Derating Factors; Derating With Temperature; Operation With Long Cables - Siemens MICROMASTER 420 Reference Manual

2.10

Derating Factors

2.10.1

Derating with Temperature

Losses within the power module rise with increasing switching frequencies, leading
to higher heatsink temperatures. Operation of the inverter outside its
recommended ambient operating temperature would normally cause the inverter to
trip with an overtemperature fault code. To avoid such tripping, the inverter
automatically reduces its switching frequency ( e.g. from 16kHz to 8kHz), thus
reducing the temperature of the heatsink, enabling the application to continue
running trip free. Should the load or ambient temperature then reduce, the inverter
will first check to see if it is safe to increase the switching frequency again and if
considered safe will then do so. See also paragraph 2.11 Cubicle Calculations, on
Page 26.
100
80
60
40
20
0
Figure 2-14
2.10.2

Operation with Long Cables

All cables have stray capacitance between conductors, and between conductors
and Earth. Current will flow in this capacitance depending on the voltage and
frequency of the signal in the cables.
The output of all PWM (Pulse Width Modulated) inverters includes fast switching
edges with very high rates of change of voltage which have a significant high
frequency component.
The total current in the stray cable capacitance is determined by the voltage level
and the switching speed, as well as the stray capacitance value.
The value of stray capacitance itself is dependent of the cable length, the type of
cable, how and where it is laid, and whether it is contained in a screen or armour
(which would normally be connected to earth).
If the resultant capacitance value and frequency content is high, peak currents may
flow in the stray cable capacitance resulting in excessive heating in the inverter, or
even in instantaneous trip values being reached, causing the inverter to shut down.
Extensive testing and experience indicates that measurements with screened, and
with loosely bundled unscreened cables give results which are valid for most
practical applications. Input cable length has no influence.
All MICROMASTER 420 inverters are fully tested and characterized for worst case
operation (50
22
-10 0 10 20 30 40 50 60
Degrees Celsius
Derating with Temperature
0
C, Full load current etc. default switching frequency) when used with
70
MICROMASTER 420 Reference Manual
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