Dimensioning the units for optimized load current

In the catalog, the components in the cabinets, for example, contactor, commutating reactor and busbars are dimensioned and installed based on the rated converter current. In practice, when referred to the actual motor load current, these components can be overdimensioned. On request, and when specifying the load cycle and the motor currents, a check can be made as to whether it makes economic sense to adapt the current-carrying components to the plant requirements.

Determining the dynamic overload capability

Function overview

The rated DC current specified on the unit rating plate (max. permissible continuous DC current) may be exceeded in operation. The extent to which this value is exceeded and how long this lasts are subject to certain limits, which are explained in more detail in the following.

The absolute upper limit for the absolute value of the overload currents is 1.8x the rated DC current. The max. overload duration depends on the time characteristic of the overload current as well as on the load history of the unit and also depends on the specific unit.

Each overload cycle must be preceded by an underload cycle (load cycle with load current < rated DC current). Once the max. permissible overload duration has elapsed, the load current must return to at least an absolute value ? the rated DC current.

The dynamic overload duration is made possible by thermally monitoring the power section (I²t monitoring). I²t monitoring uses the time characteristic of the actual load current to calculate the time characteristic of a substitute value for the increase of the depletion layer temperature of the thyristors above the ambient temperature. In this case, unit-specific properties (e.g. thermal resistances and time constants) are incorporated in the calculation. When the converter unit is switched on, the calculation process starts with the initial values that were determined before the shutdown/line supply failure. The environmental conditions (ambient temperature and installation altitude) must be taken into account when setting a parameter.

I ² t monitoring responds when the calculated substitute depletion layer temperature rise exceeds the permissible value. Two alternatives can be parameterized as response:

• Alarm with a reduction of the armature current setpoint to the rated DC current or
• Fault with unit shutdown

The I²t monitoring can be disabled. In this case, the armature current is limited to the rated DC current.

Configuring for the dynamic overload capability

The configuring sheets contain the following information:

• The max. overload duration t_an when starting with a cold power section and specified, constant overload,
• The max. zero current interval t_ab (max. cooling down time) until the "cold" thermal state of the power section is reached, and
• Limit characteristic fields for determining the overload capability during thermally stabilized, intermittent operation with overload (periodic duty cycles)

Remark: The power section is considered to be "cold" if the calculated substitute depletion layer temperature rise is less than 5 % of its max. permissible value. This state can be queried using a binary user-assignable output.

Structure of the limit characteristic fields for intermittent operation with overload

The limit characteristic fields refer to a duty cycle of the intermittent overload operation with a total duration (time period) of 300 s. Such a duty cycle comprises two time sections - the base-load duration (armature current actual value ? rated DC current) and the overload duration (armature current actual value ? rated DC current).

Each limit characteristic represents a unit-specific max. base-load current for a specific overload factor (limiting base-load current, specified as a % of the rated DC current) over the min. base-load duration (limiting base-load duration). For the remaining duration of the duty cycle, the max. permissible overload current is determined by the overload factor. If no limit characteristic has been specified for the required overload factor, then it will be subject to the limiting characteristic for the next highest overload factor.

The limit characteristic fields of the particular SINAMICS DC MASTER DC Converter are shown in Catalog D 23.1. The characteristic for the corresponding DC Converter should be used for the SINAMICS DC MASTER Cabinet. Derating data that influence the rated DC current of the unit and therefore the associated characteristic, for example, the installation altitude and the ambient temperature, should be taken from this Catalog D 23.2 Chapter "Technical data".

The limit characteristic fields are valid for a duty cycle of 300 s. Using basic algorithms, duty cycles can be configured with duty cycle durations of longer than or shorter than 300 s. This will now be shown using two basic tasks.

Characteristic example for basic tasks 1 and 2

Basic task 1

• Given:
  Unit, cycle duration, overload factor, overload duration
• To be found:
  (Min.) base-load duration and max. base-load current
• Solution:

Example for basic task 1

• Given:
  
  • Unit with 30 A
  • Cycle duration 113.2 s
  • Overload factor 1.45
  • Overload duration 20 s
• To be found:
  
  • (Min.) base-load duration
  • Max. base-load current
• Solution:
  
  • Limit characteristic for a unit with 30 A
  • Overload factor 1.5
  • Overload duration₃₀₀ = 300 s/113.2 s)  ? 20 s = 53 s > (see the characteristic example for basic tasks 1 and 2)
  • Max. base-load current = 44 % I_rated = 13.2 A

Basic task 2

• Given:
  Unit, cycle duration, overload factor, base-load current
• To be found:
  Maximum overload duration, minimum base-load duration
• Solution:

Example for basic task 2

• Given:
  
  • Unit with 30 A
  • Cycle duration 140 s
  • Overload factor 1.15
  • Base load current = 0.6 ? I_rated = 18 A
• To be found:
  
  • Maximum overload duration
  • Minimum base-load duration

Solution:

• Limit characteristic for a unit with 30 A
• Overload factor 1.2
• Base load current = 60 % I_rated > (see characteristic example for basic tasks 1 and 2)
• Overload duration₃₀₀ = 127 s
• Max. overload duration = 140 s / 300 s ? 127 s = 59 s
• Min. base-load duration = 140 s - 59 s = 81 s

Base-load duration₃₀₀ = min. base-load duration for 300 s cycle duration (300 s overload duration)

Overload duration₃₀₀ = max. overload duration for 300 s cycle duration

6RM8013?6DV62?0AA0 15 A/four-quadrant operation 400 V, 6RM8013?6FV62?0AA0 15 A/four-quadrant operation 480 V

Load classes

In order to be able to adapt the SINAMICS DC MASTER as simply as possible to the load profile of the driven machine, in addition to the individual dimensioning using the limit characteristics of the dynamic overload capability, these can also be dimensioned using pre-selected load cycles that are simple to parameterize.

Note:

SINAMICS DC MASTER does not monitor whether the load class - set using parameters - is maintained. If the power section permits it, the unit can operate for overload durations in excess of those defined by the load class. This means that the driven machine of the mechanical system is not protected against overload!

The overload duration that is actually permitted for the power section in question is always longer than the duration defined by the load class, and the SINAMICS DC MASTER does monitor whether the overload duration that is actually permitted for the power section is being adhered to.

Load class (parameter)	Load for the converter	Load cycle
DC I	I DC I continuous (IdN)
DC II	I DC II for 15 min and 1.5 ? IDC II for 60 s
DC III	I DC III for 15 min and 1.5 ? IDC III for 120 s
DC IV	I DC IV for 15 min and 2 ? IDC IV for 10 s
US rating	I US for 15 min and 1.5 ? IUS for 60 s Note: With this setting, for all drive cabinet types, an ambient and/or cooling medium temperature of 40 °C is permissible.

Duty cycles for two-quadrant operation

¹⁾ For other temperatures, see under "Coolant temperature and installation altitude" in the catalog part "SINAMICS DC MASTER Cabinet", Section "Ordering and technology" – "Technical data".

Duty cycles for four-quadrant operation

¹⁾ For other temperatures, see under "Coolant temperature and installation altitude" in the catalog part "SINAMICS DC MASTER Cabinet", Section "Ordering and technology" – "Technical data".