Danfoss VLT Series Design Manual page 90

5. How to Install
5.2.5. Motor Cables
See section
length.
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5.2.6. Electrical Installation of Motor Cables
Screening of cables
Avoid installation with twisted screen ends (pigtails). They spoil the screening effect at higher
frequencies.
If it is necessary to break the screen to install a motor isolator or motor contactor, the screen must
be continued at the lowest possible HF impedance.
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 LC filters to reduce the acoustic noise from a
motor, the switching frequency must be set according to the LC filter instruction in
Aluminium conductors
Aluminium conductors are not recommended. 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 connected.
Furthermore, the terminal screw must be retightened after two days due to the softness of the
aluminium. It is crucial to keep the connection a gas tight joint, otherwise the aluminium surface
will oxidize again.
90
General Specifications for correct dimensioning of motor cable cross-section and
Use a screened/armoured motor cable to comply with EMC emission specifications.
Keep the motor cable as short as possible to reduce the noise level and leakage currents.
Connect the motor cable screen to both the decoupling plate of the frequency converter
and to the metal cabinet of the motor.
Make the screen connections with the largest possible surface area (cable clamp). This
is done by using the supplied installation devices in the frequency converter.
Avoid mounting with twisted screen ends (pigtails), which will spoil high frequency
screening effects.
If it is necessary to split the screen to install a motor isolator or motor relay, the screen
must be continued with the lowest possible HF impedance.
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
®
VLT HVAC Drive Design Guide
Par. 14-01.