Motor Reactors; Reduction Of The Voltage Rate Of Rise Dv/Dt At The Motor Terminals; Reduction Of Additional Current Peaks When Long Motor Cables Are Used - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information
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Motor reactors

Reduction of the voltage rate of rise dv/dt at the motor terminals

As described in detail in the section "Effects of using fast-switching power components (IGBTs)", very high voltage
rate of rise dv/dt occurs at the inverter output and the motor terminals.
This rate of rise can be reduced through the use of motor reactors.
In systems without motor reactors, the voltage edges at the inverter output which have a rate of rise dv/dt of typically
3 kV/μs – 6k V/μs, move along the cable towards the motor and reach the motor terminals with a virtually unchanged
rate of rise. The resultant voltage reflections cause voltage spikes which can reach up to twice the DC link voltage,
see Figure a) in diagram below.
As a result, the motor winding is subjected in two respects to a higher voltage stress than would normally be imposed
by a sinusoidal supply. The voltage rate of rise dv/dt is very steep and the voltage spikes V caused by the reflection
PP
are also very high.
a) without motor reactor b) with motor reactor
Voltage v(t) at the inverter output and at the motor terminals
When motor reactors are installed, the reactor inductance and the cable capacitance are forming an oscillating circuit
which reduces the voltage rate of rise dv/dt. The higher the cable capacitance is, i.e. the longer the cable is, the
greater the reduction in the rate of rise. When long, shielded cables are used, the voltage rate of rise drops to just a
few 100 V/μs, see Figure b) in diagram. Unfortunately, however, the oscillating circuit built by the reactor inductance
and the cable capacitance is only relatively weakly damped so that severe voltage overshoots can occur. The voltage
spikes at the motor terminals are therefore of the same order of magnitude as the spikes caused by reflections in
systems without motor reactor.
Since the motor reactor reduces only the voltage rate of rise dv/dt rather than the voltage spikes V , there is no
PP
fundamental difference in the quality of the voltage stress for the winding as compared to systems without a motor
reactor.
The use of a motor reactor is thus not a suitable solution for improving the voltage stress for the winding of the motor
with line supply voltages of between 500 V and 690 V to such an extent that it is possible to dispense with special
insulation in the motor. This level of improvement can be achieved only by means of dv/dt filters plus VPL or sine-
wave filters (see sections "dv/dt filters plus VPL" and "Sine-wave filters").
Although the reduction of the voltage rate of rise attenuates the bearing currents in the motor, this is not sufficient to
completely obviate the need for an insulated NDE bearing in the motor.

Reduction of additional current peaks when long motor cables are used

As a result of the high voltage rate of rise of the fast-switching IGBTs, the cable capacitance of long motor cables
changes polarity very quickly with every switching operation in the inverter, thereby loading the inverter output with
high additional current peaks.
The use of motor reactors reduces the magnitude of these additional peaks because the cable capacitance changes
polarity more slowly due to the reactor inductance, thereby attenuating the amplitudes of the current peaks.
Suitably dimensioned motor reactors or series connections of motor reactors therefore offer a solution which allows a
higher capacitance and thus also longer motor cables to be connected.
SINAMICS Engineering Manual - May 2008
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