Variable frequency drive: what it is, how it works and what it is used for
A variable frequency drive controls the speed and torque of an AC motor by adjusting the output frequency and voltage. It is used to adapt the motor to the process, regulate pumps and fans, smooth starts and improve installation control.
What does a variable frequency drive do in an electric motor?
A variable frequency drive is a power electronic device installed between the mains supply and the motor. It receives alternating current at a fixed frequency, converts it internally and generates a new AC output with the frequency and voltage needed to control speed, torque, acceleration and deceleration.
In an induction motor, frequency determines the speed of the rotating magnetic field. The drive also manages voltage, flux and current to generate the torque demanded by the load. That is why it is not simply a speed regulator: it is a power converter, a motor controller and an integration point for PLCs, sensors and industrial networks.
You will also find it referred to as VFD (Variable Frequency Drive), variable speed drive, VSD, frequency converter or AC drive. In common industrial use, these terms usually describe electronic drives used to control AC motors.
What is a variable frequency drive and what does it control?
It is a power electronic device that modifies the frequency and voltage applied to an AC motor. This allows it to adapt motor speed, torque and motion ramps to the actual demand of the machine or process.
It controls the motor supply, not just its speed
The setpoint can come from a keypad, potentiometer, PLC, analog input, industrial network or an internal controller using process sensors.
Supply voltage and frequency are practically fixed.
It rectifies, filters, switches, calculates and protects within the functions provided by the manufacturer.
Speed, torque or process variable adapted to demand.
How a variable frequency drive works: rectifier, DC bus and inverter
The most common configuration first converts the AC input into direct current, stabilizes this energy in an intermediate circuit and then converts it back into a controlled AC output using power semiconductors.
Four stages working in coordination
The diagram is conceptual. The exact topology, components and functions vary by manufacturer, power rating, voltage, and whether the drive is regenerative or low-harmonic.
- RectificationConverts the AC input into a DC voltage.
- Intermediate circuitConditions and temporarily stores energy.
- InversionElectronic switching to synthesize the motor output.
- ControlExecutes the algorithm, ramps, limits and communications.
Power supply available to the drive.
Converts the input AC into DC.
Capacitors and, depending on the design, inductors to stabilize the circuit.
Semiconductors that generate the controlled three-phase output.
The inverter switches thousands of times per second
In many drives, power transistors—commonly IGBTs in many conventional product ranges—connect the motor phases to the DC bus following a modulation strategy. The width and sequence of the pulses determine the equivalent voltage and frequency.
Control algorithms such as scalar V/f control, vector control or other manufacturer-specific strategies operate on top of this power stage. The choice depends on speed accuracy, torque response, motor type and the required dynamics.
How does it regulate the speed and torque of an electric motor?
The synchronous speed of an AC motor depends directly on electrical frequency and the number of poles. In an induction motor, the rotor turns slightly below this speed because of slip.
Frequency, poles and revolutions
The formula gives the speed of the magnetic field. The actual mechanical speed of an asynchronous motor depends on slip, which changes with load and the applied control.
ns = 120 · f / pns: synchronous rpm · f: Hz · p: number of poles| Poles | 50 Hz | 60 Hz |
|---|---|---|
| 2 | 3,000 rpm | 3,600 rpm |
| 4 | 1,500 rpm | 1,800 rpm |
| 6 | 1,000 rpm | 1,200 rpm |
| 8 | 750 rpm | 900 rpm |
Estimate motor speed
Change the frequency, number of poles and an indicative slip value. The result does not replace motor data or commissioning.
Slip is not constant, and the drive may estimate or compensate for it depending on the control algorithm. Synchronous and reluctance motors follow different control considerations.
What is a variable frequency drive used for? Functions and benefits
Its main function is to adapt the motor to what the process needs at any given time. This makes it possible to regulate speed, torque, flow or pressure, reduce mechanical stress, automate sequences and avoid unnecessary energy consumption when the load does not require full-speed operation.
Regulate speed and production
It allows speed, flow, pressure, conveying rate, dosing or linear speed to be varied without relying solely on valves, dampers, gearboxes or other mechanical elements.
Control torque
It can limit or control motor torque to adapt it to acceleration, variable loads, material tension, extrusion, lifting or processes at risk of jamming.
Start and stop with ramps
It reduces abrupt speed changes and can limit current peaks and the stresses associated with direct-on-line starting. In pumps, a properly configured ramp can also help reduce hydraulic transients.
