Servo cables, shielding, grounding and EMC

Basics 01

How should I route power and signal cables?

Motor power cables are a strong source of noise because of the drive's fast switching. Run them in separate ducts from encoder, network, pulse and analog cables; where they must cross, cross at right angles, and avoid long parallel runs. This one rule prevents a large share of unexplained faults in the field.

Commissioning 02

How should I ground the cable shield?

The shield of the motor power cable must be grounded at both ends, at the drive and at the motor, with a wide-area (360°) clamp; connecting it through a thin pigtail wire largely cancels the shield at high frequency. Inside the cabinet, all grounds should meet on a common, conductive mounting plate.

• A painted surface is not ground; the clamp must sit on bare metal
• For low-level analog signals, the shield may be tied at one end if a ground loop appears

Basics 03

Which cable should I use on moving axes?

Cables flexing continuously in a drag chain must be of the flexible type designed for it; a fixed-installation cable soon breaks inside and first shows up as random encoder errors. Respect the cable's minimum bending radius in the chain and lay the cables side by side without tying them together.

Basics 04

What do an EMI filter and a line reactor do, and when do I need them?

An EMI filter stops the drive's high-frequency noise from travelling back into the mains and upsetting other equipment on the same supply; it is usually needed for the machine's EMC compliance. A line reactor reduces harmonics and current surges on the supply side and protects the drive's input stage. Both should be mounted as close to the drive as possible and well grounded.

Basics 05

How long can the motor cable be?

A long motor cable brings two problems: leakage current that grows with the cable's capacitance, and voltage spikes at the motor terminals. Do not exceed the longest cable length the drive manual gives; if a long run cannot be avoided, move the drive closer to the motor or use an output filter. The encoder signal has a length limit of its own.