By Arthur R. von Hippel
Reissued to commemorate the fortieth anniversary of the unique booklet of "Dielectrics and Waves". A complete assurance of a topic that crosses the fields of chemistry, physics and engineering and may be useful to somebody with an curiosity in dielectric engineering.
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This type of conducted interference is termed common-mode interference. MAINS SUPPLY APPLIANCE GENERATING EMI N EQUIPMENT BEING INTERFERED WITH Γ" ELECTRICAL Γ ELECTRICAL INTERFERENCE CURRENTS I CIRCUITRY ! < ίθ. CIRCUITRY 42i STRAY CAPACITANCE' STRAY CAPACITANCE INTERFERENCE CURRENT If two capacitors are connected, one between the live and earth, and the other between neutral and earth at the appliance, then the common-mode interference currents are offered an alternative path. Less of the common-mode interference currents will flow to the other equipment.
Inductance is increased in proportion to the number of turns squared. Inductance of a long coil L A / μ = inductance in Henrys = cross sectional area of the coil in square metres = length of the coil in metres = magnetic permeability of the coil's core (=4π x 10 7 for air) This formula is only approximate because it assumes that all the magnetic flux lines link all the turns of the coil. In practice some of the flux lines fail to link some of the turns near the ends of the coil. If the length of the coil is lOx its diameter, then the true inductance is 4% less than that given by the formula.
It is now the 100 ohm resistor R2 which is dictating the current flowing from the voltage source as the impedance presented by the transformer is much lower than R2. Tl is now connected as a current transformer as it is transforming the current set by R2 across to the resistor Rl. A current transformer can be removed from a system and replaced with a short circuit and the remaining system will be unaffected. This cannot be done with a voltage transformer. In practice, current transformers are designed with a low number of turns on the primary (often 1/2 turn) and a much larger number of turns on the secondary.