Specifications

5-15
5.3.2 Inter-winding capacitance (C)
The inter-winding capacitance between the primary and the secondary is measured by connecting
one side of each winding to the instrument as shown in Figure 5-21.
5.3.3 Mutual inductance (M)
Mutual inductance (M) can be obtained by using either of two measurement methods:
(1) The mutual inductance can be derived from the measured inductance in the series aiding and
the series opposing configurations (see Figure 5-22 (a).) Since the combined inductance (La) in
the series aiding connection is La = L
1
+ L
2
+ 2M and that Lo in the series opposing connection
is Lo = L
1
+ L
2
– 2M, the mutual inductance is calculated as M = (La – Lo)/4.
(2) By connecting the transformer windings as shown in Figure 5-22 (b), the mutual inductance
value is directly obtained from inductance measurement. When test current (I) flows through
the primary winding, the secondary voltage is given by V = jwM x I. Therefore, the mutual
inductance can be calculated from the ratio between the secondary voltage (V) and the primary
current (I.) However, the applicable frequency range of both measurement techniques is limited
by the type and the parameter values of transformer being measured. These methods assume
that the stray capacitance effect, including the distributed capacitance of windings, inter-
winding capacitance, and test lead capacitance, is sufficiently small. To minimize the cable
capacitance effect for the method shown in Figure 5-22 (b), the Hp test lead length should be
made as short as possible. It is recommend to use both techniques and to cross-check the
results.
Figure 5-21. Inter-winding capacitance measurement
Figure 5-22. Mutual inductance measurement
L
a
M =
L
a
-L
o
4
L
o
L
1
L
2
L
1
L
2
L
a
= L
1
+ L
2
+ 2M
L
o
= L
1
+ L
2
-2M
(a) Series aiding and series opposing
H
c
L
p
L
c
H
p
M
I
V
M =
V
jwI
V = jwMI
(b) Direct connection technique
H
c
L
p
L
c
H
p
C