# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 482

470
π⋅
+ ⋅
⋅µ
=
2
25.0
d
a2 ln
L
0
'
cable
Eq. 13.13
'
cable
L
: Cable inductanceper length


µ
m
H
µ
0
:
Vacuum permeability
m
H 10 4
7
⋅π⋅
a:
Distancebetween conductors [m]
d:
Conductor diameter [m]
aswell as the cable capacitance per unit of length
d
a2 ln
2
C
r 0
'
cable
ε⋅ε⋅π⋅
=
Eq. 13.14
'
cable
C
: Cable capacitance per length


m
pF
ε
0
:
Vacuum permittivity
m
F 10
85419 .8
12
ε
r
:
Relative static permittivity
a:
Distancebetween conductors [m]
d:
Conductor diameter [m]
this results in:
'
cable
'
cable
cable
C
L
Z
=
Eq. 13.15
Common values formotor cable impedances are in the rangeof 50
to75
.
The amplitude of the reflected voltageU
ref
can be defined as:
cable
motor
cable
motor
inverter
ref
Z Z
Z Z U U
+
=
Eq. 13.16
U
inverter
is the output voltage of the inverter. If one assumes the motor impedance to be
significantly higher than the cable impedance the
s simplified to:
inverter
ref
U U
=
Eq. 13.17
This means that the reflected voltage at themotor terminals has the same value as the
incoming voltage from the inverter. Both the voltages are added at the motor terminals
and results in a total peak voltage of
inverter
ref
inverter
motor
U2 U U U
⋅ = +
=
Eq. 13.18
Now it has to be observed that the voltage reflected from themotor terminals returns to
the inverter terminals where it is again reflected and runs back to the motor terminals.
Because thewave impedance of the inverter is virtually 0 for higher frequencies – since
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