What is a Variable Frequency Drive?
A variable frequency drive (VFD), also called a frequency inverter, is a power electronic device that continuously controls the speed and torque of an AC motor by adjusting the frequency and voltage of the motor supply. Instead of running a motor at the fixed mains frequency (50 Hz in Europe) and throttling the process with valves or dampers, you let the motor turn exactly as fast as the process requires. Variable frequency drives are the most common type of drive; they mainly control the induction motor, but increasingly also permanent magnet motors.
Variable frequency drives are widely used in industrial installations to save energy, improve process control and reduce mechanical wear. As networked devices with firmware, they are also a full part of your OT environment.
🧠 How does a variable frequency drive work?
A voltage-source drive consists of three stages:
- Rectifier - a three-phase diode bridge (the so-called six-pulse rectifier) converts the mains voltage into DC.
- DC link - capacitors buffer that DC voltage; on a 400 V supply it is roughly 560 V DC.
- Inverter - IGBT transistors switch the DC voltage on and off thousands of times per second. By varying the pulse width (pulse width modulation, PWM), the drive creates a new AC voltage with adjustable frequency and amplitude. In low-voltage drives the switching frequency is typically between 2 and 16 kHz.
The speed of an induction motor follows from n_s = 60 · f / p. If the drive halves the frequency, the speed halves too. To keep the magnetic flux in the motor constant, the drive lowers the voltage proportionally: at 400 V and 50 Hz that is 8 V per Hz (the V/f ratio).
| Output frequency | Voltage (constant V/f) | Synchronous speed of a 4-pole motor | Operating region |
|---|---|---|---|
| 10 Hz | 80 V | 300 rpm | Constant torque, cooling critical |
| 25 Hz | 200 V | 750 rpm | Constant torque |
| 50 Hz | 400 V | 1500 rpm | Rated point |
| 60 Hz | 400 V | 1800 rpm | Field weakening: constant power, torque falls |
Above the rated frequency the drive cannot raise the voltage any further, so the motor runs faster but with less torque. At low speeds the shaft-mounted fan of a standard motor cools poorly, so long periods at full torque call for a separately powered fan.
Most drives use V/f control or sensorless vector control, in which a motor model estimates flux and torque. For precise speed or torque control, for example in lifts, cranes or winders, feedback from an encoder is added and the drive becomes part of a Closed-loop control loop. One of the first PWM drives was developed from the late 1960s at the Finnish company Strömberg, which later became part of ABB; the first installations followed in the 1970s. The IGBT, commercially available since the mid-1980s, became the standard switching device in low-voltage drives during the 1990s.
🏭 Where do you use a variable frequency drive?
- Pumps and fans - energy savings by matching speed to flow or pressure, often with a PID controller acting on a setpoint
- Conveyors - controlled start, stop and synchronised speeds
- Centrifuges, mixers and agitators - variable speed for different process phases
- Compressors - lower peak demand, fewer start cycles and less wear
- Hoisting installations - controlled acceleration, braking and positioning
- HVAC - air handling units and chilled water pumps in building automation
The largest gains come from pumps and fans. According to the affinity laws, flow is proportional to speed, pressure to the square of speed and power to the cube:
| Speed | Flow | Head | Power drawn (theoretical) |
|---|---|---|---|
| 100% | 100% | 100% | 100% |
| 90% | 90% | 81% | 73% |
| 80% | 80% | 64% | 51% |
| 63% | 63% | 40% | 25% |
| 50% | 50% | 25% | 13% |
In practice the savings are smaller when a pump works against a large static head, because the speed cannot drop very far. Always measure the actual load profile before you calculate a payback period.
