2 Engineering
2.2 Electrical power network
DA1 Variable Frequency Drives
08/18 MN04020005Z-EN
www.eaton.com
49
2.2.3 Voltage balance
Unbalanced voltages and deviations from the ideal voltage shape may occur
in three-phase AC supply systems if the conductors are loaded unevenly and
if large output loads are connected directly. These supply voltage unbalances
may cause the diodes in the variable frequency drive’s rectifier bridge
converter to be loaded unevenly, resulting in premature diode failure.
2.2.4 Total Harmonic Distortion (THD)
The THD value (THD = Total Harmonic Distortion) is defined in standard
IEC/EN
61800-3 as the ratio of the rms value of all harmonic components to
the rms value of the fundamental frequency.
￫
In the project planning for the connection of three-phase
supplied variable frequency drives (DA1-3…), consider onlyAC
supply systems that handle permitted asymmetric divergences
in the mains voltage
≦
+3 %.
If this condition is not fulfilled, or symmetry at the connection
location is not known, the use of an suitable main choke is
recommended.
￫
In order to reduce the THD value (up to 30 %, it is
recommended to use a DX-LN… mains choke
￫
FS5, FS6, and FS7 DA1 variable frequency drives feature chokes
in their DC link. Using mains chokes in order to reduce current
harmonics is not necessary in this case.
￫
Only for FS8 DA1 variable frequency drives:
DA1-34370FB-B20C, DA-34450FB-B20C
If it is not guaranteed that the system percentage impedance is
greater than or equal 1 %, a mains choke must be connected.
Your u
K
value should fall between 1 % and 4 %.
Examples:
DA1-34370FB-B20C
￫
DX-LN3-370
DA1-34450FB-B20C
￫
DX-LN3-450
