A Variable Frequency Drive (VFD), also called an AC drive or frequency converter in many industrial applications, is a power-electronic system that controls an AC motor by managing the frequency and voltage supplied to it. Describing a VFD simply as a device that “changes frequency” misses much of what actually happens inside the drive. A modern AC drive combines a power conversion stage, a DC bus, semiconductor switching devices, a real-time motor-control algorithm, protection functions, I/O and, in many models, industrial communication interfaces.

In the common voltage-source VFD topology, incoming AC power is first rectified to DC. DC-link capacitors store and smooth that energy. The inverter stage then switches the DC bus through devices such as IGBTs to synthesize an AC output with the frequency and effective voltage required by the motor. A useful simplified power path is therefore AC input → rectifier → DC link → inverter → motor.

Why does frequency change motor speed? The synchronous speed of the rotating magnetic field can be approximated by n_s = 120 × f / p, where f is electrical frequency and p is the number of motor poles. A four-pole motor therefore has a synchronous speed of 1500 rpm at 50 Hz and 750 rpm at 25 Hz. An induction motor normally turns slightly below synchronous speed when producing torque. The difference is known as slip, so electrical frequency should not be treated as an exact measurement of rotor speed.

Important: The interactive model below is an educational illustration. Actual slip, torque, current and thermal behaviour depend on the motor equivalent circuit, load characteristics, drive algorithm and parameter settings. It is not a drive-sizing calculator.

Motor-control strategy and PWM are not the same thing

This distinction matters. PWM is primarily a power-stage modulation technique: the inverter switches its semiconductor devices in patterns that synthesize the required motor voltage. V/Hz control, sensorless vector control, field-oriented control and closed-loop vector control, on the other hand, describe how the drive regulates motor flux, torque or speed. Modulation and motor control work together, but they should not be presented as interchangeable concepts.

V/Hz control is simple, robust and suitable for many applications where extreme low-speed torque accuracy or very fast dynamic response is unnecessary. Applications demanding stronger low-speed performance and better response can use sensorless vector control or vector control. Where the actual shaft speed must be measured directly, closed-loop vector control can use feedback from a rotary encoder.

Vector-control methods depend more heavily on an accurate electrical model of the motor. Correct motor nameplate data therefore matters. Many drives provide an auto-tuning routine to identify or refine parameters used by that model. Auto-tuning should not be described as a drive continuously “learning by itself”; what is measured, whether the shaft rotates during the test and which parameters are identified vary by manufacturer and drive family.

Base frequency and the field-weakening region

Below the motor's base-frequency region, a conventional V/Hz strategy raises voltage approximately in proportion to frequency so that motor flux can be maintained. This is commonly referred to as the constant-flux or constant-torque region.

The inverter cannot increase output voltage indefinitely. Once its practical voltage limit has been reached, increasing frequency further causes the volts-per-hertz ratio to fall. Motor flux decreases and the amount of torque that can be produced also begins to fall. This is the field-weakening region. Consequently, doubling the frequency does not mean that a motor can automatically run at twice its rated speed while producing the same torque. Mechanical speed limits, rotor balance, bearings, cooling, insulation and manufacturer data also have to be checked.

Acceleration, deceleration and catching a spinning motor

A VFD controls more than the final speed command. Acceleration and deceleration times define how quickly the drive moves between operating points. Proper ramps can reduce mechanical shock and unnecessary current peaks, while an unrealistically short deceleration ramp on a high-inertia machine can drive regenerative energy back into the DC link.

A fan or other rotating load may continue to coast after a power interruption. Applying a zero-speed starting sequence to a motor that is already rotating can produce high current or torque shock. Flying start, also called speed search or catch-on-the-fly by different manufacturers, attempts to identify the rotating motor's existing speed and direction and then brings the inverter output into a suitable operating point before normal control resumes.

What happens to energy during braking?

When a rotating motor and machine are decelerated, their kinetic energy does not disappear. Depending on the load, energy can flow back through the motor and inverter into the DC bus, raising the DC-link voltage.

With dynamic braking, a brake chopper connects a correctly sized braking resistor to the DC link and converts regenerative energy into heat. A regenerative drive uses a different power topology and can return suitable energy to the supply instead of dissipating it as heat. These should not be described as different names for the same braking unit. For the energy-flow concept itself, see regenerative energy.

Brake-resistor sizing is not determined by motor kW alone. The minimum permissible resistance, chopper current and voltage limits, peak braking power, inertia, initial and final speed, braking time, repetition rate and average thermal duty all matter. High-cycle cranes, hoists, centrifuges, presses and winding systems therefore require application-specific braking calculations.

Does a VFD always save energy?

No universal percentage applies. The greatest opportunities are often found on centrifugal fans and pumps. Under ideal similarity conditions, flow varies approximately with speed, pressure or head with the square of speed, and required shaft power with the cube of speed. Replacing mechanical throttling with speed control can therefore provide substantial savings when the process regularly operates below full demand.

