Common mode current

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Common mode current is the portion of conductor currents that are unmatched with the exactly opposite and equal magnitude currents. [1] Common mode current cause multiconductors to act or behave like a single conductor. In electromagnetic compatibility (EMC), there are two common terms that will be found in many electromagnetic interference discussions or considered as fundamental concepts, those are Differential Mode and Common Mode. Those terms are related to coupling mechanisms. Many electrical systems contain elements that are capable to act like an antenna. Each element is capable of unintentionally emitting Radio Frequency energy through electric, magnetic, and electromagnetic means. [2] Common Mode coupling as well as Differential Mode coupling can occur in both a conducted and radiated way. [3]

Contents

Definitions

Differential mode (DM) is where the signal or power propagation through a conductor and return using the intended path by the designer or flowing differently in opposition to each other. Meanwhile common mode (CM) is where the parasitic circuit (unwanted) is formed between the desired circuit (main and return path) and the structure of the circuit within which it is located. The signal or power propagates in the same direction in the same circuit. [3]

Henry Ott remarked something similar in his book. Differential mode is the result of the normal operation of the circuit and results from electric current flowing around loops formed by the electrical conductors of the circuit. Common mode is the result of parasitics in the circuit and results from undesired voltage drops in the conductors. [4]

Clayton R. Paul provide a simple illustration that explains CM and DM terms on his book. [5] A pair of parallel conductors with current Î1 and Î2 flowing on each conductor, which can be decomposed into CM and DM current respectively.

Fig. 1. CM and DM Current Illustration on Pair Conductors. 23.06.2020 Current Illustration.jpg
Fig. 1. CM and DM Current Illustration on Pair Conductors.

As shown in the figure above, the relations between Î1 , Î2 and modal current are given:

Î1= ÎC + ÎD
Î2= ÎC - ÎD

From those two equations, the modal current were obtained as follows:

ÎD= 1/2(Î1 - Î2)
ÎC= 1/2(Î1 + Î2)

The CM current flowing in each conductor is equal in magnitude and directed in the same direction, while DM current has equal magnitude but is directed in different direction.

Fig. 2. Illustration for Relative Radiated Electric Field from DM and CM Current. 23.06.2020 E Field.jpg
Fig. 2. Illustration for Relative Radiated Electric Field from DM and CM Current.


The radiated electric field from both conductors can be superimposed to obtain the total radiated electric field. For Differential Mode Current, since the conductors are not located in close vicinity, the fields do not exactly cancel each other, but the resultant is a small net radiated electric field. Different from DM current, CM current is directed in the same direction and results in a much higher electric field because fields from both conductors will be added. So a small CM current has a much higher potential towards producing radiated emissions compared to DM current. [5] For conducted interference, if the interference doesn't appear between conductors, it will appear between each conductor to a third reference point, for example a structure near the conductor. [3]

Conducted CM interference causes more problems compared to DM interference because of the possible third reference point that could include any structure that is normally not designed for the purpose. Therefore:

Measurement

Common Mode current measurement is carried out to determine the conducted interference or radiated interference that happened in an electrical system due to the high probability of unwanted field emission to the environment. It is also said that most failures are due to common mode currents on cable and the wire assemblies. Note that some common mode current returns through a third point path that could be an adjacent cable, a ground plane or another unexpected return path. [3] Common mode currents in a circuit don't necessarily follow the designed schematics.

Henry Ott consultants [6] explained a simple setup on measuring common mode current by putting a high frequency current clamp from Fischer Custom Communications [7] on multi-conductors and connect it to a spectrum analyzer. [8] It is assumed that all of the common mode current flowing on those multiconductors will travel using another return path that is unknown.
With a known transfer impedance, the common mode current measured from the multi-conductors can be determined by looking at the voltage shown at the spectrum analyzer. That measurement technique can work on both shielded and unshielded cables.

There are many improvisation on common mode measurement method nowadays. Here are some examples: Measurement of common mode current vnd Voltage can be done simultaneously without needing to do it in separate measurements. [9] Measurement for both common mode and differential mode current can be done using two single path Line Impedance Stabilization Networks. [10] Radiated emission from a power cable prediction using a common mode current measurement also done in United Kingdom. [11] Electromagnetic radiation emission from a wind turbine also performed by measuring common mode current from all of the power cables and neutral cable. [12]

Related Research Articles

In telecommunications and professional audio, a balanced line or balanced signal pair is an electrical circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths, to ground, and to other circuits. The primary advantage of the balanced line format is good rejection of common-mode noise and interference when fed to a differential device such as a transformer or differential amplifier.