🔄 What is the difference between a VFD and a soft starter?
| Aspect | Direct-on-line (DOL) | Soft Starter | Variable frequency drive (VFD) |
|---|---|---|---|
| Speed control | No | No, only ramp-up and ramp-down | Yes, across the full range |
| Starting current | 6–8 × rated | approx. 2–4 × rated | ≤ approx. 1.5 × rated |
| Energy savings in operation | No | No, only lower inrush current | Yes, especially at partial load |
| Complexity | Simple | Simple, little tuning | Advanced, many parameters |
| Mains pollution | None during operation | Briefly during start-up | Continuous harmonics, filtering needed |
| Cost | Lowest | Low | Higher |
Choose a soft starter when the motor runs at fixed speed and you only need to limit starting current or pressure surges (water hammer). Choose a variable frequency drive as soon as the process needs varying speed.
⚡ What side effects does a variable frequency drive have?
A variable frequency drive is a switching power electronic load. That has consequences on both the supply side and the motor side:
| Problem | Cause | Common measure |
|---|---|---|
| Harmonics on the supply | The six-pulse rectifier draws pulsed current, mainly with 5th, 7th, 11th and 13th harmonics | DC choke or line reactor, 12- or 18-pulse rectifier, active front end (AFE), passive or active filters |
| EMC interference | Steep switching edges of the IGBTs | Shielded motor cable with 360° termination, mains filter, correct EMC category C1–C4 under IEC 61800-3 |
| Voltage peaks at the motor | Reflected waves on long motor cables, up to roughly twice the DC link voltage | Shorter cable, dv/dt filter or sine filter, inverter-duty motor |
| Bearing currents | High-frequency common-mode currents discharge through the bearings and erode the races | Symmetrical shielded cable, insulated bearings, shaft grounding, common-mode cores |
| Braking energy | When decelerating, the motor acts as a generator and the DC voltage rises | Braking resistor, or an AFE that feeds the energy back to the supply |
On the supply side, IEC 61000-3-12 applies to equipment with an input current between 16 and 75 A per phase connected to the public low-voltage network and, in international projects, IEEE 519. At the point of common coupling, the latter limits total voltage distortion to 8% and each individual voltage harmonic to 5% at voltages up to 1 kV; between 1 and 69 kV the limits are 5% and 3%. Good meshed earthing and a tidy layout of the control cabinet prevent many disturbances in PLC inputs and fieldbuses.
🧭 How do you select and commission a variable frequency drive?
- Determine the load type - pumps and fans have quadratic torque; conveyors, extruders and cranes need constant torque and often high starting torque.
- Size on current, not only on kW - select the drive based on the rated motor current on the nameplate. Many manufacturers give a normal duty rating (about 110% overload for 60 seconds) and a heavy duty rating (about 150% for 60 seconds) for demanding applications.
- Choose EMC category, filters and cable - pay attention to cable length and to harmonics relative to the rating of the supply transformer.
- Enter the motor data - voltage, current, speed, power factor and frequency from the nameplate, followed by a motor identification run (autotune) in which the drive determines its motor model.
- Set the process limits - acceleration and deceleration times, minimum and maximum frequency, current limit and skip frequencies to avoid mechanical resonances.
- Wire the safety functions - connect Safe Torque Off (STO) under IEC 61800-5-2 to the emergency stop or safety circuit; the level achieved depends on product and wiring.
- Integrate and document - connect the drive to the controller, back up the parameter set and record firmware versions in your asset inventory.
Do not forget the heat: a variable frequency drive is roughly 97 to 98% efficient, so the remaining losses end up as heat inside the control cabinet. Manufacturers usually specify the rated output up to an ambient temperature of about 40 °C and an altitude of 1,000 m; above that you must oversize the drive (derating) or add cabinet cooling.
Diagnostic data from the drive, such as motor current, torque, temperature and operating hours, enables condition monitoring and predictive maintenance without extra sensors. Through the PLC or directly over the network, that data reaches SCADA, a DCS or, in buildings, the BMS, so operators can see speed, energy use and faults for every VFD in one place.
🔐 How do you secure a variable frequency drive in the OT network?
A variable frequency drive includes built-in protections against overload (a thermal motor model), overtemperature, over- and undervoltage, short circuit and phase loss. Drive parameters must be set correctly so as not to exceed motor and cable limits, which matters for machine safety as well as availability.
