Shielding concept
Why do cables have to be shielded?
Cables are equipped with shielding in order to keep electrical or magnetic fields away from technical devices, buildings or rooms. This shielding protects the environment from interference emanating from the cables.
Electromagnetic waves are a combination of electrical and magnetic fields. These waves have magnetic and electrical components. Cables are shielded in order to prevent interference within the cable and outside the cable.
Shielding is used to improve electromagnetic compatibility (EMC) and to ensure trouble-free signal transfers.
The effect of motor or signal line shields can be measured as impedance. The impedance indicates the relationship between sinusoidal alternating voltage and sinusoidal alternating current. The temporal shift of the phase angle between the these parameters (current and voltage) is the phase shift angle (power factor).
Loop impedance
The loop impedance is the sum of all impedance values in a closed circuit through which interference currents flow.
The interference currents of the loop impedance are composed of:
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- the non-grounded conductor between the current source and the fault location
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- the return wire to the current source
Line connection to the AX5000
A further cause of electromagnetic interference is incorrect line connection. As already mentioned in section “Cables”, the shield braid should be connected to the metallic housing of the plug connector or a special shroud via a surface that is as large as possible. When the plug connector is screwed to the servo drive, ensure that low-resistance contact to the shield-carrying components with the metallic housing (potential) of the servo drive is created.
Cable lengths
With longer motor cables the resulting commutation currents can lead to EMC faults. Mains chokes or mains filters may have to be used, depending on the application. The fitting dimensions for any mains chokes or mains filters etc. that may be required should be taken into account when dimensioning the control cabinet. Run the power and signal cables in separate metallic cable ducts, as shown in our Potential equalization sample. If a common cable duct is used, a grounded, metallic partition between the cables should be used, as shown in our Potential equalization sample.
| More detailed information and overview tables for the cable lengths
can be found in section 9.15: “Motors and cables for servo drives” of the AX5000 system manual! |
Motor connector, brake and temperature contact
| For the motor connector the connection of the shroud (1) is particularly important in terms of interference immunity. The shroud (1) is connected to the metallic housing of the AX5000 via two straps (2). To ensure safe contact, tighten the two screws (3) with a suitable tool. |
| The brake temperature plug (4) must be securely connected with the two screws (5). The shield connection is established via the PE pin, which is internally connected to the metallic housing of the AX5000. |
Feedback connector
| The shield is connected via the trapezoidal sheet (8), which is connected to the metallic housing (7). In the AX5000 the counterpart is internally connected to the metallic housing of the AX5000. The screws (6) must be tightened securely, in order to ensure that the trapezoidal sheet (8) is connected to the counterpart with low resistance. |
| Please note the tightening torques for the motor and feedback connectors! Detailed information can be found in sections: “Motors” and “Feedback” of the AX5000 system manual! |
Shield concept
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Pre-assembled power and signal cables! The company Beckhoff Automation offers pre-assembled power and signal cables. The cables are tested with regard to material, shielding and connection type. If such pre-assembled cables are used, Beckhoff guarantees proper functioning and compliance with statutory regulations such as EMC, UL etc.
The use of other cables or application of custom cable shield can result in unexpected interference and may void the warranty!
Please refer to other relevant sections in this chapter! |
In practice, pre-assembled Beckhoff motor and signal cables are often shortened or re-configured on site.
Re-configuration or shortening of the cables can lead to the following problems:
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- the shielding effect of the cables becomes inadequate;
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- high-frequency interference currents enter brake lines etc.
If the motor cable is only connected on one side, the shielding effect is inadequate!
Motor cable ZK4509-0019-xxxx | Item no. with explanation |
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| Highly flexible inner shield for particularly dynamic requirements |
| External shield for the brake line |
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Feedback cable ZK4509-0019-xxxx | Item no. with explanation |
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| Highly flexible inner brake shield for particularly dynamic requirements |
| Highly flexible inner shields for the thermal contact / OCT for particularly dynamic requirements |
| Wire isolation (thermal contact / OCT) |
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| Motor cables of servo drives are inherently subjected to strong EMC interference sources and belong to cable category 1. The precise specifications of the motor cable can be found in the “AX5000 user manual → Accessories → Cables”. The main characteristics of EMC-compliant cables are described below. |
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| The surrounding shield braid (1) is flush with the metallic shielding ring (2). The PE conductor ends in a metallic sleeve (3). Ensure the sleeve (3) is fitted securely in the receptacle (4) when fitting the inner part to the outer part. The shielding ring (2) should be flush with the perimeter edge (5). The joint (6) must be screwed securely to the outer part (7), in order to ensure that the shielding ring (2) and the sleeve (3) have maximum contact with the outer part (7). |
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| The wires of the motor connector (9) and the two pairs of wires (10) of the brake temperature connector (8) are brought together in one line (11). The whole line (11) is covered by a shield braid (1) and secured to the shroud with a shield clamp (12). Ensure that the shield braid (1) is protected with copper foil (14) and secured with the shield clamp (12) on the shroud (13). The two pairs of wires (10) of the brake temperature connector (8) are pair-wise twisted and covered with a shield braid (15). Two wires are soldered to this shield braid and screwed to the PE pin of the brake temperature connector. |
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| Feedback cables of servo drives are inherently non-interfering and EMC sensitive and belong to cable category 4. The precise specifications of the feedback cables can be found in the “AX5000 user manual → Accessories → Cables”. The main characteristics of EMC-compliant cables are described below, using an encoder cable as an sample. |
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| The surrounding shield braid (1) is flush with the metallic shielding ring (2). The joint (4) must be screwed securely to the outer part (3), in order to ensure that the shielding ring (2) has maximum contact with the outer part (3). |
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| Ensure that the surrounding shield braid (5) has maximum contact with the strain relief. Both screws (6) must be flush in the housing. The two screws (7) must be tightened securely, in order to ensure that the shield braid (5) has maximum contact with the housing (8) via the strain relief. |
Single-sided connection of the inner shield of the motor cable can lead to damage of the servo drive in the long term and result in damage to the brake coil as a consequence.
A TF line with shielding that is not grounded on both sides can trigger a feedback error.
Apply the shields of the motor and signal cables on both sides!
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Intermediate shield! In general it is not recommended to apply an additional shield at the earthing points (intermediate shield). |
| Using motors with OCT feedback Only pre-assembled OCT cables from Beckhoff Automation GmbH & Co. KG must be used in conjunction with motors with OCT feedback. Please note that the use of custom-assembled OCT cables voids any warranty offered by Beckhoff Automation GmbH & Co. KG! |