<span class="mw-page-title-main">Electromagnetic compatibility</span> Electrical engineering concept

Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage to operational equipment. The goal of EMC is the correct operation of different equipment in a common electromagnetic environment. It is also the name given to the associated branch of electrical engineering.

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around 20 kHz to around 300 GHz. This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies. These are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves, so they are used in radio technology, among other uses. Different sources specify different upper and lower bounds for the frequency range.

<span class="mw-page-title-main">Twisted pair</span> Type of wiring used for communications

Twisted pair cabling is a type of communications cable in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation from the pair and crosstalk between neighbouring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham Bell.

In an electrical system, a ground loop or earth loop occurs when two points of a circuit are intended to have the same ground reference potential but instead have a different potential between them. This is typically caused when enough current is flowing in the connection between the two ground points to produce a voltage drop and cause two points to be at different potentials. Current may be produced in a circular ground connection by electromagnetic induction.

<span class="mw-page-title-main">Electromagnetic interference</span> Disturbance in an electrical circuit due to external sources of radio waves

Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of a data path, these effects can range from an increase in error rate to a total loss of the data. Both human-made and natural sources generate changing electrical currents and voltages that can cause EMI: ignition systems, cellular network of mobile phones, lightning, solar flares, and auroras. EMI frequently affects AM radios. It can also affect mobile phones, FM radios, and televisions, as well as observations for radio astronomy and atmospheric science.

<span class="mw-page-title-main">Differential signalling</span> Method for electrically transmitting information

Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conductors can be wires in a twisted-pair or ribbon cable or traces on a printed circuit board.

<span class="mw-page-title-main">Shielded cable</span> Electric cable with metal jacket (shield) to prevent magnetic interference

A shielded cable or screened cable is an electrical cable that has a common conductive layer around its conductors for electromagnetic shielding. This shield is usually covered by an outermost layer of the cable. Common types of cable shielding can most broadly be categorized as foil type, contraspiralling wire strands or both. A longitudinal wire may be necessary with dielectric spiral foils to short out each turn.

The Comité International Spécial des Perturbations Radioélectriques was founded in 1934 to set standards for controlling electromagnetic interference in electrical and electronic devices and is a part of the International Electrotechnical Commission (IEC).

In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit or channel to another.

<span class="mw-page-title-main">Choke (electronics)</span> Inductor used as a low-pass filter

In electronics, a choke is an inductor used to block higher-frequency alternating currents (AC) while passing direct current (DC) and lower-frequency ACs in a circuit. A choke usually consists of a coil of insulated wire often wound on a magnetic core, although some consist of a doughnut-shaped ferrite bead strung on a wire. The choke's impedance increases with frequency. Its low electrical resistance passes both AC and DC with little power loss, but its reactance limits the amount of AC passed.

<span class="mw-page-title-main">Line Impedance Stabilization Network</span> Tool used in emissions testing

A line impedance stabilization network (LISN) is a device used in conducted and radiated radio-frequency emission and susceptibility tests, as specified in various electromagnetic compatibility (EMC)/EMI test standards.

In the field of EMC, active EMI reduction refers to techniques aimed to reduce or to filter electromagnetic noise (EMI) making use of active electronic components. Active EMI reduction contrasts with passive filtering techniques, such as RC filters, LC filters RLC filters, which includes only passive electrical components. Hybrid solutions including both active and passive elements exist. Standards concerning conducted and radiated emissions published by IEC and FCC set the maximum noise level allowed for different classes of electrical devices. The frequency range of interest spans from 150 kHz to 30 MHz for conducted emissions and from 30 MHz to 40 GHz for radiated emissions. Meeting these requirements and guaranteeing the functionality of an electrical apparatus subject to electromagnetic interference are the main reason to include an EMI filter. In an electrical system, power converters, i.e. DC/DC converters, inverters and rectifiers, are the major sources of conducted EMI, due to their high-frequency switching ratio which gives rise to unwanted fast current and voltage transients. Since power electronics is nowadays spread in many fields, from power industrial application to automotive industry, EMI filtering has become necessary. In other fields, such as the telecommunication industry where the major focus is on radiated emissions, other techniques have been developed for EMI reduction, such as spread spectrum clocking which makes use of digital electronics, or electromagnetic shielding.

Common-mode signal is the voltage common to both input terminals of an electrical device. In telecommunication, the common-mode signal on a transmission line is also known as longitudinal voltage.

