Increased Voltage Stress On The Motor Winding As A Result Of Long Motor Cables - Siemens SINAMICS G130 Engineering Manual

Increased voltage stress on the motor winding as a result of long motor cables

The IGBTs used in SINAMICS inverters connect the DC link voltage V
≥ 0.1 μs. In the case of a 690 V supply with a DC link voltage of virtually 1000 V, this corresponds to a voltage
T
r
jump with a rate of rise of dv/dt ≤ 10 kV/μs. The typical average values of voltage rate of rise for SINAMICS inverters
are dv/dt = 3 kV/μs to 6 kV/μs. If the inverter output is connected directly to the motor cable, i.e. no motor-side
options such as motor reactors, dv/dt filters plus VPL or sine-wave filters are installed at the inverter output, this
voltage jump moves along the motor cable towards the motor with a velocity of about 150 m/μs (half the speed of
light).
Since the impedance Z
W Motor
voltage jump arriving at the motor winding is reflected, causing brief voltage spikes on the motor winding which can
reach values of twice the magnitude of the DC link voltage V
Inverter output
(dv/dt ≤ 10 kV/μs)
Characteristic voltage at the inverter output and the motor when a long motor cable is used
The voltage reflection on the motor winding reaches its maximum value when the propagation time t
motor cable is greater than the rise time T
With a minimum rise time of the voltage jump of T
v = 150 m/μs, the critical cable length at which the voltage reflection reaches its maximum value can be calculated as
The voltage reflection can therefore reach its maximum value already at relatively short motor cable lengths of 15 m
and above. We must therefore assume that in most applications where the inverter output is directly connected to the
motor cable and no motor reactors or motor filters are installed, significant voltages spikes of up to twice the
magnitude of the DC link voltage will occur on the motor caused by reflections.
In cases where motor cable lengths of > 15 m are used and voltage spikes due to reflections must be expected, the
absolute magnitude of the reflection-related voltage spikes on the motor are depending on two influencing variables,
•
the DC link voltage V
•
the reflection factor r at the motor terminals.
The DC link voltage V
DCLink
•
the line supply voltage V
•
the type of Infeed (Basic Infeed / Smart Infeed or Active Infeed), and
•
the operating conditions of the drive (normal motor operation or braking operation using
the V
DC max
Fundamental Principles and System Description
of the motor is significantly higher than the impedance Z
Motor cable length
(> 15 m)
of the voltage jump at the inverter output, i.e.
r
l
Cable
< =
T t
r Pr op
v
r
m
>
v =
l T 150
*
Cable *
μ
r
s
of the inverter and
DCLink
of the inverter is itself depending on three influencing variables,
of the drive,
Line
controller or a braking unit).
to the inverter output with a rise time of
DCLink
.
DCLink
= 0.1 µs and a propagation velocity along the motor cable of
μ
=
0 1 , s 15 m
SINAMICS Engineering Manual - May 2008
Engineering Information
of the motor cable, the
W Cable
Voltage spikes due
to reflections
along the
Prop
77/396
© Siemens AG