Detailed Description
The Single-Cable Advantage
Traditional servo motor connection uses two separate cables:
- Power cable: 4-core (3P+PE), 1.5–35 mm², 0.6/1 kV — connects the drive's power output to the motor's stator windings. Typically unshielded or with overall braid shield for EMC compliance
- Feedback cable: Multi-pair shielded signal cable — connects the motor's encoder to the drive's feedback interface. Double-shielded, low capacitance
The composite servo cable integrates both into one cable. One drag chain compartment, two connectors (one at each end — combined power+feedback connector), one cable to manage. The space saving in the drag chain is typically 25–35% compared to two separate cables of equivalent specification.
Power-to-Feedback Crosstalk — The Design Challenge
Putting 3-phase motor power (400–480 V, 10–40 A, 4–16 kHz PWM switching) and millivolt-level encoder signals (RS-422, ±2 V differential) in the same cable creates the worst-case scenario for capacitive crosstalk. The solution is three-fold:
1. Physical Layer Separation
The power cores are stranded in the innermost concentric layer. A separator layer — typically PU or aramid-reinforced filler rods — physically separates the power core layer from the feedback pair layer. This increases the distance between the noise source (power cores) and the victim (feedback pairs), reducing the parasitic capacitance between them. The separation layer thickness is typically 1.0–2.0 mm.
2. Individual Pair Shielding
Each feedback pair has its own aluminium/polyester foil shield with drain wire. This individual shielding provides the first stage of protection — capacitively coupled noise from the power cores is intercepted by the foil and shunted to ground via the drain wire, rather than coupling into the signal conductors.
3. Overall Braid Shield
The overall tinned copper braid provides the second stage — it intercepts any noise that penetrates past the individual pair shields and provides a low-impedance path to ground for the full cable length. The braid also serves as the cable's EMC shield for the motor power conductors — containing the PWM switching noise radiated from the power cores.
Cable Selection by Motor Power
The composite cable's power core cross-section must match the servo motor's rated current:
| Motor Power (Typical) | Continuous Current | Power Core Cross-Section | Feedback Pairs | Approx. Cable OD |
|---|---|---|---|---|
| 0.5–2 kW | 1.5–5 A | 4 × 1.5 mm² | 4 pairs (incremental encoder) | 12–14 mm |
| 2–5 kW | 5–12 A | 4 × 2.5 mm² | 4 pairs (incremental encoder) | 14–16 mm |
| 5–10 kW | 12–25 A | 4 × 4 mm² | 6 pairs (absolute encoder) | 17–19 mm |
| 10–20 kW | 25–45 A | 4 × 6 mm² | 6 pairs (absolute encoder) | 19–22 mm |
| 20–40 kW | 45–80 A | 4 × 10 mm² | 8 pairs (absolute + resolver) | 23–26 mm |
| 40–70 kW | 80–130 A | 4 × 16 mm² | 8 pairs | 28–32 mm |
Larger motors (≥70 kW) typically use separate power and feedback cables because the composite cable OD becomes impractical for standard drag chains and connectors.
Yichi Composite Servo Cable Manufacturing
- Power-to-feedback capacitance verified: Capacitance between each power core and each feedback pair is measured on the first article of every new design — the design target is ≤10 pF/m per power-feedback pair. Excessive coupling indicates insufficient physical separation
- Overall braid tested as EMC shield: The overall braid is tested for shielding effectiveness at the motor PWM frequency (4–16 kHz) and harmonics. The braid must provide ≥40 dB attenuation at 30 MHz for the cable to serve as the motor power cable's EMC shield
- Combined connector termination: Composite cables require combined power+feedback connectors (typically Siemens Signal/Power hybrid, Bosch Rexroth hybrid, or custom — manufacturer-specific). We offer pre-terminated cables with molded connectors to customer specification
- Drag chain test with power applied: Flex life test (10 000 000 cycles) is performed with the power cores energized at rated current — this simulates the thermal + mechanical stress of real operation, not just cold flexing