What are drives?
Drives are power-electronic devices that control the speed, torque and position of an electric motor by adjusting the voltage, current and frequency of its supply. Instead of a motor being simply “on or off”, it runs exactly as fast and as hard as the process requires. Drives sit in almost every machine, pump, fan and conveyor and form the link between the controller (PLC, motion controller) and the motor. Because modern drives come with network ports, firmware and often a web interface, they are also a full member of the OT network, and therefore of your security story.
🧠 What exactly does a drive do?
A drive translates a command from the controller (“run at 1,200 rpm” or “move to position 350 mm”) into the right electrical power for the motor. Its main functions are:
- Speed and torque control - stepless adjustment instead of a fixed mains frequency of 50 Hz
- Controlled starting and stopping - adjustable acceleration and deceleration ramps that prevent mechanical shocks and pressure surges
- Motor protection - a thermal motor model, current limiting and monitoring of phases, overvoltage and short circuits
- Energy savings - especially on pumps and fans, where power consumption drops sharply at lower speed
- Energy recovery - regenerative drives feed braking energy back into the grid instead of burning it in a braking resistor
- Safety functions - integrated functions such as Safe Torque Off (STO)
- Diagnostics and data - current, torque, temperature, operating hours and fault codes for condition monitoring
🔌 What types of drives are there?
“Drives” is the umbrella term. Which type you choose depends on the motor and the accuracy the process demands.
| Type | Motor | Controls | Typical application |
|---|---|---|---|
| VFD / variable frequency drive | Induction motor, also PM motors | Speed and torque | Pumps, fans, compressors, conveyors |
| Servo drive | Servomotor with encoder | Position, speed and torque in a closed loop | Robots, packaging machines, pick-and-place |
| Stepper driver | Stepper motor | Position in steps, usually open loop | Small positioning tasks, dosing systems |
| DC drive | DC motor | Speed and torque via a thyristor converter | Older rolling mills, cranes and retrofit installations |
| Soft starter | Induction motor | Only the starting and stopping current | Large fixed-speed pumps and compressors |
Strictly speaking, a soft starter is not a speed controller: it only limits the inrush current and then bypasses to the mains. A simple motor starter (direct-on-line or star-delta) controls nothing at all.
🔧 How does a variable frequency drive work inside?
The most common drive, the voltage-source frequency converter, has three stages:
- Rectifier - converts the AC mains voltage into DC, usually with a diode bridge
- DC link - capacitors buffer the DC voltage (around 560 V DC on a 400 V supply)
- Inverter - IGBT transistors switch that DC voltage on and off thousands of times per second (pulse-width modulation, PWM), creating a new AC voltage with adjustable frequency and amplitude
The control method determines how precisely the motor responds:
| Control method | How it works | Suitable for |
|---|---|---|
| V/f control | Voltage and frequency rise proportionally | Pumps and fans, simple applications |
| Sensorless vector control | A motor model estimates flux and torque | Conveyors, mixers, higher starting torque |
| Closed-loop vector control | Feedback from the actual shaft | Lifts, cranes, winders, accurate torque |
| Servo control | Cascade of position, speed and current loops | Motion control and robotics |
In a servo drive the current loop often runs in tens of microseconds. That requires real-time communication with the controller, for example via EtherCAT, ProfiNET IRT or SERCOS III.
🔁 How do drives communicate with the PLC and SCADA?
Drives are controlled via digital and analogue I/O or, increasingly, via a fieldbus or Industrial Ethernet. To allow vendor-independent operation, the IEC 61800-7 series describes four standard drive profiles:
- CiA 402 - originally for CANopen, also used on EtherCAT (via CoE) and Powerlink, with modes such as cyclic synchronous position (CSP), velocity (CSV) and torque (CST)
- CIP Motion - the profile for IP in the Rockwell Automation world
- PROFIdrive - the profile for Profibus and PROFINET, widely used by Siemens
- SERCOS - the profile of SERCOS III
Many drives also offer a Modbus interface, particularly in HVAC and building automation. Through the PLC, drive status and alarms eventually reach SCADA and the historian.
🔐 What safety and security requirements apply to drives?
Functional safety. IEC 61800-5-2 defines safety functions built into the drive itself: STO (Safe Torque Off), SS1 (Safe Stop 1), SLS (Safely-Limited Speed), SBC (Safe Brake Control) and more. STO corresponds to stop category 0 in IEC 60204-1 and in many machines replaces a contactor in the emergency stop circuit. The level you achieve, SIL 3 under IEC 62061 or PL e under ISO 13849, depends on the certified product and the wiring: with single-channel STO wiring many manufacturers rate a lower level, often SIL 2 or PL d. Safety functions can also run over the network, using PROFIsafe or CIP Safety.
