Effects Of Using Fast-Switching Power Components (Igbts); Increased Current Load On The Inverter Output As A Result Of Long Motor Cables - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information
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Effects of using fast-switching power components (IGBTs)

IGBTs (Insulated Gate Bipolar Transistors) are the only type of power semiconductors used in the power units of the
SINAMICS motor-side inverters. One of the characteristics of these modern power components is that they are
capable of very fast switching. As a result, the power losses incurred in the inverter with every switching operation
are kept low and the inverter can therefore be operated at a relatively high pulse frequency. It is thus possible to
obtain a motor current which is very close to sinusoidal and the oscillating torques and stray losses caused in the
motor by the converter operation remain low.
The fast switching of the IGBTs does, however, cause undesirable side effects.
When long motor cables are used, the substantial motor cable capacitances are charged and discharged very quickly
with every switching operation of the IGBTs, thereby loading the inverter output with high additional current peaks.
Furthermore, the propagation time of the electromagnetic waves moving along the motor cable causes voltage
reflections at the motor terminals which increase the voltage on the motor winding.
Another effect is the increased current flow in the motor bearings caused by the high voltage rate of rise at the motor
terminals of converter-fed drives.
All these effects need to be considered when the drive is configured to prevent the inverter from shutting down with
the error message "Overcurrent" before it reaches its configured output current and to protect the motor against
premature failure due to winding or bearing damage.
The individual side effects and appropriate corrective actions are discussed in more detail below.

Increased current load on the inverter output as a result of long motor cables

The cable capacitance of motor cables is in proportion to their length. The cable capacitance on very long motor
cables is therefore substantial, particularly if the cables are shielded or several cables are installed in parallel in the
case of drives with high power ratings.
This capacitances are charged and discharged with every switching operation of the IGBTs in the inverter, as a result
of which additional current peaks are superimposed on the actual motor current, as the diagram below illustrates.
Instantaneous values of inverter output voltage and inverter output current with long motor cables
SINAMICS Engineering Manual - May 2008
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