Yes, it is critically important for servo motor power cables to be shielded in industrial automation applications. Shielding minimizes high-frequency switching-induced electromagnetic interference (EMI), ensuring system stability, data integrity, and environmental compliance. This is an indispensable requirement, particularly in systems that demand precise control and high performance.
Should Servo Motor Power Cables Be Shielded? What Is It?
Servo motors are dynamic devices at the heart of industrial automation that provide precise motion control. Whether the power cables supplying these motors are shielded or not is a critical factor in terms of the system’s overall performance, reliability, and electromagnetic compatibility (EMC). A shielded cable contains a barrier that surrounds the conductors and is designed to reduce electromagnetic interference (EMI) originating from external sources or radiating from within. In servo systems, the power sent to the motors by the drive is typically switched at high frequencies using Pulse Width Modulation (PWM). This rapid switching generates significant amounts of high-frequency harmonics and electromagnetic radiation within and around the cable. This radiation can adversely affect nearby control signals, data communication lines, sensors, and even other electronic equipment, leading to malfunctions or system failures.
Therefore, it is essential for servo motor power cables to be shielded in order to limit such unwanted electromagnetic interactions and ensure that the system operates in compliance with established EMC standards. Shielding reduces both radiated and conducted emissions, allowing the automation system to operate in harmony with its surroundings. Additionally, by preventing external noise from infiltrating the cable, it ensures the motor operates smoothly and maintains the accuracy of feedback signals.
Operating Principle and Technical Data
The operating principle of servo systems is that a servo drive precisely controls the motor according to incoming commands. The drive continuously adjusts the voltage and current applied to the motor windings. This adjustment is typically achieved using high-speed semiconductor switching elements, such as IGBTs (Insulated Gate Bipolar Transistors), with PWM signals at kilohertz levels (for example, from 4 kHz to 16 kHz). The rapid rise and fall times of PWM signals (dv/dt and di/dt) cause high-frequency currents and voltages to form in the power cables. This causes the cables to act like antennas and radiate electromagnetic energy.
Shielding works by capturing this radiated electromagnetic energy and directing it to ground. A cable shield typically consists of braided copper wire, aluminum foil, or a combination of both. This shield acts as a barrier against electromagnetic fields radiating from the conductors inside the cable and external electromagnetic fields. A properly grounded shield transfers these noise currents to ground via a low-impedance path, thereby significantly reducing the noise level to which signal conductors and surrounding equipment are exposed. The effectiveness of shielding depends on the type of material used, the coverage ratio (in the case of braided shields), the quality of the grounding, and the frequency of the noise.
| Parameter | Value/Description |
|---|---|
| Shielding Type | Braided Copper Shield, Aluminum Foil Shield, Combined (Braided + Foil) |
| Shield Coverage Ratio | Min. 80% (for braided), 100% (for foil). Higher percentages provide better protection. |
| Communication Frequency | PWM switching frequency (typically 4 kHz – 16 kHz); harmonics can reach MHz levels. |
| EMI Reduction | Between 20 dB and 60 dB (varies depending on the application and shield type). |
| Grounding | 360° grounding (on the drive side) and proper connections on the motor side are critical. |
| Cable Impedance | Must be compatible with the driver output impedance to minimize reflections. |
| Application Areas | Robotics, CNC machines, packaging, textiles, medical devices, general automation. |
Field Considerations
- Proper Grounding and Shield Termination: The effectiveness of shielding depends on proper grounding. It is essential that the cable shield be connected to the grounding bar or chassis via a low-impedance path on both the drive side and the motor side. On the drive side, the shield must be brought into full contact with the drive’s metal housing, typically using special EMC cable glands or 360° shield clamps. On the motor side, grounding is performed through the motor housing. Incorrect or incomplete grounding can render the shield ineffective or cause additional problems by creating ground loops.