Cyber risks arise as soon as the drive is connected via Ethernet IP, ProfiNET or Modbus TCP. Whoever can manipulate the drive or the controlling PLC directly changes the physical process. Stuxnet made infected Siemens PLCs command Vacon and Fararo Paya drives over Profibus that ran between 807 and 1,210 Hz, periodically setting the frequency to 1,410 Hz, then 2 Hz and then 1,064 Hz. A rootkit on the PLC hid the modified code from engineers. In their factory settings, many drives also accept parameter changes without a password, and some have a built-in web server enabled by default. Take these security measures:
- Segment - place drives in their own zone behind the PLC using VLANs, a firewall and segmentation in line with the Purdue Model and IEC 62443
- Restrict access - disable unused web servers and protocols and protect parameters with a password or write lock
- Manage firmware - follow vendor advisories and include drives in your patch management
- Monitor - alert on unexpected parameter changes and periodically compare the active parameters with the approved backup
📜 Which standards and regulations apply to variable frequency drives?
| Standard or regulation | Subject |
|---|---|
| IEC 61800-3 | EMC requirements and categories C1–C4 for adjustable speed drives |
| IEC 61800-5-1 | Electrical, thermal and energy safety |
| IEC 61800-5-2 | Functional safety: STO, SS1, SLS and other safety functions |
| IEC 61800-7 | Standardised drive profiles such as PROFIdrive and CiA 402 |
| IEC 61800-9-2 | Efficiency classes IE0–IE2 for drives |
| Ecodesign Regulation (EU) 2019/1781 | Since 1 July 2021 at least IE2 for three-phase drives from 0.12 to 1,000 kW |
| Machinery Regulation (EU) 2023/1230 | Applies from 20 January 2027, with requirements on protection against corruption of control systems |
| Cyber Resilience Act | Networked drives are products with digital elements; reporting obligations from 11 September 2026, full application from 11 December 2027 |
❓ Frequently asked questions
What is the difference between a VFD and a variable speed drive?
A VFD (variable frequency drive) is a type of variable speed drive that controls an AC motor by varying frequency and voltage. Variable speed drive is the broader term and also covers DC drives and other speed control methods. In everyday industrial usage the terms VFD, AC drive, frequency inverter and frequency converter are often used interchangeably.
How much energy does a variable frequency drive save?
A variable frequency drive saves the most on pumps and fans, because their power demand falls roughly with the cube of speed. At 80% speed, theoretically only about 51% of the power is needed. With a large static head, or a load that already runs at nearly full speed most of the time, the savings are much smaller.
Can any motor run on a variable frequency drive?
Most modern three-phase induction motors can run on a variable frequency drive, but you should check insulation, bearing currents and cooling at low speed. With long motor cables or older motors, a dv/dt or sine filter is often needed. Single-phase capacitor motors are generally not suitable for a standard variable frequency drive.
Why does a variable frequency drive cause harmonics?
A variable frequency drive with a six-pulse rectifier only charges its capacitors at the peaks of the mains voltage and therefore draws pulsed current. That current mainly contains 5th, 7th, 11th and 13th harmonics. Chokes, multi-pulse rectifiers, filters or an active front end limit this mains pollution.
Can a variable frequency drive run a motor above 50 Hz?
A variable frequency drive can run a motor above 50 Hz, but the motor then operates in field weakening: speed rises while available torque falls. Check the maximum speed of the motor, bearings and driven machine beforehand. For pumps and fans, the required power also rises steeply above 50 Hz.
Is a variable frequency drive a cybersecurity risk?
A networked variable frequency drive is a cybersecurity risk if it is left unprotected, because it directly determines the speed and torque of a machine. Stuxnet damaged centrifuges by manipulating the output frequency of variable frequency drives. Segmentation, disabled web servers, parameter passwords and firmware management reduce that risk.
📌 In summary
A variable frequency drive controls the speed and torque of an AC motor by adjusting frequency and voltage, which makes it a critical component in modern drive technology. It saves energy, improves process control and extends the service life of motors and mechanics, but it requires attention to harmonics, EMC, motor protection and, as a networked OT device, to segmentation and security.