Free Load Duty Cycles - Siemens SINAMICS G130 Engineering Manual

Definition of the standard load duty cycle low overload
1.13.3

Free load duty cycles

Many applications require load duty cycles which deviate to a greater or less extent from the standard load duty
cycles defined above. The section below therefore explains the physical correlations and design criteria which must
be taken into account.
Load duty cycles must fullfill the following criteria in order to prevent overloading of the power unit and thus avoid
immediate fault tripping, initiation of an overload reaction or reduction in the life of the converter:
·
The magnitude of the short-time current must generally be limited to permissible values in order to prevent
initiation of an overload reaction.
·
In the case of periodically recurring load duty cycles, the frequency and/or magnitude of variations in the
IGBT chip temperature must be limited to permissible values during the load duty cycle to prevent premature
failure of power units.
·
The average power losses in the power unit must generally be limited during the duty cycle to the value
permissible for steady-state continuous operation in order to prevent initiation of an overload reaction.
The magnitude of the short-time current must generally be limited for a number of reasons: The first reason is that a
sufficient margin must be maintained between the current during the overload period and the overcurrent tripping
threshold of the power unit in order to prevent the unit from shutting down immediately on overcurrent. The second
reason is that the chip temperature in the IGBT rises during the overload period. Since this rise is proportional to the
square of the short-time current, there is a disproportionate reduction in the permissible overload period as the short-
time current rises. As a result, a very high short-time current will very quickly result in initiation of an overload reaction
or tripping of the unit due to chip thermal overloading.
In the case of periodic load duty cycles, the magnitude and/or frequency of variations in the IGBT chip temperature
must be limited during the load duty cycle in order to prevent premature failure of power units. This is necessary
because the number of permissible temperature cycles of an IGBT is limited and is further reduced as the
temperature swing ΔT
Chip
the rise of the temperature swing ΔT
inverter power units". The following therefore applies to periodically recurrent load duty cycles:
If the ratio ΔI between short-time current and base load current is small in periodic load duty cycles, and the resultant
temperature swings ΔT
Chip
only slightly limited in addition to the criteria stated above for the purpose of preserving lifetime as the duration of the
duty cycle decreases.
If the ratio ΔI between short-time current and base load current is large in periodic load duty cycles, and the resultant
temperature swings ΔT
Chip
addition to the criteria stated above for the purpose of preserving lifetime as the duration of the duty cycle decreases.
The average power losses in the power unit during the load duty cycle must be limited generally and must not exceed
the corresponding power loss in steady-state continuous operation at the permissible output current for which the
power unit is thermally rated. This is necessary in order to prevent initiation of an overload reaction or tripping of the
unit due to excessive chip temperature.
Fundamental Principles and System Description
increases. The lifetime of the IGBT therefore also decreases accordingly in proportion to
, as described in section "Power cycling capability of IGBT modules and
Chip
are correspondingly low, the magnitude of the short-time current needs not be limited or
are correspondingly high, the magnitude of the short-time current must be limited in
Engineering Information
Definition of the standard load duty cycle high overload
SINAMICS Engineering Manual – November 2015
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