A brand‑new CNC milling machine in your workshop in Malaysia is producing out‑of‑spec parts. The mechanics are fine, the drive is tuned, but the controller is getting ghost signals, resulting in poor surface finishes and scrapped material. The cause? The encoder feedback cable is picking up electrical noise from the adjacent drive power cable—a direct result of improper cable selection.
In high‑precision machinery, the servo motor cable is as critical as the motor and drive it connects. Choosing the wrong cable leads to signal noise, motor insulation damage, and costly stoppages. This guide demystifies the selection process, showing engineers in Southeast Asia how to match the right cables to their drive system, ensuring precision, reliability, and maximum uptime.
The Two‑Cable System: Power vs. Feedback
A modern servo system relies on a constant conversation between the drive and the motor. This conversation happens over two physically separate and functionally different cables:
The Power Cable: This is the muscle. It carries high‑voltage, high‑frequency power from the servo drive (inverter) to the motor. It is also a major source of electromagnetic interference (EMI). Maintain physical separation from signal and network cables wherever possible and use correct shield bonding for EMC.
The Feedback Cable: This is the nervous system. It carries low‑voltage signals from the motor’s encoder back to the drive, providing precise position and speed data. This signal is highly susceptible to EMI and must be routed and terminated with care (360° shield termination, correct pair impedance, strain relief).
Using the wrong cable for either job, or failing to understand how they interact, is the root cause of many performance issues in the industrial parks of Thailand and across the region.
Selecting the Right Power Cable: 3 Critical Factors
A servo motor power cable is not a standard motor cable. It must be specifically designed to handle the punishing electrical environment created by a PWM servo drive.
1. Low‑Capacitance Insulation to Protect Your Motor
Servo drives don’t produce a clean sine wave; they use high‑speed switching (pulse‑width modulation) that creates high‑frequency voltage edges. Modern drives commonly switch up to around 16 kHz and can produce kV/µs‑scale dv/dt at the inverter output; reflections on long runs can cause over‑voltage at the motor terminals (often approaching 2× the DC bus).
The solution is a cable that uses a low‑dielectric polypropylene (PP) layer (εr ≈ 2.2) to minimise capacitive charging currents compared with PVC. In practice, low‑capacitance servo power cables—such as ÖLFLEX® SERVO FD 796 CP—help curb insulation stress and bearing currents and can extend permissible lead length and motor life.
2. Superior Shielding to Contain EMI
High‑frequency switching currents make the power cable behave like an antenna, broadcasting electrical noise that can disrupt sensors and networks. A high‑coverage tinned‑copper braid provides robust suppression of radiated emissions in the servo power range. Some variants and data/feedback families add an aluminium‑laminated foil plus braid for enhanced high‑frequency shielding. ÖLFLEX® SERVO FD 796 CP combines robust shielding with chain‑suitable construction to keep emissions low in dynamic applications.
3. Drag‑Chain Duty for Constant Motion
In high‑speed packaging machines and CNC gantries, the power cable lives in a constantly moving drag chain. A standard cable will quickly fail under this mechanical stress. For these applications, use a purpose‑built, high‑flex servo power cable designed for dynamic motion—e.g., ÖLFLEX® SERVO FD 796 CP, engineered for long service life in energy chains, with typical values of ≥ 5 million cycles, minimum bend radius ~7.5 × OD, and support for accelerations up to ~50 m·s⁻² (application‑dependent).
Selecting the Right Feedback Cable: Precision is Key
The encoder feedback signal is a low‑voltage, high‑precision signal that is extremely vulnerable to corruption.
Correct Pairing & Shielding
A proper encoder feedback cable uses a “pairs‑in‑shield” construction: the individual signal pairs are twisted (and often individually shielded); an overall shield protects the bundle from external noise and internal crosstalk. Correct 360° shield terminations at the connectors, solid earthing practice, and physical separation from power runs are essential.
System Compliance
Major drive manufacturers set specific electrical requirements. For Siemens DRIVE‑CLiQ® links, use Category 5e, 100 Ω balanced pairs—LAPP’s ETHERLINE® SERVO DQ family is designed for this interface, including flexible variants for drag‑chain use. For single‑cable encoder systems such as HIPERFACE® DSL (and other RS‑485‑based encoder links), select a hybrid cable with the appropriate ~110 Ω differential pair—LAPP’s ÖLFLEX® SERVO FD 7DSL integrates power, brake, and a shielded data pair in one jacket.
The All‑in‑One Solution: Single‑Cable Technology
For modern, compact machine designs, a new generation of hybrid cables combines power and feedback into a single cable. Products like ÖLFLEX® SERVO FD 7DSL provide power, brake, and a shielded data pair in one jacket, and are approved for runs up to 100 m (protocol‑dependent). This reduces space and weight and simplifies installation while preserving signal integrity.
Quick Answers to Common Questions
“Why can’t I just use a standard shielded inverter cable?”
A generic inverter cable may have adequate shielding, but it often lacks the low‑capacitance PP insulation needed to protect the motor from high dv/dt stress and reflected‑wave over‑voltage. A true servo power cable delivers both electrical performance and mechanical chain duty.
“What happens if my feedback cable is too long?”
Long runs increase attenuation and susceptibility to EMI. Always follow the drive maker’s maximum lengths and EMC guidance (e.g., DRIVE‑CLiQ Cat.5e limits; HIPERFACE DSL typically to 100 m) and fit any specified ferrites, terminations, or line drivers per their manuals. (See also the ETHERLINE® SERVO DQ FD P Cat.5e datasheet – LAPP.)
“Do I need different cables for different motor brands?”
For the power cable, match the servo inverter’s ratings and the mechanical duty (fixed vs. chain). For feedback, use a cable explicitly compliant with the drive/encoder system (e.g., DRIVE‑CLiQ 100 Ω Cat.5e; HIPERFACE DSL ~110 Ω differential pair in a hybrid cable).
The Bottom Line: Your Cable is a Performance Component
In the competitive world of machine‑building, the cables you choose are critical components that directly impact your machine’s precision and reliability. By selecting the right purpose‑built power and feedback cables from the ÖLFLEX® SERVO and ETHERLINE® SERVO portfolios, you can eliminate a major source of failure.
Ensure precision and reliability. Consult with a JJ‑LAPP application engineer for a tailored recommendation. JJ‑LAPP is LAPP’s sole distributor in ASEAN—with local stock, regional support, and German quality.
Local stock. German quality. Reliably Connecting ASEAN.