Danfoss VLT HVAC Operating Instructions Manual page 29

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VLT HVAC Drive Operating Instructions
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Cable length and cross-section
The frequency converter has been tested with a given length of cable and a given cross-section
of that cable. If the cross-section is increased, the cable capacitance - and thus the leakage current
- may increase, and the cable length must be reduced correspondingly.
Switching frequency
When frequency converters are used together with sine wave filters to reduce the acoustic noise
from a motor, the switching frequency must be set according to the sine wave filter instruction in
Par. 14-01.
Aluminium conductors
Aluminium conductors are not recommended for cable cross sections below 35 mm². Terminals
can accept aluminium conductors but the conductor surface has to be clean and the oxidation
must be removed and sealed by neutral acid free Vaseline grease before the conductor is con-
nected.
Furthermore, the terminal screw must be retightened after two days due to the softness of the
aluminium. It is crucial to ensure the connection makes a gas tight joint, otherwise the aluminium
surface will oxidize again.
Connect the motor cable screen/armour to both the decoupling plate of the frequency
converter and to the metal of the motor. (Same applies to both ends of metal conduit if
used instead of screen.)
Make the screen connections with the largest possible surface area (cable clamp or by
using an EMC cable gland). This is done by using the supplied installation devices in the
frequency converter.
Avoid terminating the screen by twisting the ends (pigtails), as this will spoil high fre-
quency screening effects.
If it is necessary to break the continuity of the screen to install a motor isolator or motor
relay, the continuity must be maintained with the lowest possible HF impedance.
MG.11.A4.02 - 09.10.06. VLT ® is a registered Danfoss trademark
4. Electrical installation
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