AC

What is AC?

AC (Alternating Current) is electric current whose voltage and direction of flow reverse periodically, following a sine wave that in Europe completes 50 cycles per second. AC is the basis of virtually every public electricity grid and powers motors, variable frequency drives, PLC power supplies, HVAC installations and entire factories. For OT professionals, AC is more than “power from the socket”: power quality, protection and energy monitoring directly affect the availability of industrial automation.

🧠 How does alternating current work?

With alternating current, the voltage follows a sine wave: v(t) = V̂ · sin(2π · f · t), where V̂ is the peak value, f the frequency and t the time. At 50 Hz one full cycle takes 20 milliseconds, so the current reverses direction 100 times per second.

Voltages are stated as effective (RMS) values: the DC voltage that would deliver the same power into a resistor. The familiar 230 V is such an RMS value; the peak is 230 × √2 ≈ 325 V. That difference matters when you select insulation, surge protection and the voltage rating of components.

AC beat direct current (DC) at the end of the nineteenth century because transformers can step alternating voltage up and down easily. High voltage means low current and therefore low cable losses; the hydroelectric plant at Niagara Falls supplied alternating current to Buffalo from 1896 and was seen as the breakthrough.

🔄 What is the difference between AC and DC?

Both meet inside a control cabinet: the main supply is AC, while a power supply converts it to 24 V DC for the PLC, relays and sensors. A UPS and a variable frequency drive internally convert AC to DC and back again.

🔌 What are single-phase and three-phase AC?

Single-phase (230 V between phase and neutral) is sufficient for lighting, office equipment and small loads. Three-phase (3 × 400 V) is the industrial norm: three sine waves shifted 120 degrees relative to each other deliver nearly constant power together and create a rotating magnetic field that lets a motor start without auxiliary means. For the same power, less copper is needed as well.

Voltage levels in the Netherlands:

For comparison: in the United States 120 V at 60 Hz is common, and Japan uses 100 V with 50 Hz in the east and 60 Hz in the west. Machine builders who deliver worldwide have to take this into account.

⚙️ How does AC drive industrial motors?

A three-phase motor runs at a synchronous speed n = 120 × f / p, where p is the number of poles. At 50 Hz that is 3,000 rpm for a two-pole motor and 1,500 rpm for a four-pole motor. The robust induction motor runs slightly slower because of slip, for example around 1,450 rpm.

Because speed is tied to grid frequency, a variable frequency drive controls speed by generating a variable frequency. For pumps and fans this yields large savings, because the power drawn falls roughly with the cube of the speed. For switching and starting motors, contactors, soft starters and drives are used, often grouped in an MCC.

Since 1 July 2021 the European Ecodesign Regulation (EU) 2019/1781 has required efficiency class IE3 for most three-phase motors from 0.75 to 1,000 kW, and since 1 July 2023 IE4 for 2-, 4- and 6-pole motors from 75 to 200 kW. Variable speed drives must meet class IE2.

⚡ What are active power, reactive power and power factor?

Motors and transformers need a magnetic field. The current that builds this field does no useful work, yet it still loads cables and transformers.

The ratio P/S is called the power factor (equal to cos φ for a pure sine wave). Without further agreements, the Dutch grid code (Netcode elektriciteit) assumes a power factor at the connection point between 0.85 inductive and 1.0. If the power factor drops below that, the grid operator may charge for the excess reactive consumption. Correction is done with capacitor banks, active filters or drives with an active front end.

🔧 Power quality in practice: which values apply?

Power quality determines whether sensitive OT equipment keeps running reliably. The European standard EN 50160 describes what you may expect from the public grid:

• Frequency - 50 Hz ± 1 % (49.5 to 50.5 Hz) for 99.5 % of the year (measured as 10-second averages), and always within 47 to 52 Hz
• Voltage - within ± 10 % of nominal for 95 % of the 10-minute averages per week
• Harmonic distortion - total harmonic distortion (THD) of the voltage of at most 8 %
• Unbalance - up to 2 % between phases

In the Continental European grid, transmission system operators fully activate the primary reserve (FCR) at a deviation of ± 200 mHz. Automatic load shedding starts at around 49 Hz. Factories mainly notice grid problems as voltage dips, which can make drives and contactors drop out.

