric motors control pumps (injection/transport), compressors (gas transmission), drilling systems, winches, and cooling fans. Variable speed operation reduces energy consumption, prevents water hammer, enables precise pressure and flow control, and improves process safety.
Vector control / FOC with encoder feedback is widely used for hoisting and winch applications. AFE drives can regenerate braking energy in cyclic processes.
Key considerations:
– ATEX environments (explosion safety)
– robust EMC design
– harmonic mitigation in weak grids
– redundant control systems
See Wikipedia: petroleum, natural gas, and the Fluxcon wiki.
In wind turbines, VFDs and electric motors are used for yaw and pitch drives, cooling fans, and auxiliary pumps. Variable speed enables precise positioning (yaw) and fast blade angle control (pitch), optimizing power output and reducing mechanical stress.
Reliable drive control minimizes maintenance intervals and extends component lifetime.
Critical factors:
– EMC compliance
– harsh environmental conditions (temperature, humidity, salt)
– redundancy and remote monitoring
In solar farms, VFDs and electric motors are used in solar trackers, cooling systems, and cleaning pumps. Variable speed enables precise positioning with minimal energy consumption and reduced wear.
In cooling systems, VFDs regulate airflow based on demand, significantly improving energy efficiency.
Key considerations:
– EMC and temperature
– dust protection (IP rating)
– fieldbus integration for predictive maintenance
In electric vehicles, traction inverters (a type of VFD) control the drive motor (IM, PMSM, or SynRM). The DC battery supplies power to the inverter, which uses vector control or FOC to regulate torque and speed.
Regenerative braking feeds energy back into the battery during deceleration.
Key requirements:
– high power density
– thermal management
– EMI robustness
– functional safety
In railway systems, VFDs and electric motors control traction motors under varying loads. Vector control and four-quadrant operation enable smooth acceleration, precise speed control, and efficient regenerative braking.
Systems must meet strict requirements:
– high reliability
– strong EMC performance
– vibration resistance
Ports use VFDs and electric motors in cranes, winches, conveyors, pumps, and ventilation systems.
Benefits:
– high torque at low speed (lifting)
– precise positioning with encoder feedback
– regenerative braking
– energy savings
Harsh environments require:
– high IP ratings
– corrosion-resistant coatings
– robust EMC design
In recycling plants, VFDs control shredders, conveyors, blowers, and sorting systems.
Advantages:
– process balancing
– reduced mechanical stress
– overload protection
– fast stop response
Dusty environments require strong EMC and proper cooling.
See Wikipedia: recycling.
In metal processing, VFDs and motors control rolling mills, pickling lines, winders, and extraction systems.
Key benefits:
– precise torque control (web tension)
– efficient energy use
– regeneration during braking
High power systems require:
– harmonic mitigation
– robust EMC
Mining systems use VFDs for conveyors, crushers, ventilation, and hoisting.
Benefits:
– soft start
– torque control at low speed
– energy savings
Environmental factors:
– dust, moisture, explosion risk (ATEX)
– long cable runs → EMC considerations
See Wikipedia: mining.
Cement plants use VFDs in mills, fans, conveyors, and kilns.
Benefits:
– reduced mechanical stress
– stable process control
– large energy savings in fan applications
Heavy-duty environments require:
– high IP ratings
– robust EMC design
See Wikipedia: cement.
Although frequency inverters and electric motors are designed to operate very efficiently, internal losses still occur.
The main sources of loss are conduction losses in semiconductors (IGBTs/MOSFETs), switching losses at high
frequencies, resistive losses in chokes and cabling, and losses in the DC bus capacitors. In addition,
heat is generated in control circuits and control boards.
These losses are generally low (typically 2–5% of the input power), which helps keep overall system efficiency high. Good
thermal management (heat sinks, fans) and proper filter sizing can further reduce losses.
See Wikipedia: inverter and the
Fluxcon wiki on energy.
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A heat sink is a metal structure that transfers heat away from power components such as IGBTs and MOSFETs into the surrounding environment.
In frequency inverters and electric motors, this is essential to prevent thermal overload.
