Danfoss VLT AHF005, Руководство по конструкции

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5 Selection of Advanced Harmonic Filter
This chapter will provide guidance about how to choose the
right filter size and contains calculation examples, electrical
data and the general specification of the filters.
5.1 How to Select the Correct AHF
For optimal performance the AHF should be sized for the
mains input current to the frequency converter. This is the
input current drawn based on the expected load of the
frequency converter and not the size of the frequency
converter itself.
5.1.1 Calculation of the Correct Filter Size
Needed
The mains input current of the frequency converter (I
FC,L
) can
be calculated using the nominal motor current (I
M,N
) and the
displacement factor (Cos
φ
) of the motor. Both values are
normally printed on the name plate of the motor. In case the
nominal motor voltage (U
M,N
) is unequal to the actual mains
voltage (U
L
), the calculated current must be corrected with
the ratio between these voltages as shown in the following
equation:
IFC.L = 1.1 × IM,N × cos(ρ) ×
UM,N
UL
The AHF chosen must have a nominal current (I
AHF,N
) equal to
or larger than the calculated frequency converter mains
input current (I
FC,L
).
NOTE
Do not oversize the AHF. The best harmonic performance is
obtained at nominal filter load. Using an oversized filter will
most likely result in reduced THiD performance.
If several frequency converters are to be connected to the
same filter, the AHF must be sized according to the sum of
the calculated mains input currents.
NOTE
If the AHF is sized for a specific load and the motor is
changed, the current must be recalculated to avoid
overloading the AHF.
5.1.2 Calculation Example
System mains voltage (U
L
): 380V
Motor name plate power(P
M
): 55kW
Motor efficiency (
ƞ
M
): 0.96
FC efficiency (
ƞ
FC
): 0.97
AHF effiency (
ƞ
AHF
)(worst case estimate): 0.98
Maximum line current (RMS):
PM × 1000
UL × ηM × ηFC × ηAHF × 3
=
55 × 1000
380 × 0.96 × 0.97 × 0.98 × 3 = 91.57 A
In this case a 96A filter must be chosen.
5.1.3 Voltage Boost
In stand-by and under low condition, the AHFs will boost the
input voltage with up to 5%. This means that the voltage at
the frequency converter terminals is up to 5% higher than
the voltage at the input of the filter. This should be
considered at the design of the installation. Special care
should be taken in 690V applications, where the voltage
tolerance of the frequency converter is reduced to +5%, the
boost voltage can, at low load and stand-by, be limited via
the available capacitor disconnect. For more information see
section 6.2.2.
Selection of Advanced Harmo... AHF005/010 Design Guide
MG.80.C4.02 - VLT
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