Free Load Duty Cycles - Siemens SINAMICS G130 Engineering Manual

Free load duty cycles

Many applications require load duty cycles which deviate more or less from the standard load duty cycles defined
above. For this reason, the physical design criteria on which load duty cycles are based and methods by which free
load duty cycles can be correlated mathematically with standard load duty cycles are described below.
Three criteria must always be fulfilled in each load duty cycle to prevent the power unit from becoming overloaded,
which could cause fault tripping or initiate an overload reaction or reduce the lifetime of the converter:
•
The magnitude of the short-time current must be limited.
•
The average power loss in the power unit must be limited for the duration of the load duty cycle.
•
The frequency or the magnitude of the temperature variations of the IGBT chips must be limited during the load
duty cycle to ensure that the lifetime of the power unit is not affected.
The magnitude of the short-time current must be limited for several reasons. The first reason is that there needs to be
a sufficient margin between the short-time current and the overcurrent trip level of the power unit in order to prevent
shutting down on overcurrent. The second reason is that the chip temperature in the IGBT rises during the overload
period. This rise is in proportion to the square of the short-time current, which means that the permissible overload
duration decreases more than the short-time current rises. As a result, the converter would trip extremely fast due to
thermal overloading of the chip when the short-time current is too high.
The average power loss in the power unit during the load duty cycle must be limited and must not exceed the
corresponding power loss during steady-state continuous operation with the permissible output current for which the
power unit has been dimensioned.
The frequency or magnitude of the temperature variations of the IGBT chips must be limited during the load duty
cycle to ensure that the lifetime of the power unit is not affected. This is due to the fact that the number of permissible
temperature cycles of an IGBT is limited and decreases in proportion to the rise of the temperature swing ΔT
lifetime of the IGBT, therefore, also decreases accordingly in proportion to the rise of the temperature swing ΔT
This means:
If, during a free periodic load duty cycle, the frequency of the temperature swing ΔT
cycles, the magnitude of the short-time current does not need to be limited with respect to the lifetime in addition to
the above-mentioned criteria. This applies in the case of long load duty cycles with durations of T ≥ 60 s.
If, during a free periodic load duty cycle, the frequency of the temperature swing ΔT
cycles, the magnitude of the short-time current must be limited with respect to the lifetime in addition to the above-
mentioned criteria. This applies in the case of short load duty cycles with durations of T < 60 s.
Note:
In the case of load duty cycles that are extremely short (i.e. a few seconds (T < app. 10 s)), servo control should be
used instead of vector control due to the high dynamic response.
If the above-mentioned criteria are applied to the SINAMICS G130, G150 and S150 converters and to the SINAMICS
S120 Motor Modules (Chassis and Cabinet Modules), then free load duty cycles are permissible whenever the
following conditions are fulfilled:
•
The short-time current I
(In the case of parallel connections of S120 Motor Modules, I
inverter resp. one Motor Module)
• 2
The I t value, averaged over a load duty cycle duration T of max. 300 s, must not exceed 100 %.
2
The I t value is the evaluation criterion for losses and temperature rise in the power unit for the duration of the load
duty cycle and is defined as follows:
Fundamental Principles and System Description
must not exceed 1.5
ShortTime
⎛
T
1 t I ) (
∫
⎜ ⎜
= ⋅
2
I t value
⋅
⎝
T I
Rated
0
Engineering Information
I k .
* H * D
is the short-time current of a partial
ShortTime
2
⎞
⎟ ⎟
dt
• 100 %
⎠
k
D
SINAMICS Engineering Manual - May 2008
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is low due to long load duty
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© Siemens AG
. The
.
Chip