Non-linear loads such as variable frequency drives, UPS systems, switched-mode power supplies and LED lighting cause harmonics. The consequences are overheated neutral conductors and transformers, EMC interference and unexplained trips. Limits for equipment are set in IEC 61000-3-2 (up to 16 A per phase) and IEC 61000-3-12 (16 to 75 A). Countermeasures include line reactors, harmonic filters, active front ends and proper earthing.

A practical approach for a plant:

1. Measure - install a power quality analyser on the main switchboard and relevant sub-distribution boards for at least a week.
2. Compare - check voltage, frequency, THD and dips against EN 50160 and the specifications of your equipment.
3. Correlate with faults - line up timestamps with fault messages in SCADA or the historian.
4. Mitigate - choose filters, compensation or a UPS for critical control.
5. Monitor continuously - include energy meters in monitoring and energy management according to ISO 50001.

🛡️ Which safety risks and standards apply to AC?

The key standards are NEN 1010 (low-voltage installations), NEN 3140 (safe operation), IEC 60204-1 (electrical equipment of machines) and IEC 61439 (switchgear and controlgear assemblies). Work on installations requires a lock-out tag-out procedure.

🔐 Why is AC infrastructure an OT security topic?

The electrical infrastructure itself is increasingly controlled digitally: protection relays, energy meters, variable frequency drives and substations using IEC 61850 communicate over Industrial Ethernet. The attacks on the Ukrainian grid in December 2015 and December 2016 (using Industroyer) showed that power supply can be switched off through software.

Typical cybersecurity risks are manipulation of energy management, abuse of remote access to drives and remote opening of circuit breakers. That is why power systems belong in the zone model of IEC 62443, with Defense in Depth, Zero Trust principles, network monitoring and logging to a SIEM. For critical infrastructure such as water treatment, hospitals and data centres, UPS systems, standby generators and battery storage form the last line of defence.

🏭 What does AC look like in a pumping station?

A typical drinking water pumping station follows this chain:

The PLC or DCS sets the speed through real-time control. Energy data flows to SCADA and a historian, where predictive maintenance and industrial AI detect deviations in current draw early. During grid congestion, the same data helps to shift peaks.

❓ Frequently asked questions

Why does Europe use 50 Hz and America 60 Hz?

The choice of 50 Hz in Europe and 60 Hz in North America grew historically from the first electricity companies at the end of the nineteenth century. Technically both frequencies work well, but equipment must suit the local grid frequency. An AC motor runs about 20 % faster on 60 Hz than on 50 Hz.

What exactly does 230 V mean for alternating current?

230 V is the effective (RMS) value of the AC voltage between phase and neutral in the Netherlands and the rest of Europe. The peak of the sine wave is about 325 V. Between two phases of a three-phase AC grid the voltage is 400 V.

Why is three-phase AC better for motors?

Three-phase AC delivers nearly constant power and a rotating magnetic field, so a motor starts without extra aids and runs smoothly. For the same power, less copper is needed than with single-phase alternating current. That is why virtually all industrial motors from about 1 kW upwards use a three-phase connection.

What is a good power factor?

A power factor between 0.85 and 1.0 is considered acceptable in the Netherlands; the Dutch grid code assumes that range unless other agreements are made. The closer the power factor is to 1, the less reactive current loads cables and transformers. Below that, the grid operator may charge for the excess reactive consumption.

Do variable frequency drives cause problems on the AC grid?

Variable frequency drives are non-linear loads and cause harmonic distortion on the AC grid. With many drives this can lead to overheating, EMC interference and trips of sensitive equipment. Line reactors, harmonic filters and drives with an active front end limit this effect.

Why is power supply relevant to OT security?

The AC supply of a plant is increasingly monitored and switched digitally through protection relays, smart meters and networked drives. An attacker who gains access to these can stop a process without attacking the control system itself. That is why power systems belong in the same IEC 62443 zoning as the rest of the OT.

📌 In summary

AC (alternating current) is the sinusoidal 50 Hz, 230/400 V supply that powers virtually every industrial installation in Europe, from motors and variable frequency drives to PLC power supplies. If you manage OT, you also need to understand the AC side: three-phase distribution, power factor, power quality according to EN 50160, electrical safety according to NEN 1010 and NEN 3140, and the security of digitally controlled power systems.