General information on the shielding of motor cables
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Suppression measures should be carried out as directly as possible on the source of interference in order to prevent uncontrolled propagation of the electromagnetic faults through galvanic, inductive and capacitive coupling (also coupling between cables that run in parallel). There is a series of measures that can be implemented to minimize the transient emissions.
On the one hand, through a wide range of different shielding versions:
Cable shields with nonmagnetic plastic webs or braids:
- the height or thickness of the braid is proportional to the effective cross section of the plastic web
One-sided shielding:
- must be considered for sufficient current carrying capacity – ground the shield where the electronics are most sensitive
Double-sided shielding:
- is used where alternating magnetic fields emit interference
A voltage induced in the shield leads to a current that, in turn, forms an inherent magnetic field which opposes the original cause. The residual induced effect on the cables is reduced in the shielding.
Double-sided grounding of the shielding also harbors disadvantages. It inevitably allows partial currents to flow via the shield in the case of differences in potential between the start and end of the cable. The earth loops hence reduce the actual shielding effect. In addition, couplings from the energy distribution network or lightning strike can lead to overloading of the shield.
In the case of higher frequency magnetic couplings, one side can be grounded directly and the other via a capacitor. Supply frequency currents are blocked and higher frequencies are compensated. Low loss ceramic capacitors of 10 – 100 nF are typically used.
