Practical Aspects of Motor Cables

Monday, May 18, 2015

The power components of frequency converters are designed for specific motor cable lengths. If the specified cable length is exceeded, malfunctions can occur and the converter could trip.

The capacitance of the cable used is partly responsible for this behavior. If the capacitance at the frequency converter output exceeds a specified value, transients can occur on the cables which can lead to a malfunction of the converter.


Rated voltage

Peak voltages up to three times the DC link voltage in the frequency converter occur in the motor cable. They severely stress the motor cable and the motor insulation. The stress is higher if the frequency converter output does not have a du/dt filter or sine-wave filter.

For this reason, the rated voltage specification of the motor cables should be at least U0/U = 0.6/1 kV. High-voltage insulation testing of cables with this specification is usually performed with a test voltage of at least 3,500 V AC and usually 4,000 V AC, and in practice, they have proven to have good resistance to insulation breakdown.

Cable dimensioning

The required cross-section of the motor cable depends on the output current of the frequency converter, the ambient temperature, and the type of cable installation. Overdimensioning the wire cross-section to allow for harmonics is not necessary. For the selection and dimensioning of cables and conductors, EN 60204-1 and VDE 0113-1 provide current capacity data for wire cross-sections up to 120 mm². If larger wire crosssections are necessary, useful information can be found in VDE 0298-4.

Motor cable length

In installations with long motor cables, the voltage drop over the cable must be taken into account in cable dimensioning. Design the system so the full output voltage reaches the motor, even with
a long motor cable. The length of the motor cable that can be connected to a standard frequency converter is typically 50 to 100 meters. Even with these cable lengths, products from some manufacturers cannot provide the full output voltage at the motor.

If users need cables longer than 100 m, there are only a few manufacturers that can meet this requirement with standard products. Otherwise, it is necessary to provide supplementary motor chokes or output filters.

Energy efficient

The voltage drop over a motor cable, as well as the resulting heat dissipation, is nearly proportional to its length and dependent on the frequency. To ensure an energy efficient installation, you should keep cable runs as short as possible and dimension the wire cross-sections no larger than is electrically necessary.

Cables with suitable shielding

Shielded cables should have a shield coverage of at least 80%. Some examples of suitable cable types are:

– Lapp Ölflex 100-CY
– Helu Y-CY-JB
– Helu Topflex-EMV-UV-2YSLCYK-J

Consult manufacturer

Consult the manufacturer regarding the lengths of cables that may be connected to the frequency converter and the expected voltage drop. With f.i. a standard Danfoss VLT® HVAC Drive frequency converter you can connect a shielded cable up to 150 m long or an unshielded cable up to 300 m long and still have the full voltage at the motor.