Siemens SINAMICS G130 Engineering Manual page 172

Fundamental Principles and System Description
Engineering Information
2
4. Determine the I t value of the motor current:
For the purpose of simplifying the calculation, the actual time characteristic of the motor current I(t) during starting is
approximated by three time phases, i.e. T
must be selected such that the total of phases T
duration of T=300 s. The calculation for the I
1
= ×
2
I t value
T
1
2 =
I t value
300
=
2
I t value
300
2
The calculated I t value of 98 % is slightly lower than the permissible value of 100 % and is thus just about
acceptable within the limits of accuracy of the approximations used to calculate the current characteristic over time.
Note about calculation example 1:
In addition to the motor current characteristic over time I(t) as shown by the blue curve, the characteristic of the junction
temperature over time T (t) of the IGBT chips in the power unit is also shown by the black curve in the diagram below.
j
Time characteristic of the motor current I(t) and the junction
temperature T (t) of the IGBT chips during starting
j
It is apparent from the diagram that very high temperature fluctuations occur in the IGBT at the beginning of the
startup process as a result of the initially very low converter output frequency combined with the very high output
current. This phenomenon is described in section "Power cycling capability of IGBT modules and inverter power
units". These temperature fluctuations in the IGBT increasingly decrease during startup because the output frequency
is continuously rising on the one hand, and the output current also drops towards the end of the startup process on
the other. The peak values of the junction temperature T
150° C at the beginning of the startup process which means that the selected converter doesn't quite reach the limits
of its thermal load capacity during startup and the triggering of an overload reaction is therefore narrowly avoided.
Provided that the unit is operated only occasionally for periods of less than around 2 % of the total operating time of
the drive with temperature fluctuations of this magnitude caused by a combination of low output frequencies and high
output currents (as is the case with this example), the only essential configuring requirement is to ensure that the
maximum permissible chip temperature in the IGBT is not exceeded. The influence of temperature fluctuations on the
IGBT lifetime can be ignored in this instance, i.e. k
However, if the drive is required to operate periodically with high temperature fluctuations of this magnitude caused
by severe load fluctuations and/or a combination of low output frequencies and high output currents, the influence of
temperature fluctuations on the lifetime of the IGBTs must be taken into account as a configuring factor, i.e. the
fluctuations must be limited to permissible values by the application of derating factors, as demonstrated by the
example below.
SINAMICS Engineering Manual – November 2015
172/528
Ó Siemens AG
to T , each with a constant current I
1 3
+T
1
2
t value is therefore
é
2
æ ö æ
I
ç ç ÷ ÷ ç ç
ê × +
1
T
1
×
I k I
ê
è ø è
ë
Rated D
é
2
æ ö
720 A
ê ç ç ÷ ÷
× × +
120 s
s 536 A
ê
è ø
ë
1
[
× + +
217 s 52 s 26
s
IGBT
+T does not exceed the maximum permissible load duty cycle
2 3
2
ö æ
I I
÷ ÷ ç ç
× +
2
T
2
×
k I
ø è
Rated D Rated
2
æ ö æ
500 A 250
ç ç ÷ ÷ × + ç ç
60 s
536 A 536
è ø è
295 s
]
× = ×
s 100 % 100
300 s
of the IGBT chips remain just below the permissible limit of
j
= 1.
to I . In this case, the last phase T
1 3
ù
2
ö
÷ ÷
× ú •
3
T
100 %
3
×
k
ú
ø
û
D
ù
2
ö
A
÷ ÷ × ú
•
120 s
100 %
A
ú
ø
û
=
% 98 %
3