Lenze EDB9300UES - Manual (Page 27)

Technical Data
22 BA8230
Harmonics
The mains converter consists of an uncontrolled 6-pulse bridge circuit (B6) with
diodes. Capacitors which decouple the mains converter from the motor converter are
fitted on the link and maintain the continuous flow of current to the motor converter. A
varying amount of energy is taken from the link capacitor depending on the load on
the motor, whereby the mains converter compensates for the reduction in charging
according to the mains voltage characteristic, i.e. the phase position of the current, in
contrast to the thyristor mains converter, remains almost constant and is not
dependent on the momentary value.
The effective mains impedance and the load conditions affect the shape of the mains
current curve and therefore the harmonic content, whereby the mains choke and the
ratio between the mains short-circuit voltage and the inverter apparent power are
decisive for the harmonic content.
Commutation notches
If the ratio between the mains short-circuit power to the complete rated controller
power is less than 30 : 1 with a standard mains choke, special mains chokes to limit
commutation notches in the mains voltage at public mains according to
prEN50178/VDE0160:1991 are to be used.
If the feedback is evaluated on the basis of VDEW
1)
recommendations, the ratio must
not be smaller than approx. 100 : 1 with mains chokes.
1)
VDEW = Association of German Electricity Supply Companies.
In large industrial plant it is possible to reduce the dominant 5th harmonic in 6-pulse
mains converters by connecting some of the converter load through transformers with
a phase shift of 30° electrical in relation to the other transformers. Another transformer
circuit in switching group Yy0 with a compensation winding could be implemented
instead of the Dy5 circuit often used. In this configuration, for example, the 5th
harmonic can be reduced by more than 50 % provided that both converters have the
same overlap angle.
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