Heat sinks are typically made of aluminium or copper and include cooling fins that increase surface area, thereby improving convection and heat dissipation.
In larger drives, heat sinks are combined with forced ventilation (fans) or even liquid cooling for heavy industrial applications.
The quality and sizing of the heat sink directly affect the reliability and service life of the frequency inverter.
For more background information, see Wikipedia: heat sink
and the Fluxcon wiki.
Frequency inverters and electric motors generate heat due to losses in semiconductors, capacitors and chokes.
Fans provide active cooling by moving airflow across heat sinks and internal components. This prevents overheating,
extends the life of sensitive parts and ensures stable operation.
Without fans, larger VFDs would become unnecessarily large or inefficient. In low-power applications (for example microdrives),
natural convection may be sufficient, but at higher power levels (>1–2 kW), fans are almost always required.
See Wikipedia: fan and the
Fluxcon wiki.
In frequency inverters and electric motors, capacitors in the DC bus have a dual role: they temporarily store energy
and suppress voltage ripple caused by rectification. In this way, they stabilise the DC link and provide the inverter with a
constant voltage. Without capacitors, the pulsating DC voltage would be too unstable and motors would not run smoothly.
In addition, capacitors absorb peaks during regeneration and contribute to the suppression of harmonic distortion.
Proper sizing and maintenance of capacitors are crucial: ageing or drying out can lead to faults or a shorter service life
of the frequency inverter.
See Wikipedia: capacitor
and the Fluxcon EMC wiki.
Chokes, also known as inductors, are used in frequency inverters and electric motors to reduce current ripple and harmonic
currents. They work together with the capacitors in the DC bus to ensure a stable DC voltage. In addition, they limit inrush currents
and improve capacitor life by smoothing peak currents.
In practice, both DC chokes (in the DC link) and AC line reactors (on the mains side) are used to reduce harmonic
distortion. This helps meet EMC and grid standards and prevents unnecessary losses in the installation.
See Wikipedia: inductor and
the Fluxcon EMC wiki.
An EMC filter limits electromagnetic interference that frequency inverters and electric motors can cause
in power networks or nearby electronics. Due to the fast switching of IGBTs, high dv/dt and di/dt values are generated,
which can be conducted or radiated. The EMC filter, often built from chokes and capacitors, suppresses this interference
and helps ensure compliance with EMC regulations.
EMC filters protect both the installation itself (PLCs, sensors) and the surrounding environment, while also improving system reliability.
They are often integrated into the VFD or added externally in installations with stricter requirements.
See Wikipedia: EMC
and the Fluxcon EMC wiki.
A dv/dt filter reduces the rate of voltage rise (dv/dt) caused by the fast switching in the inverter
of frequency inverters and electric motors. High dv/dt can place stress on motor winding insulation systems,
especially in older motors or in installations with long cables. A dv/dt filter, usually built from coils and capacitors,
extends the service life of motors and cables and reduces overvoltage reflections.
They are often used in installations with cable lengths over 50 metres or with motors that do not have VFD-rated insulation.
See Wikipedia: EMC
and the Fluxcon wiki.
A sine filter converts the block-shaped PWM output of frequency inverters and electric motors into an almost
perfect sine wave. It consists of LC filters that remove the high-frequency components. As a result, the voltage on the
motor side becomes much cleaner, which is especially important for older motors and extremely long cables.
Sine filters reduce insulation stress, limit bearing currents and extend motor service life. They are used
in critical installations such as marine drives, mining and long conveyor systems.
See Wikipedia: low-pass filter
and our Fluxcon wiki.
An output filter is used in frequency inverters and electric motors when the motor or the installation
is sensitive to the high switching frequencies and harmonic voltages generated by the inverter. Filters such as
dv/dt filters and sine filters reduce voltage transients and make the output voltage more sinusoidal.
This protects motor windings, extends bearing life and reduces electromagnetic interference.
They are especially useful with long motor cables (typically >50 m), older motors without VFD-rated insulation, or in
critical applications such as marine installations, mining and HVAC systems with strict EMC requirements. In some cases,
the motor manufacturer explicitly requires the use of an output filter.
