Siemens SINAMICS G130 Engineering Manual page 170

Fundamental Principles and System Description
Engineering Information
Limiter block (LIM) of free function blocks for controlling current-dependent switchover between pulse frequencies
The configured pulse frequency must never be generally increased independently of the different load conditions
using an overload reaction with pulse frequency reduction (p290 = 2 or 3), as described in section "Operation of
converters at increased pulse frequency", in order, for example, to reduce motor noise in applications involving
periodically recurrent load duty cycles with a load duty cycle duration ranging from several seconds to several
minutes. Because, when combined with a pulse frequency higher than the configured pulse frequency, high currents
initiate an overload reaction relatively quickly owing to the high power losses and thus the pulse frequency is reduced
again. However, the overload reaction is not triggered until the IGBT chip temperature T
in order to allow operation of the power unit at a high pulse frequency for as long as possible. This operating mode
with temperature-dependent pulse-frequency switchover as a function of the overload reaction with pulse frequency
reduction thus maximizes the temperature swings in the IGBTs and is therefore not suitable for minimizing
temperature fluctuations in applications with periodically recurrent load duty cycles with short load duty cycle duration
and substantial load fluctuations (current derating factor k
Figure 2 illustrates the interrelationships using the example of a periodically recurrent load duty cycle with a load duty
cycle duration of 2 minutes and load current fluctuations ranging in magnitude between I
T
I(t)
T
T (t)
Chip1
(temperature-depending variation of f
T (t)
Chip3 I
max
I
min
f (t)
Pulse
5,00 kHz
2,50 kHz
2,50 kHz
1,25 kHz
1,25 kHz
0 kHz
0 60
Figure 2: Switchover between pulse frequencies as a function of temperature
In operation at the configured pulse frequency of 1.25 kHz = constant, the temperature characteristic over time
T (t) is shown by the black curve. The associated temperature swing ΔT
Chip1
the purpose of preserving IGBT lifetime provided that the configuring rules described on the previous pages are
observed.
In operation at a pulse frequency increased to 2.5 kHz, the temperature characteristic over time T
the red curve. The inverter attempts to continue operation at the higher pulse frequency for as long as possible.
During the periods of load with I
is triggered and the pulse frequency is reduced to 1.25 kHz. Operation continues at this low pulse frequency until the
chip temperature T (t) during a period of load with I
Chip3
can be changed to 2.5 kHz again.
As a consequence of this temperature-dependent pulse frequency switchover as a function of the overload reaction
with pulse frequency reduction, the temperature swing ΔT
ΔT at a constant, low pulse frequency, with a correspondingly negative impact on the lifetime of the IGBTs in the
Chip1
power unit. The difference in the temperature swing can amount to as much as 5°C in practice, an effect which can
shorten the IGBT lifetime by a factor of 2 to 3.
SINAMICS Engineering Manual – November 2015
170/528
Ó Siemens AG
(t) with f = 1,25 kHz = constant
Chip1 Pulse
(t) with f = 2,5 kHz or 1,25 kHz
Chip3 Pulse
I
max
I I
min
2,50 kHz 2,50 kHz
1,25 kHz
120 180 240
, the chip temperature T
max
< 1.0).
IGBT
)
Pulse
ΔT < ΔT
Chip1 Chip3
min
t / s
300
Chip1
(t) reaches such a high level that an overload reaction
Chip3
has decreased far enough again that the pulse frequency
min
is significantly higher than the temperature swing
Chip3
reaches a very high level
Chip
and I .
min max
remains within acceptable limits for
(t) is shown by
Chip3