Mesures pour une installation conforme CEM

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Topic:

• What measures must be taken to build up a EMC consistent installation (CE)?

Solution:

Shielding

The term „Safety earth“ is defined as a security measure for the protection of people. The term „Shielding earth“ includes the connection points where cable shields are connected. As a rule the safety earth and the shielding earth have the same potential.

While the connection of a machine to the safety earth guarantees the protection against overvoltage in the case of an error, the connection of the cable shields to the shielding earth represents a protection against interference immission and emission.

For maximum interference suppression the cable shields must be mounted on both ends and with a resistance as low as possible. Exceptions need a special reason (e.g. sensitive analog signals with low signal level).

Most maxon controllers have a metal housing. Ideally the housing is mounted electrically connected on a metal plate. Hence, the different cable shields can easily be mounted with large connecting areas on the mounting plate giving an ideal shield connection on the controller side. If such a mounting configuration is not possible, then connect the shields according to the operating instruction with as low a resistance as possible to the „Earth“ contact or the screw on the housing.

Earth loops

Connecting cable shields on both ends can produce loops. For sensitive anlog signals the incoming 50/60Hz interference may ask for cable shields which are connected only on one end. But as a rule, cable shields should be connected on both ends. The reason for interferences with both ends connected can be a insufficient earthing concept or a bad cable routing with large loops.

HF blocking

HF blocking by means of a ferrite core on a cable improves the interference stability against external interference couplings. This can be helpful to recognise when interference suppression has to be done on a machine exhibiting insufficient interference resistance during operation.

As a rule only complete units comprising of all individual components (motor, sensor, amplifier, position control, power supply, EMC/EMI filter, cables, etc.) should be submitted to an EMC test for CE conform, interference free operation.

Connections

Motor

• The motor cables must be shielded when used with a PWM (= pulse width modulated) power stage of a motor controller.
• Please find some information how the cable's shielding is attached to a broad Earth contact the best way by the following Support Center document:
  -> How to connect a cable's shielding to Earth potential? 
  • Such a broad Earth contact using Earth clamps like mentioned by the linked document has to be preferred instead of wiring the cable's shielding to just a single Earth pin (which may be present by a motor controller's connector too). Such kind of single braid wiring of a cable's shielding to an Earth pin of a connector can even act like a small antenna and even increase emission in worst case. 
  • Take care that there is a broad Earth contact present which it is not blocked by some painting or anodized metal.
• Depending on the cable length the shield has to be connected on both sides (at the motor side on the housing or on an electrically connected mechanical construction).
  
  
• Just for DC motors which are supplied by a battery or linear DC voltage:
  For DC motors without CLL, a suppression with decoupling capacitors directly at the motor is recommended due to the arc production in the commutator.

Important note / restriction:

Adding external capacitors, as shown in the diagram above, can lead to sporadically faulty motor current measurement and control in the case of a PWM output stage.

When using modern motor controllers with PWM power stages (such as maxon's ESCON, EPOS, MAXPOS), no additional external capacitors should be added to the motor and, if possible, no DC motors with CLLs should be used!

An additional EMC motor filter which partly or just consists of capacitors can distort the current measurement of PWM power stages permanently or sporadically. Finally this can result in an unpredictable and faulty control behavior. Therefore adding capacitors as an EMC filter is just recommended in case of DC motors which are supplied directly by a power supply or battery (i.e. without a controller with a PWM power stage).

Hall sensors

• Use shielded cables.
• Shield connected on both sides.
• Use a separate cable (not in motor cable).

Encoder

• The pairs "channel A, channel A\", "channel B, channel B\" und "index channel I, index channel I\" must be twisted in pairs.
• At strong external interference level a shield (connected on both ends) may be necessary.

Resolver

• The resolver cables must be shielded.
• The pairs "cos+/cos-", "sin+/sin-" and "resolver primary/GND primary" must each be twisted and shielded in pairs.
• On the resolver side, the shielding may not come into contact with the plug housing (motor housing) to prevent earth loops.

Analog signals (set values)

• A shielded cable is recommended.
• As a rule, shield connected on both ends. If problems with 50/60Hz interference occur, connect shield on one end only.

Digital I/O signals

• As a rule no shielding is necessary.

Related documents

Preventing defects by correct earthing concepts

How to connect a cable's shielding to Earth potential?

Measures in case of long cables