See Wikipedia: low-pass filter
and the Fluxcon wiki.
For practical guidance, see the
FLC500 manual p.139–140.
The block diagram of a frequency inverter shows the sequential conversion stages: first the
rectifier (AC → DC), then the DC bus with capacitors and chokes, and finally the
inverter (DC → AC with variable frequency via PWM). In addition, the block diagram includes auxiliary functions
such as control electronics, measuring circuits, EMC filters, brake chopper and communication interfaces.
This diagram clearly shows how energy flows through the VFD and where filtering, cooling and protective functions
are integrated. It forms the basis for both design and maintenance.
Example diagrams can be found in the
Fluxcon Applications Manual p.14
and Wikipedia: frequency inverter.
The internal structure of frequency inverters and electric motors combines power electronics and
control circuits. Physically, VFDs consist of three main sections: a rectifier module, a
DC link with capacitors and chokes, and an inverter module with IGBTs or MOSFETs. In addition,
there are cooling systems (heat sinks, fans or liquid cooling), EMC filters, I/O modules, displays and
communication cards.
The design is modular, which simplifies maintenance and enables different power classes and functions.
In heavy-duty applications, redundant fans or expansion modules may be added.
More details can be found in the
Fluxcon Applications Manual p.16
and on the Wikipedia page about inverters.
An H-bridge is a circuit of four electronic switches (for example MOSFETs or IGBTs) that together
convert a DC voltage into an AC voltage for one motor phase. By opening and closing the switches in pairs,
the voltage polarity can be reversed, allowing the current direction to alternate and a sine-shaped voltage
to be built up using PWM.
In a three-phase inverter, three H-bridges are used, one for each phase. This configuration forms the basis of
almost all frequency inverters and electric motors.
See Wikipedia: H-bridge and
our Fluxcon wiki.
Check the available options and follow the BMS commissioning procedure (device instance, object list).
More information: Fluxcon wiki — Interfaces.
HVAC examples: VFD for fans.
Step 1: Wiring: use shielded twisted pair cable, 120 Ω termination at both ends, correct A/B polarity, and 360° shielding at cabinet entry.
Step 2: Parameters: set slave address, baud rate, parity and stop bits.
Step 3: Bus discipline: use a line topology with minimal stubs.
Minimise EMC influence by separating power and signal cables and crossing them at 90°.
Verify communication using a Modbus tester and log timeouts/CRC errors.
See I/O and communication commissioning:
Applications Manual p.18–22,
and EMC guidelines: Fluxcon wiki — EMC.
On RS485/Modbus RTU, typically up to 32 nodes can be connected without repeaters (depending on transceivers and cable quality).
With repeaters or line drivers, this can be extended.
Keep the bus layout simple and short, document addresses clearly, and ensure correct termination and biasing.
On Ethernet-based systems (Modbus TCP, Profinet, EtherNet/IP), limits are mainly determined by network load and polling frequency.
Guidelines and examples: Fluxcon wiki — Interfaces.
Diagnostics and logging on the drive side: FLC500 p.86–92.
Activate pump mode or the application wizard and select PID control with pressure or flow feedback.
Configure: setpoint source (BMS or analogue input), feedback source (4–20 mA transducer), PID parameters, minimum/maximum frequency, sleep/wake, anti-jam, and optionally multi-pump (master/assist/standby).
HVAC and pump examples:
VFD on pump.
PID and I/O setup: Applications Manual p.18–22.
Select fan mode (HVAC) and configure PID control for pressure, flow or temperature (e.g. VAV or duct systems).
Set minimum and maximum frequency, night mode, energy-saving mode (flux optimisation), and optionally fire mode where required.
Best practices and examples:
Fluxcon — Fans.
PID configuration and I/O: Applications Manual p.18–22.
Enable the multi-pump function: define master and assist/standby pumps, select cascade logic, configure equal runtime or lead–lag, and set sleep/wake parameters.
Connect the pressure transducer to the master drive and share status via digital I/O or fieldbus.
This balances energy consumption, runtime and redundancy.
Examples and PID tips:
Fluxcon — Pump.