A TEM or transverse electromagnetic cell is a type of test chamber used to perform electromagnetic compatibility (EMC) or electromagnetic interference (EMI) testing. It allows for the creation of far field electromagnetic fields in a small enclosed setting, or the detection of electromagnetic fields radiated within the chamber.

<span class="mw-page-title-main">Conducted emissions</span>

Conducted emissions are the effects in power quality that occur via electrical and magnetic coupling, electronic switch of semiconductor devices, which form a part of electromagnetic compatibility issues in electrical engineering. These affect the ability of all interconnected system devices in the electromagnetic environment, by restricting or limiting their intentional generation, propagation and reception of electromagnetic energy.

Switching Control Techniques address electromagnetic interference (EMI) mitigation on power electronics (PE). The design of power electronics involves overcoming three key challenges:

  1. power losses
  2. EMI
  3. harmonics
<span class="mw-page-title-main">Low-frequency electromagnetic compatibility</span>

Low-frequency electromagnetic compatibility is a specific field in the domain of electromagnetic compatibility (EMC) and power quality (PQ), which deals with electromagnetic interference phenomena in the frequency range between 2 kHz and 150 kHz. It is a special frequency range because it does not fit in the PQ problems, with range of up to 2 kHz, where relative levels of voltage and current can have massive impact on efficiency and integrity of electric systems, and neither in the conducted EMC range, which starts at 150 kHz and influences mainly informational systems, and already too far from radiated EMC range, which starts at 30 MHz and goes up to 1 GHz.

References

  1. ↑ Tom, Tom. "Balun Basics: Common Mode vs. Differential Mode". DX Engineering.
  2. ↑ Montrose, Mark I.; Nakauchi, Edward M. (2004). Testing for EMC compliance : approaches and techniques. John Wiley. ISBN   0-471-43308-X.
  3. 1 2 3 4 Williams, Tim; Armstrong, Keith (2000). EMC for systems and installations. Newnes. ISBN   0750641673.
  4. ↑ Ott, Henry W. (2009). Electromagnetic compatibility engineering. New Jersey: John Wiley & Sons. ISBN   978-0-470-18930-6.
  5. 1 2 Paul, Clayton R. (2006). Introduction to electromagnetic compatibility (2nd ed.). Ney Jersey: Wiley-Interscience. ISBN   978-0-471-75500-5.
  6. ↑ Ott, Henry W. "EMC Consulting and Training". Henry Ott Consulting. Retrieved 23 June 2020.
  7. ↑ CC, Fischer. "Company Website". FCC. Retrieved 23 June 2020.
  8. ↑ Ott, Henry W. "Measuring CM Currents on Cable". Henry Ott Consultant. Retrieved 23 June 2020.
  9. ↑ Kobayashi, Ryuichi; Nagao, Atsushi; Ito, Hidenori; Hirasawa, Norihito (June 2019). "Simultaneous and Non-invasive Probe for Measuring Common-mode Voltage and Current". 2019 Joint International Symposium on Electromagnetic Compatibility, Sapporo and Asia-Pacific International Symposium on Electromagnetic Compatibility (EMC Sapporo/APEMC). pp. 645–648. doi:10.23919/EMCTokyo.2019.8893871. ISBN   978-4-8855-2322-9. S2CID   207972348 . Retrieved 23 June 2020.
  10. ↑ Li, Jinlong; Ma, Shiping; Yin, Xuebin; Qin, Xiazhen (June 2019). "Measurement of Common-Mode and Differential-Mode Noise Source Impedances Using a Current Probe and Single Path LISNs". 2019 Joint International Symposium on Electromagnetic Compatibility, Sapporo and Asia-Pacific International Symposium on Electromagnetic Compatibility (EMC Sapporo/APEMC). pp. 641–644. doi:10.23919/EMCTokyo.2019.8893676. ISBN   978-4-8855-2322-9. S2CID   207973282 . Retrieved 23 June 2020.
  11. ↑ Wright, M.A. (August 1990). "Common mode current measurements and radiated emissions from long cable systems". Seventh International Conference on Electromagnetic Compatibility, 1990: 19–23.
  12. ↑ Koj, Sebastian; Reschka, Cornelia; Fisahn, Sven; Garbe, Heyno (August 2017). "Radiated electromagnetic emissions from wind energy conversion systems". 2017 IEEE International Symposium on Electromagnetic Compatibility & Signal/Power Integrity (EMCSI). pp. 243–248. doi:10.1109/ISEMC.2017.8077874. ISBN   978-1-5386-2229-2. S2CID   38134771.
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