Maintain a setpoint
Many drives include PID control to maintain pressure, flow, level, temperature or another variable using a sensor signal.
Integrate with automation
Inputs and outputs, industrial communications, diagnostics, alarms, logs and monitoring allow the motor to be coordinated with PLC, BMS, SCADA or maintenance systems.
Reduce unnecessary work
When the process does not need 100% speed, the drive can reduce the energy delivered to the motor. The potential is especially significant in centrifugal pumps and fans operating for many hours at partial load.
A drive does not by itself guarantee energy savings, functional safety, complete motor protection, maintenance elimination or compatibility with every motor. Available functions, limits and external measures depend on the model and the project.
Variable frequency drive applications and load types
To select a drive correctly, it is necessary to know how the resisting torque changes with speed. Two motors with the same rated power may require different drives if they operate a fan, conveyor, extruder, pump or winder.
Centrifugal pumps and fans
Torque rises approximately with the square of speed and power with the cube, within similarity-law conditions. These are the applications with the best-known energy-saving potential.
- Ventilation and air conditioning
- Water pumping
- Cooling towers
- Some centrifugal compressors
Conveying, mixing and processing
The load requires similar torque over a wide speed range. Power varies approximately linearly with speed, and selection must pay particular attention to current and overload capacity.
- Conveyor belts
- Mixers and extruders
- Positive-displacement pumps
- Reciprocating or rotary compressors depending on the process
Winding and certain machines
As speed increases, the required torque decreases to maintain approximately constant power. The field-weakening region is often used.
- Winders and unwinders
- Some machine-tool spindles
- Material-tension processes
- Specific traction applications
Variable frequency drive or soft starter: which solution is best?
They are not equivalent solutions. The decision depends on whether speed must be regulated during operation, starting current limited, torque controlled, braking performed, energy saved, or the motor simply brought up to rated speed.
| Criterion | Direct-on-line starting | Star-delta | Soft starter | Variable frequency drive |
|---|---|---|---|---|
| Speed during operation | Fixed | Fixed | Normally fixed | Adjustable |
| Starting control | Abrupt | Stepped reduction | Voltage or torque ramp | Frequency, voltage and control ramp |
| Starting current | High | Lower than direct-on-line | Limitable | Controlled according to design and load |
| Torque control | No | Very limited | During start/stop depending on model | Throughout the entire cycle |
| Energy-saving potential through speed reduction | No | No | No when bypassed at full speed | Yes, if the process allows it |
| Complexity and initial cost | Minimal | Low | Medium | Higher |
| Typical application | Small motor or simple fixed-speed process | Compatible motor at fixed speed | Fixed speed with smooth starting and stopping | Variable speed, process control or torque control |
The table is conceptual. Actual behavior depends on the motor, load, mains supply, protections, configuration and specific device. In many full-speed applications, a soft starter can be a simpler solution than a drive.
How to select a variable frequency drive step by step
kW is only the starting point. Sizing should be based on motor current, voltage, load, overload capacity, ambient conditions, speed range, braking, electromagnetic compatibility and control integration.
Start with the motor nameplate and the machine’s actual duty cycle
Rated current is a decisive criterion because the drive must be able to supply the current required by the motor in operation and during the expected overloads.
Supply and motor voltage
Check phases, mains voltage and frequency, output voltage, star/delta connection and motor compatibility. A drive does not necessarily increase the available voltage.
Current and overload
Select according to the permissible current for the required duty. Distinguish normal-duty from heavy-duty service and validate overload duration and percentage.
Load type
Centrifugal fan or pump, constant torque, constant power, lifting, high inertia or regenerative load all imply different criteria.
Speed range and cooling
At low speed, a self-ventilated motor cools less effectively. It may be necessary to reduce the load, add independent ventilation or select a suitable motor.
Acceleration, inertia and braking
Calculate dynamic torque and braking energy. Depending on the cycle, a resistor, chopper, shared DC bus or regenerative solution may be required.
Environment and enclosure
Temperature, altitude, dust, humidity, corrosion, vibration, enclosure ventilation and IP rating may require derating or a different mounting type.
Cable, motor and filters
Cable length, motor insulation and voltage may make dV/dt, sine-wave or common-mode filters, or bearing-related measures, necessary.
Control, communications and safety
Define I/O, signals, encoder, fieldbus, PID, STO or other functions. Always verify the safety level and architecture required by the machine.
