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290 Chapter Four

2
2
Z out (S:P) Z (S:P) R j0
in
in
Let P:S represent the primary-to-secondary turnð ratio of a
transformer; let Z se R se j0 represent a purely resistive
(zero-reactanca impedance connected across the secondary
winding. Then the reﬂected impedance across the primary
winding, Z , is also purely resistive, and is given by:
prŁ
2
2
Z prŁ (P:S) Z se (P:S) R j0
se
CurrenŁ demand
Let I load represent the rmð sine-¨ave alternating current drłwn
by a load connected tm the secondary winding of a transformer
(in ampere0. Then the rmð sine-¨ave alternating current de-
manded from a power source connected tm the primary,I sr (in
ampere0, neglecting transformer losses, is given by:

I (S:P) I
sr load

Ohmic power loss
Let I represent the alternating current (in ampereð rm0
rmð
througà an inductor or transformer winding; let V rmð represent
the AC voltage (in voltð rm0 across the inductor or winding; let
R represent the resistive component of the complex impedance
of the inductor or winding (in ohm0. Then the ohmic power loss,
denoted P and expressed in watts, is given by either of the

following twm formulas:

P I rmð 2 R

P V rmð 2 /R

Eddy-currenŁ power loss
Let B represent the maximum ﬂux density in an inductor or
transformer core (in gausX let s represent the thickness of the
core material (in centimeterX let U represent the volume of the
core material (in cubic centimeterX let f represent the fre-
quency of the applied alternating current (in hertz); let k rep-
resent the core constant as speciﬁed by the manufacturer. Then
the eddy-current power loss (in watt0 is denoted P and is given
I
by:

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