Reducing harmonics from AC drives12 October 2023

AC power drives ABB (Image credit: AdobeStock by djoronimo)

Generally speaking, a harmonic is a sinusoidal wave with a frequency that is a whole number multiple of the fundamental frequency of a periodic signal. In an electrical system, a harmonic is voltage or current disturbance with a sinusoidal wave of frequency that is some multiple of the AC frequency. The second harmonic, or the second order harmonic, has a frequency of twice the AC frequency. The third harmonic has three times the AC frequency, and so forth (see diagram, bottom right).

“Any non-linear load causes harmonics. Examples of those are non- incandescent lighting, computers, copier machines, battery chargers, anything that takes AC power and converts it to DC,” says Jeff Fell, ABB application engineering manager.

Harmonic distortion is typically measured by the total harmonic distortion (THD). THD is defined as the root mean square (RMS) value for all harmonics, divided by the RMS value for the fundamental component (the AC waveform). Effects are typically calculated up to the 40th or 50th order. Higher-order harmonics are usually of negligible power and so can be omitted from the calculation.

There are many common non-linear loads that can cause current harmonics. Semiconductor devices such as transistors and diodes are all non-linear loads and many electronic and power supply devices therefore cause harmonics. Significant devices include motor starters, variable speed drives, power supplies, computers, welding equipment and some types of lighting. AC drives are a very significant cause of harmonics. Some new drives claim to filter out or eliminate harmonic distortion at source.


Power system harmonics increase current and therefore lead to overheating. Harmonics can cause overheating in many different devices and power system components, especially transformers, cables, motors, generators and capacitors. Other symptoms may include flickering of electronic displays and lighting, circuit breakers tripping, failures in electronic devices including computers and false readings from electrical meters.

When harmonics are causing significant issues within a plant, it is very likely that the source of the harmonic distortion is also within the plant. The effects of a disturbance are usually seen within the plan before they affect the wider distribution network.

Continues Fell: “That transformer that has to supply the fundamental currents also has to supply the harmonic currents within your system. Think of the harmonics currents as waste in your system… if I don’t take care of the harmonics I’m going to have to size my equipment larger. I’m going to have to size my conductors larger, my fusing larger… this can also cause component heating if you don’t do it correctly.”

A number of standards set limits of harmonic currents and have been widely adopted. The International Electrotechnical Commission (ICE) standards form the basis for conformity within Europe. CE marking indicates that devices meet these standards. EN/IEC 61800-3 covers adjustable speed electrical power drive systems, setting limits on the level of harmonic disturbance that they can cause. Other important IEC standards include IEC 61000 parts 2, 3 and 4. The Institute of Electrical and Electronics Engineers (IEEE) forms the basis for requirements in the USA and elsewhere. IEEE 519 covers recommended practices and requirements for harmonic control in electrical power systems and is widely adopted in the USA. Although this standard suggests limiting the harmonic injection from individual customers, it does not set limits on individual devices.


An AC drive system typically consists of a transformer, rectifier, inverter and motor. Factors which affect harmonics include the rated power and impedance of the transformer, the type of rectifier and inverter, reactor and filter inductance, and the rated power and load of the motor. The harmonics injected into the system by drive systems can be reduced by either structural changes to the drive system itself, or by using external filtering.

Keeping the non-linear load below 30% of the transformer’s maximum capacity can greatly reduce harmonics. If power factor correction capacitors are being used, resonance can reduce the limiting level to 15%.

Also, harmonics can be significantly reduced by attaching a choke or reactor to the AC input or DC bus between rectifier and inverter. These devices consist of a coil of wire around a magnetic core, increasing impedance in the line and reducing the harmonic disturbance that is injected upstream into the electrical system. Which location will give the greatest reduction in harmonics depends on which orders of harmonics are most significant.

An AC line reactor is positioned before the rectifier of a drive, in series with the incoming line. They therefore also act as a buffer for surges and other transients. Their main disadvantage is that they cause a voltage drop that changes the drive output, and they are larger and more expensive than DC chokes.

A DC choke is positioned on the DC bus between rectifier and inverter. They are typically somewhat smaller and cheaper than AC line reactors, and do not cause a voltage drop.

The configuration of rectifier will also influence the harmonics it produces. Most three-phase AC drives use a six-pulse diode bridge, with six diodes, a choke and a capacitor forming a low-pass filter to smooth the DC current. By combining two six-pulse rectifiers in parallel, a 12-pulse rectifier is created. If this is used together with a special three-winding transformer, some low order harmonics cancel each other out. With a 24-pulse rectifier and two three-winding transformers, low-order harmonics are virtually eliminated. While these solutions require special transformers that can add significant cost, they can be economical for large installations.

An ISU is a switch-mode power supply that can produce very low harmonic disturbance. These devices use insulated-gate bipolar transistors (IGBT) instead of diodes. IGBTs can switch on and off extremely quickly and with very high efficiency. Although lower order harmonics (below the 50th harmonic) are greatly reduced, this can come at the cost of a significant increase in higher-order harmonics. There are also significant ripple effects which may be removed with additional filtering.


Nidec/Leroy-Somer uses passive rectifier systems such as 12, 18 and 24-pulse systems, as well as active rectifier systems, to reduce harmonics. ABB produces ultra-low harmonic drives that combine an active supply unit with an integrated low harmonic line filter, as well as multi-pulse arrangements.

The new Sinamics G220 drives from Siemens are claimed to reduce harmonics by up to 97%. How this is done is not specified, although the supplier says that they don’t use an AC line reactor or DC choke. Siemens has, however, published general information on harmonic mitigation in the past

( that includes isolation transformers, AC line reactors, DC chokes, passive filters, multi-pulse rectifiers, active filters and active front ends (IGBT supply units).

With good power system design and filtering where required, harmonics can be reduced to very low levels. However, larger transformers, more sophisticated rectifier systems, and additional filtering all come at a cost. Different approaches are also better suited to dealing with different types of harmonics.

Jody Muelaner

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