Motor-Side Reactors And Filters; 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

1.10 Motor-side reactors and filters

1.10.1

Motor reactors

1.10.1.1 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/ms – 6k V/ms, 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
reflection are also very high.
a) without 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/ms, see Figure b) in diagram. Unfortunately, however, the oscillating circuit built by the reactor inductance
and the cable capacitance is relatively weakly damped so that severe voltage overshoots occur. If a motor reactor is
installed, the voltage peaks at the motor terminals are therefore typically only around 10 % to maximum 15 % lower
than those produced by reflections without motor reactor..
While the motor reactor significantly reduces the voltage rate-of-rise dv/dt, it dampens the voltage spikes V
a limited extent, and the difference in the quality of the voltage stress by comparison with systems without a motor
reactor is therefore only minimal.
As a result, the use of a motor reactor is not generally a suitable solution for reducing the voltage stress on the motor
winding with line supply voltages of 500 V to 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, dv/dt filters
compact plus VPL or sine-wave filters (see sections "dv/dt filters plus VPL and dv/dt filters compact 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.

1.10.1.2 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.
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b) with motor reactor
caused by the
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to only
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