- Cable Routing and Separation: Servo motor power cables are sources of high-frequency noise. Therefore, they must be physically separated from control signal cables (encoder, communication, etc.) and other low-voltage, sensitive signal cables. Power and signal cables should be routed in separate cable channels or, at a minimum, parallel to each other at a specific minimum distance (typically 20–30 cm). If they must cross, the crossing should be at a 90-degree angle, and the contact distance should be kept to a minimum. This helps minimize inductive and capacitive coupling.
- Cable Quality and Type Selection: Not all shielded cables offer the same performance. High-quality, flexible, oil- and abrasion-resistant EMC-compliant power cables specifically designed for servo applications should be preferred. These cables typically include a braided shield with high coverage, as well as an additional foil shield. Additionally, the cable’s bend radius and suitability for moving applications (e.g., robotics or cable carrier systems) should be taken into account. Selecting cables that comply with manufacturer specifications and relevant international standards (such as EN 61800-3) ensures long-term reliability.
Common Problems and Solutions
Issues related to the shielding of servo motor power cables are typically caused by EMC incompatibilities. Here are some common problems and suggested solutions:
- Problem: Noise in Feedback Signals and Motor Vibration/Position Error
Explanation: EMI radiated from the power cable leaks into nearby feedback signal cables—such as those from an encoder or resolver—causing signal distortion. This can lead to unstable motor operation, vibration, or the motor failing to reach its target position.
Solution: Check whether the power cable’s shield is properly and low-impedance grounded at both ends. Physically separate the signal cables from the power cables and route them through separate cable channels. Ensure that the signal cables are also shielded and that their shields are properly grounded. If necessary, use high-quality, double-shielded (foil + braid) servo power cables.
- Problem: Malfunctions or Communication Interruptions in Nearby Equipment (PLCs, Sensors, HMIs)
Explanation: High-frequency noise emitted by servo power cables can cause sensitive electronic equipment—such as nearby PLCs, sensors, human-machine interfaces (HMIs), or industrial network devices—to exhibit abnormal behavior, data loss, or temporary malfunction.
Solution: Ensure that all power cables are shielded and that their shields are securely connected to the drive chassis via a 360° ground connection. To prevent ground loops, ensure that all equipment is grounded to a single star point. If the noise level is still high, consider installing ferrite beads around the power cable. Ensure that the equipment meets its own EMC requirements.
- Issue: Overcurrent or Overvoltage Error Codes in the Driver
Explanation: Insufficient shielding or cable damage can lead to an unbalanced current distribution or high voltage spikes between the drive and the motor, causing the drive to enter error mode.
Solution: Check the cable insulation and shielding for any physical damage. Ensure the cable is properly connected to the drive and motor terminals and that there are no loose connections. Verify that the cable impedance is compatible with the driver’s requirements. For excessively long cables, follow the driver manufacturer’s recommendations (e.g., use of a DC reactor).
Expert Advice
In the complex and dynamic world of industrial automation, shielded servo motor power cables are not an option—they are a necessity. The high-frequency switching (PWM) techniques inherent in high-performance servo systems inevitably generate significant amounts of electromagnetic interference (EMI). This interference not only disrupts the servo system’s own precise control loops (particularly feedback signals) but can also negatively impact the operation of other critical automation equipment in the vicinity (PLCs, sensors, communication networks). This situation leads to production downtime, quality issues, difficulties in fault diagnosis, and significant long-term costs.
As a matter of best practice, you should always opt for shielded solutions for servo motor power cables. However, simply using shielded cable is not enough; the effectiveness of the shielding is directly related to proper installation and, in particular, a complete and low-impedance ground connection. Ensuring a 360-degree ground connection at both ends of the cable (driver and motor) using appropriate EMC cable glands or clamps is the key to minimizing electromagnetic noise. In addition, physically separating power cables from control and signal cables, routing them correctly in cable channels, and using the shortest possible cable lengths will also significantly improve the system’s EMC performance. Selecting high-quality cables that comply with industrial standards and are manufacturer-approved is the best investment you can make for long-lasting and trouble-free operation. These measures will not only enhance the reliability of your current system but also lay a solid foundation for future expansions and potential EMC certification processes.