More About Motor Cable Length

Saturday, December 27, 2014

The power components of a Frequency Converter are designed for specific motor cable lengths. If the specified cable length is exceeded, malfunctions can occur and the FC could trip with an error/alarm message. The capacitance of the cable used is partly responsible for this behavior. If the capacitance at the FC output exceeds a specified value, transients can occur on the cables that can lead to a malfunction of the FC.

Most manufacturers prescribe shielded cables for their FC’s to prevent potential EMC problems. If the user decides on other suitable measures for compliance with EMC requirements then unshielded cables can be used. Since the unshielded cable places a lower capacitive load on the FC, a longer cable length is possible in this case.

Typically cable lengths that can be used are 50m / 75m (shielded) or 150m /300 m (unshielded).

Not using shielded motor cables can only be recommended if other measures are taken. Even if an installation operates properly during its acceptance test without shielded motor cables, EMC problems can occur sporadically, or as a result of modifications or extensions to the installation. The financial expenditure then required to eliminate such problems is usually greater than the money saved by using unshielded cables.

When installing cables, care must be taken to avoid additional inductance resulting from routing cables in the form of an air-core coil and additional capacitance resulting from parallel conductors.

If several motors are connected in parallel to the output of a FC, the lengths of the individual motor cables must be added together to determine the connected cable length. Here it should be noted that some manufacturers specify the geometrical addition of the individual cable lengths. In such cases, daisy-chaining the motor cable is advisable. A star formation can cause problems due to the additional capacitance between the individual conductors.