Two motors with the same power can have different currents, speeds, efficiencies and torque demands. In addition, a drive may have a different current rating depending on overload class, temperature, altitude or switching frequency.
How to install a drive: cooling, EMC, harmonics and motor considerations
A good selection can fail if the installation does not follow the manual. Physical layout, grounding, cable routing, ventilation, protections and filters are part of the drive system.
Mains side
Drive inputMotor side
PWM outputElectrical safety: the DC bus can retain voltage after disconnection
Installation, commissioning and maintenance must be carried out by qualified personnel. Follow the discharge time specified by the manufacturer, lock out energy sources, verify absence of voltage and apply the safety rules for the machine and installation. Stopping the motor from the keypad is not the same as isolating the drive from the mains.
Standards and efficiency of variable frequency drives
In Europe, drives are part of the ecodesign framework for motors and drive systems. In addition, the IEC 61800 series covers aspects such as drive-system definitions, safety, electromagnetic compatibility, functional safety and energy efficiency.
The drive must be evaluated as part of the complete system
A drive system includes power conversion, control, the motor, cabling and the driven machine. Compliance of one component does not remove the responsibility to integrate the complete system correctly.
Official sources on definition and operation
ABB — What is a variable speed drive? Explains the position of the drive between the mains and the motor and the rectification, DC-bus and inversion stages.
Schneider Electric — What a drive is and how to select one Summarizes AC–DC–AC conversion and the basic selection criteria.
Official sources on sizing, loads and braking
ABB — Technical guide book Brings together guides on drives, harmonics, sizing, bearing currents, braking and motion control.
ABB — Dimensioning of a drive system Explains motor and drive selection according to process, torque, speed and current.
ABB — Power regeneration Describes the use of regenerative drives in cyclic or continuous braking applications.
Official source on loads and energy efficiency
U.S. Department of Energy — Adjustable Speed Drive Part-Load Efficiency Explains variable- and constant-torque loads and the need to evaluate the duty cycle.
Official sources on standards and compatibility
EUR-Lex — Regulation (EU) 2019/1781 European ecodesign framework for electric motors and variable speed drives.
IEC 61800-3:2022 Electromagnetic compatibility of adjustable speed drive systems.
IEC 61800-5-1:2022 Electrical, thermal and energy safety requirements.
IEC 61800-9-2:2023 Determination and classification of energy efficiency for drive systems.
Frequently asked questions about variable frequency drives
What exactly is a variable frequency drive?
It is a power electronic converter installed between the mains and the motor that regulates output frequency and voltage to control speed, torque, acceleration and deceleration.
Do VFD and VSD mean the same thing?
In common industrial use for AC motors, they are often used as synonyms. Technically, VSD is a broader term for any variable speed drive, while VFD specifically emphasizes frequency variation.
Does a drive always reduce the electricity bill?
No. It saves energy when it allows work that the process does not need to be reduced. The potential is very high in many centrifugal pumps and fans at partial load, but it can be small or zero in applications that must always operate at full speed and full load.
What does it mean that a pump follows the cube law?
Under ideal similarity conditions, flow varies with speed, pressure with the square, and power with the cube. For example, at 80% speed, theoretical power is approximately 51.2%. The actual system curve can change this result significantly.
What is the difference compared with a soft starter?
A soft starter primarily regulates voltage during starting and, depending on the model, stopping. Afterwards, the motor normally runs at mains frequency. A drive controls frequency, voltage and motion throughout operation.
Can I choose the drive based only on kW?
No. You should review at least current, voltage, load type, overload, speed, cooling, braking, environment, cable, motor, filters and control.
Can a motor run faster than its rated speed?
Some motors can operate above base frequency, but available torque decreases in field weakening and the mechanical, thermal, bearing and driven-machine limits must be respected. This should not be done without validation by the manufacturer and the project engineer.
Why can a motor overheat at low speed?
In a self-ventilated motor, the shaft-mounted fan turns more slowly and provides less cooling. If the load continues to demand high torque, it may be necessary to limit the load, use forced ventilation or select a motor suitable for the duty.
When is a dV/dt or sine-wave filter required?
It depends on voltage, motor, insulation, cable length and type, switching frequency and the manufacturer’s instructions. Long cables and older motors are situations that deserve specific review.
Does a drive replace all motor protections?
No. It may include thermal models, current limits and alarms, but protection coordination, isolation, machine safety and any necessary sensors must be defined in the project.