The actual result depends on the system curve, static head, duty profile, motor efficiency, drive losses and the control method being replaced. A blanket claim such as “installing a VFD saves 30%” is therefore not technically sound. On conveyors, cranes, extruders and other constant-torque processes, the primary reasons for using a drive may instead be process control, controlled acceleration, torque management and reduced mechanical stress.

Why VFD selection cannot be based on kW alone

A good starting point is usually the motor rated current and the application's required current/torque profile, not just the motor's kW number. The same motor power can be used on machines with completely different overload requirements. When selecting a VFD, consider supply voltage and phase, motor rated current, normal-duty or heavy-duty requirements, overload magnitude and duration, minimum and maximum speed, low-speed torque, braking, feedback, communications, ambient temperature, altitude, enclosure cooling, EMC environment and motor-cable length.

A centrifugal fan and a crane can both use motors carrying the same kW rating while requiring very different drive sizing. The drive's continuous current, overload current, overload duration and duty classification must be matched to the actual machine.

Motor cables, switching and EMC

A VFD output is not a normal utility sinewave. The inverter creates a PWM waveform using fast semiconductor switching. Fast voltage edges can interact with motor cables and motor insulation, particularly on long cable runs. Reflected-wave effects, high dv/dt, common-mode currents and EMC behaviour may therefore become important. Cable construction, shielding, protective-earth connections, shield termination, motor insulation and any required dV/dt or sine-wave filters should follow the drive and motor manufacturers' instructions.

An EMC/RFI filter, an input line reactor and a surge-protection device are not interchangeable accessories. They address different electrical phenomena. VFD installations also fall within the broader technical landscape covered by the industrial standards section, including the IEC 61800 family for adjustable-speed electrical power drive systems.

STO is not an isolator or a mechanical holding brake

STO – Safe Torque Off is a drive safety function intended to prevent the drive from producing motor torque through its power stage. Activating STO does not mean that every terminal inside the drive is electrically dead, and STO should not be treated as a substitute for electrical isolation. It also does not replace a mechanical holding brake on a suspended load.

A drive's STO capability does not automatically give the complete machine a particular SIL or PL rating. The entire safety function—from sensing and logic through the final actuator—has to be assessed as a system.

Commissioning: the first checks that matter

Before commissioning, enter the motor's rated voltage, rated current, frequency, power and rated speed correctly. Select the appropriate motor-control mode and, where required, perform the manufacturer's specified auto-tuning procedure. Initial testing should verify rotation direction, motor current, acceleration and deceleration behaviour, DC-bus behaviour and the mechanical machine response under controlled conditions.

Drive cooling should also be treated as a thermal-design problem rather than simply adding a fan whenever the cabinet feels warm. Drive losses, other heat sources, ambient temperature, enclosure dimensions, airflow and manufacturer-specified clearances all influence cabinet temperature. Blocked ventilation paths and contaminated cooling fans can eventually force a drive into thermal derating or overtemperature trips.

VFD questions in short

Can every AC motor be operated from a VFD? No. Motor type, insulation, speed range and the control modes supported by the drive have to be compatible. Induction motors are extremely common, while suitable drives may also support PMSM, SynRM and other motor technologies.

Can a VFD hold full torque at zero speed? It depends on the drive, motor and control strategy. Basic open-loop V/Hz control should not be expected to provide precision zero-speed torque. Encoder feedback or a more advanced control system may be required, and vertical loads can still require a mechanical brake.

Does increasing frequency increase motor power without limit? No. Field weakening, drive current limits, motor thermal capability and mechanical speed limits constrain the available torque and power.

Is a VFD simply a more advanced soft starter? No. Both can provide controlled starting, but a soft starter primarily manages the starting and stopping transition. A VFD can continuously regulate operating speed and the motor's working point.

Can a contactor be switched freely between the drive and motor? Not necessarily. Opening or closing the motor circuit while a drive is actively producing output can create faults, overvoltage and stress. Output-side switching must follow the drive manufacturer's instructions.

External technical references

For the basic VFD power-conversion chain, see ABB's Variable Speed Drive technical overview. For the speed, flow and power relationships of centrifugal machinery, Danfoss provides a concise explanation of the Affinity Law. A practical manufacturer example of encoder feedback used for closed-loop vector control can be found in Yaskawa's PG / Encoder Feedback documentation.

In summary: a VFD is not merely an electronic speed knob. It is a real-time power conversion and motor-control system that manages how electrical energy is delivered to a motor. Correctly selected and commissioned, it can improve process control, reduce mechanical stress, provide major energy savings in suitable fan and pump applications and enable sophisticated torque and speed control. Poor selection or incorrect parameterization can instead introduce motor, braking, thermal and EMC problems. The better engineering question is therefore not simply “How many kW?” but “What motor, what load, what speed-torque profile and under what operating conditions?”