01 March, 2007

Admittance and Susceptance

Admittance :
In electrical engineering, the admittance (Y) is the inverse of the impedance (Z). The SI unit of admittance is the siemens. Oliver Heaviside coined the term in December 1887.Admittance is a measure of how much current is admitted in a circuit. The admittance has its most obvious utility in dealing with parallel AC circuits.

Y is the admittance, measured in siemens
Z is the impedance, measured in ohms.

Therefore the expression for admittance in terms of voltage and current can also be wriiten as
Y = I/V = G + j B
G, the real part of the admittance, is the conductance of the circuit, and B, the imaginary part of the admittance, is the susceptance of the circuit. The units of admittance are called siemens or mhos (reciprocal ohms).

Substituting the expression of Z = R + j X in Y = 1/Z after simplifying and equating to G + j B the expressions are

The magnitude of admittance is given by:

Susceptance :
In electrical engineering, the susceptance (B) is the imaginary part of the admittance. In SI units, the susceptance is measured in siemens. Oliver Heaviside first defined this property, which he called permittance, in June 1887.Susceptance is the measure of how much a circuit is susceptible to conducting a changing current.

It can also be defined as the opposite of reactance.Just as there's capacitive reactance and inductive reactance, so too there is capacitive susceptance (BC) and inductive susceptance (BL). Just like with conductance, both of these are the reciprocal of their corresponding reactances. That is to say, capacitive susceptance is 1 divided by the capacitive reactance, and inductive susceptance is 1 divided by the inductive reactance.

Thus, capacitive susceptance can be simplified into the following equation: BC = 2*pi*f*C.

Inductive susceptance, meanwhile, becomes exactly like the formula for capacitive reactance, except that it of course uses inductance rather than capacitance: BL = 1/2*pi*f*L

For more details on this topic visit the following sites :
http://www.allaboutcircuits.com/vol_2/chpt_5/5.html http://www.geocities.com/siliconvalley/2072/eleccsa.htm

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