Siemens SINAMICS G130 Engineering Manual page 178

Fundamental Principles and System Description
Engineering Information
3.1. Determine the current derating factor k
·
k : 1.0 (ambient temperature ≤ 40 °C, installation altitude < 1000 m, degree of protection IP20),
Temp
·
k : 1.0 (factory-set pulse frequency)
Pulse
·
k : 1.0 (not a parallel connection of S120 Motor Modules)
Parallel
·
k : This factor must be taken into account because the press is periodically started up and shut down.
IGBT
According to diagram 2, the following apply:
- Current ratio ΔI = I
- Load duty cycle duration T = T
With ΔI = 4.87 and T = 120 s, the current derating factor is according to derating characteristic 1
which is applicable to the SINAMICS S150 converter with V = 690 V and I
The current derating factor k
= ×
k k k
D Temp Pulse
3.2. Determine the permissible short-time current during periodic startup and shutdown:
With a base load current I
H
= ×
I 1 5 . k
ShortTime
This value is lower than the required maximum motor current of 730 A during periodic startup and shutdown and is
therefore not permissible in terms of preserving the IGBT lifetime.
As a result, the initially selected converter rated for V = 690 V and I
514 A and thus a short-time current I
purpose of ensuring an acceptable IGBT lifetime.This means that the next larger size of converter rated for V = 690 V
and I = 735 A must be used which also has a factory-set pulse frequency in vector control mode of 1.25 kHz. The
rated
calculation must be checked according to the same principle described above (determination of the short-time current
2
and I t calculation). This calculation is not presented in detail here, but confirms that the new converter has been
correctly selected.
Note about calculation example 3:
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 (factory setting) in operating states with very high current. In operating states
with very low current, the drop in temperature can be minimized by operating the converter at 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.
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 type characteristic of this example –
this mode of operation is not a suitable method of reducing temperature fluctuations or prolonging the life of the
IGBTs.
SINAMICS Engineering Manual – November 2015
178/528
Ó Siemens AG
during startup and shutdown:
D
/ I
ShortTime BaseLoad
is therefore calculated as follows:
D
× × = ×
k k 1 0 . 1
Parallel IGBT
= 514 A and a derating factor k
× = × ×
I 1 5 . . 0 91 514 A
D H
of 1.5 • I
ShortTime
IGBT
. This is because the temperature rise can be minimized by operating the
Chip
= 730 A / 150 A = 4.87.
+ T + T +T = 120 s.
0 1 2 3
× × =
0 . 1 0 . . 0 91 . 0 91
.
= 0.91, the permissible short-time current is
D
=
702 A
.
= 575 A, which has a base load current I
rated
= 771 A according to catalog D 21.3, is not suitable for the
H
is < 1.0:
= 575 A: k = 0.91.
rated IGBT
of
H
in the
Chip