Siemens SINAMICS G130 Engineering Manual page 175

Note about calculation example 2:
In addition to the absolute motor current value |I(t)| as shown by the blue curve and the motor frequency f(t) as shown
by the red curve, the characteristic of the junction temperature over time T
also shown by the black curve in the diagram below.
Absolute value of motor current |I(t)|, motor frequency f(t) and characteristic of junction temperature
over time T (t) of the IGBT chips during the periodic load duty cycle of the centrifuge
j
With the peak values of the junction temperature T
are well below the permissible limit of 150° C in this periodic load duty cycle. But the temperature fluctuations in the
IGBT are relatively high as a result of two effects. One effect is the very high variations in current which occur
alternately in the different phases of the load duty cycle (low current during filling and spinning, high current during
acceleration and braking), and the other effect is the relatively long discharge phase which is characterized by a
combination of low output frequency and relatively high current.
Both effects have been taken into account in this example by the application of the correct derating factors with the
result that the temperature fluctuations of around 35°C remain within acceptable limits with respect to preservation of
the IGBT lifetime.
In order to minimize to the greatest possible extent the inevitable temperature fluctuations which occur as a result of
load changes within a load duty cycle, it is advisable in this instance to configure the drive according to the criteria
described above and also to note the following additional points at the commissioning stage because the result of the
configuring calculation of the current derating factor k
The drive must be commissioned with the configured pulse frequency (which corresponds to the factory setting in this
example), or with meaningfully selected, current-dependent switchover between different pulse frequencies.
Current-dependent switchover between different pulse frequencies is the best solution for achieving an additional
reduction in the temperature swings ΔT
converter on a very low pulse frequency in operating states with very high current (acceleration phase T
deceleration phase T ). In operating states with very low current (filling phase T
4
temperature reduction can be minimized by operating the converter on a very high pulse frequency. By using
switchover between different pulse frequencies as a function of current, it is possible to reduce temperature
fluctuations and thus prolong the lifetime of the IGBTs.
Furthermore, the temperature fluctuations caused by the low output frequency during the discharge phase T
be minimized through selection of the low pulse frequency corresponding to the factory setting and by attempting
(within the limits imposed by the process) to keep the output frequency as high as possible during the discharge
phase T and setting the output current to the lowest possible value. Even very small increases in the output
5
frequency, e.g. from 5 Hz to 7 Hz, or minor reductions by a few 10 A in the output current can effect a substantial
reduction in the temperature fluctuations in this instance.
The configured pulse frequency should never be generally increased independently of the different load conditions
using an overload reaction with pulse frequency reduction (p290 = 2 or 3). Because, when combined with an
increased pulse frequency, high currents trigger an overload reaction very quickly owing to the high power losses.
However, this overload reaction is not triggered until the IGBT chip temperature reaches a very high level in order to
allow operation of the power unit at the increased pulse frequency for as long as possible. This mode of operation
with temperature-dependent pulse frequency switchover therefore maximizes the temperature swings ΔT
IGBTs. With load duty cycles involving substantial load current fluctuations – of the type characteristic of this example
with ΔI = 4.5 – this mode of operation is not a suitable method of reducing temperature fluctuations or prolonging the
life of the IGBTs.
Fundamental Principles and System Description
of the IGBT chips measuring around 105° C, it is clear that they
j
is 0.8 < 1.0:
IGBT
. This is because the temperature rise can be minimized by operating the
Chip
SINAMICS Engineering Manual – November 2015
Engineering Information
(t) of the IGBT chips in the power unit is
j
and spinning phase T
1
Ó Siemens AG
and
2
), the
3
should
5
in the
Chip
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