How to connect the shielding of cable to a earth potential?
All modern power stages of motor controllers use PWM voltages to control the motor supply. Such PWM power signals can result in some EMI radiation and can have a negative impact on sensitive sensor signals (e.g. hall sensors, analog sensors, video signals). Therefore it is recommended to use shielded motor cables. This recommendation is especially important in case of ...
- ... long motor cables.
- ... motor cables which are placed close to sensor cables.
- ... high motor currents.
The cable shielding has to be attached to earth at least on one side of the cable. If the cable length exceeds 1 or 2 meters, it is recommended to attach the cable shielding on both cable ends to the earth potential.
A motor cable shield can be quite easily attached to an earthed part of a metal structure (e.g. of a cabinet or machine) by so-called earthing clamps. The earthing clamps should be placed close to the controller and motor. This is the same solution like in use for more powerful servo drives and frequency converters which can be used for maxon motors and controllers too.
- Strip the insulation close to the motor cable ends.
(Typically within the last 10 cm of the cable).
- Mount the earthing clamps on an earthed metal piece of the cabinet or your machine close to the controller and motor.
- Plug the shielding of the cable into the earthing clamp.
Ensure that the diameter of the earth clamp fits to the diameter of the cable and there is a close and extensive contact in between the cable shielding and the earth clamp.
Please ensure that ...
- ... earth wires which may be in use to connect different machines structures or earth bars (holding the earth clamps) have a sufficient cable cross section. Typically there will be a cross section of minimum 6 mm^2 (copper wire) up to 25 mm^2 in use. Earth cables with a 16 mm^2 cross section are quite commonly in use standard by machine builders and electrical cabinets.
- ... earth contacts are not blocked or limited by some anodized coating or paint, e.g. if the contact is done by a screw, nut, cable lug, or washer used to contact the metal structure or housing of a motor.
The main point is that the earth contact, earth bar, earth wire and cable shield (and any connection in between) needs to have a much lower resistance (i.e. better conductibility) than any other GND contact or wire in the system. Equalization and stray currents always follow the lines of the least resistance and these have to be the earth wires. A thick cross section and broad earth / shield contact points ensures that the current flow and potential equalization actually goes through the earth bars, metal structure, and earth wires. Finally any contact points should have the same earth potential which can be achieved by extreme low wire and connector resistances and a resulting mimized voltage drop by the earth path.
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