Cybersecurity. Drives are an attractive target because they directly influence the physical process:
- Stuxnet targeted frequency converters from Vacon and Fararo Paya running between 807 and 1,210 Hz and switched their output frequency to 1,410 Hz and then 2 Hz while operators saw normal values
- Vulnerabilities - CISA published, among others, advisory ICSA-19-087-01 on the PowerFlex 525, where specially crafted, unauthenticated CIP packets could make the drive unavailable (CVE-2018-19282)
- Open parameters - many drives accept parameter changes over the network without a password, such as a higher maximum speed or a disabled current limit
So include drives in your asset inventory, place them in their own zone behind the PLC (network segmentation in line with IEC 62443), disable unused web servers and protocols, and include firmware in your patch management.
🇪🇺 Which European rules affect drives?
| Rule | What it means for drives | Since |
|---|---|---|
| Ecodesign Regulation (EU) 2019/1781 | Three-phase drives from 0.12 to 1,000 kW must reach at least efficiency class IE2 (25% lower losses than the IE1 reference) | 1 July 2021 |
| Same regulation, motors | Three-phase motors 0.75–1,000 kW (2 to 8 poles) at least IE3; 75–200 kW (2, 4 and 6 poles) at least IE4 | 1 July 2021 / 1 July 2023 |
| EMC standard IEC 61800-3 | Categories C1 to C4 determine whether a drive may be used in residential or industrial environments | Ongoing |
| Machinery Regulation (EU) 2023/1230 | Replaces the Machinery Directive; requirements for safety components and protection against corruption of the control system | 20 January 2027 |
| Cyber Resilience Act | Networked drives are products with digital elements | Reporting 11 September 2026, full application 11 December 2027 |
Regenerative drives, low-harmonic drives (THD below 10%), single-phase drives, DC drives and medium-voltage drives are excluded from the IE2 requirement of the Ecodesign Regulation.
🏭 How much energy do drives save in practice?
Electric motor systems use roughly half of all electricity worldwide; in 2011 the IEA estimated their share at 43 to 46%. On pumps and fans, the power drawn roughly follows the cube of the speed (the affinity laws). If you run a fan 20% slower with a drive instead of throttling it with a damper, in theory you need only about 51% of the power. In systems with a large static head, such as pumping to a fixed height, the savings are lower.
A practical approach for a Dutch production site:
- Inventory large fixed-speed motors that are controlled with valves, dampers or bypasses
- Measure the actual load profile over several weeks
- Choose the right type and EMC category, and take harmonic distortion on the grid into account
- Integrate the drive via a standard profile and expose diagnostic data for predictive maintenance
- Secure the drive as an OT asset: zone, passwords, parameter backup and firmware management
❓ Frequently asked questions
What is the difference between a drive and a variable frequency drive?
Drive is the collective name for all electronic motor controls. A variable frequency drive or VFD is the most common type of drive and controls AC motors by varying frequency and voltage. Servo drives, DC drives and stepper drivers are also drives, but not VFDs.
Is a soft starter a drive?
A soft starter is often grouped with drives, but it does not control speed. It only limits the current during starting and stopping and then connects the motor directly to the mains. For variable speed you need a variable frequency drive.
What is Safe Torque Off on a drive?
Safe Torque Off (STO) is a safety function from IEC 61800-5-2 that ensures the drive no longer delivers torque to the motor. STO corresponds to stop category 0 in IEC 60204-1. Depending on the product and wiring, STO achieves up to SIL 3 or PL e.
Why are drives a cybersecurity risk?
Drives directly determine how fast and how hard a motor runs, so manipulation can damage machines or create unsafe situations. Stuxnet, discovered in 2010, demonstrated this by covertly changing the frequency of the drives powering Iranian uranium-enrichment centrifuges. Many drives also have Ethernet, a web server and parameters that can be changed without authentication.
Which efficiency class must a drive have in the EU?
Since 1 July 2021, three-phase drives from 0.12 to 1,000 kW in the EU must reach at least efficiency class IE2 under Ecodesign Regulation (EU) 2019/1781. IE2 means 25% lower losses than the IE1 reference from IEC 61800-9-2. Regenerative, low-harmonic and medium-voltage drives are exempt.
Which protocols do drives use?
Drives communicate via fieldbuses and Industrial Ethernet, such as PROFINET, EtherCAT, EtherNet/IP, CANopen and Modbus. The IEC 61800-7 series describes four drive profiles for this: CiA 402, CIP Motion, PROFIdrive and SERCOS. This lets a controller command drives from different vendors in the same way.
📌 In summary
Drives control the speed, torque and position of electric motors and are therefore the muscles of every automated installation. From variable frequency drives to servo drives, they save energy, spare mechanical parts and deliver valuable data, but as networked devices with safety functions they also belong in your functional safety and OT security approach within industrial automation.