magnetic field calculation

Electromagnetic interference (EMI) is the disruption of operation of an electronic device when it is in the vicinity of an electromagnetic field (EM field) in the radio frequency (RF) spectrum that is caused by another electronic device.

All of us encounter EMI in our everyday life. Common examples are:

magnetic field effect

• Disturbance in the audio/video signals on radio/TV due an aircraft flying at a low altitude

• Noise on microphones from a cell phone handshaking with communication tower to process a call

• A welding machine or a kitchen mixer/grinder generating undesired noise on the radio

• In flights, particularly while taking off or landing, we are required to switch off cell phones since the EMI from an active cell phone interferes with the navigation signals.

Type of Electromagnetic interference

a) In which there is physical contact between the source and the affected circuits, and

b) Radiated – this is caused by induction.

The EMI source experiences rapidly changing electrical currents, and may be natural such as lightning, solar flares, or man-made such as switching off or on of heavy electrical loads like motors, lifts, etc. EMI may interrupt, obstruct, or otherwise cause an appliance to under-perform or even sustain damages.

Recent technologies like Wi-Fi are more robust, using error correcting technologies to minimize the impact of EMI.

All integrated circuits are a potential source of EMI, but assume significance only in conjunction with physically larger components such as printed circuit boards, heat sinks, connecting cables, etc. Mitigation techniques include the use of surge arresters or transzorbs (transient absorbers), decoupling capacitors, etc.

RFI detection with software is a modern method to handle in-band RFI. It can detect the interfering signals in time, frequency or time-frequency domains, and ensures that these signals are eliminated from further analysis of the observed data. This technique is useful for radio astronomy studies, but not so effective for EMI from most man-made sources.

EMI is sometimes put to useful purposes as well, such as for modern warfare, where EMI is deliberately generated to cause jamming of enemy radio networks to disable them for strategic advantages.

Type of Electromagnetic interference on the basis of frequency:

High frequency -(Khz Mhz called RF)

Low frequency –electrical system standard frequency

Electrical gadget produces interference which may affect operation of electronic equipment.

High frequency interference:

It affects wireless transmission such as radio and TV. The interference is generally generated by device like variable frequency drive. Frequency in the drive is controlled by means of a technique called pulse width modulation (PWM).

typical pulse output of inverter

Above wave form contains harmonics.. the harmonic greater than 100KhZ can scape from the drive in two manners-

Radiation- this is an electromagnetic radiation. the drive acts as screen.

Conduction-the conducted interference will travel to other electrical equipment connected to the network i.e. to the mains through the incoming cable and to the motor through the cable.

Government wants to keep the radio, TV transmission frequency band, and wireless communication frequency band and telephone system free of interfrerence.IEC standards are available and adopted by the government. Generally the following equipment s don’t requires any certification –

-Switch mode power supply (SMPS)

-Electronic transformer or ballast

-Commutator or slip ring motor

-Broadcast service local

-Speed controller

-Microprocessor or small device

Low frequency interference

a)      Mal operation of sensitive device and medical equipment

b)      Flicker on computer screen , exception LCD based computers

c)      Interference with communication system

d)      Interference with low current and voltage system such as instrumentation cable and PLC cabling.

e)      5-10 milli gauss can disturb the electronic gadgets.

The interference is caused by electromagnetic wave generated by electrical wiring. Cables and overhead lines carry current and rated frequency (50-60 Hz) and may carry harmonic currents. The current in cables and overhead line produces electromagnetic field and its strength is measured in Tesla or Gausss, 1T=10⁴ Gauss.

calculation of magnetic field

The frequency of the magnetic field is the same as frequency of the current and magnitude is the directly proportional to the magnitude of current.

The typical example of interference -

a)       cable running in the ceiling space

b)      electronic items near with switchboard ,transformer etc

c)      any communication cable running parallel with electrical cable

d)      power and communication cable running on same pole

Calculation of magnetic field in milli Gauss-

I= ampere, d= distance between conductor

Field at a distance R in meter

a)      field due to a single conductor

B=2 X (I/R)

b)      field due to two  conductor in opposite direction

B=2 X (I x d /R²)

c)      three phase three wire

B=3.046 X (I x d /R²)

d)      three phase 4 wire

B=2 X (I/R) (I=unbalance current in neutral)

EXAMPLE

a)      field due to a single conductor

say, I=250A, distance R=2M Then B=2x250/2 =250 milli Gauss

b)      field due to two conductor in opposite direction, 150mm apart

Say, I=250A, distance R=2M Then B=2x (250x150)/1000x2² =18.75 milli Gauss

c)      three phase three wire

Three wire circuit with 250A current per phase, 150mm distance between 2 adjacent conductors, at 2 meter

B=3.046 X (250 x 0.15 /2²) = 28.56milli Gauss

Generally this situation finds at high voltage terminal of transformer.

d)      three phase 4 wire

Neutral current =50A, field at 2 meter distance

B=2 X (50/2) =50milli Gauss

The magnetic induction may be reduced by –

a)      Increasing the distance between the conductor

b)      Reducing the distance between 2 conductors

c)      Balancing the load of three phase supply

Typical magnetic field -

a)      LV switchboard =0.5 to 1.5 milligauss

b)      11KV switchboard =0.5 to 1.5 milligauss

c)      Distribution Transformer without enclosure =10 to 80 milligauss

d)      Distribution Transformer with enclosure =0.5 to 1.5 milligauss

Protection against interference for low voltage circuit

a)      Maintain distance 300 mm -600 mm between power and control cable

b)      Reduce gap by 150 mm

c)      Prepare separate route for power and control cable

d)      Use screen cable for signal /communication cable

Table 1. Reference levels for occupational exposure

Frequency	B limit (H limit), milligauss  (Am-1)	E limit, kVm-1
50 Hz	10.0 (800)	10
60 Hz	10.0 (800)	8.3

Table 2. Reference levels for general public exposure

Frequency	B limit (H limit), milligauss (Am-1)	E limit, kVm-1
50 Hz	2.0 (160)	5
60 Hz	2.0 (160)	4.2

Ref: ITU-T, Geneva