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Servo Motor Cable Selection: Combining Power, Feedback, and Brake Conductors in One Cable
A servo cable carries noisy power, fragile encoder signals, and brake current in the same jacket. Getting the shielding and separation right is what keeps your axis moving without glitches.
★ Motion Control — Servo Cabling GuideIf there’s one cable that needs to do three jobs at once, it’s the servo motor cable. Inside a single jacket it must carry motor power (with all the PWM switching noise that comes with it), encoder feedback (low-voltage signals that fail at the slightest interference), and often brake current (a 24 V path that must stay reliable).
I’ve diagnosed far too many "mysterious" servo glitches — axis drift, random position errors, intermittent brake failure — that turned out to be a cable where the power and signal conductors weren’t properly separated or shielded. The cable is often the last thing anyone suspects, and the hardest to troubleshoot because the fault is intermittent.
Let me walk through what actually goes into a servo cable, why shielding separation is the make-or-break detail, and how the connector interface (M23 and friends) fits in.
Servo Cable Components
A composite servo cable is really three cables sharing one jacket. Each element has its own job and its own design constraints.
| Component | Function | Typical Construction | Design Constraint |
|---|---|---|---|
| Motor power | Drives the 3-phase servo motor (U/V/W + PE) | 4 cores, e.g., 4 × 1.5 mm² up to 4 × 6 mm² | Carries high current and PWM switching noise — must be shielded away from signals |
| Encoder / feedback | Returns position/speed data to the drive | Twisted pairs (e.g., 4 × 2 × 0.25 mm²), impedance matched to the feedback interface (120 Ω RS-485 / 110 Ω Hiperface DSL) | Low-level differential signals — extremely sensitive to noise |
| Brake | Controls the holding brake (typically 24 V) | 2 cores, e.g., 2 × 0.75–1 mm² | Switching current can couple noise — often given its own shield |
| Thermal sensor (optional) | Monitors motor temperature | Small signal pair, often inside the encoder bundle | Low level, needs the same protection as feedback |
★ Why composite works: A single composite cable with power, feedback, and brake means one cable run, one connector, and one routing path on a moving axis — far cleaner than three separate cables. The trade-off is that all that noise and all those sensitive signals share a jacket, so the internal shielding and separation have to be excellent. That’s the detail that separates a good servo cable from a bad one.
The Importance of Shielding and Isolation
This is the heart of servo cable design — and where most failures originate.
Here’s the noise problem. PWM switching on the motor power conductors can induce substantial common-mode and differential noise on unprotected feedback pairs, with noise bursts aligned to the PWM switching edges. Your encoder signal is a tiny differential voltage — noise of this scale will corrupt it instantly if the cable doesn’t suppress it.
| Shielding Element | What It Does | Why It Matters for Servo |
|---|---|---|
| Twisted pairs | Encoder signal pairs are twisted so external noise couples equally to both wires (common mode) | Differential receiver cancels the common-mode noise — the fundamental noise-fighting technique |
| Individual foil shield per pair | Each feedback pair wrapped in aluminium/PET foil with a drain wire | Stops power-line noise from coupling directly into a specific pair |
| Overall braid shield | A braided copper layer around the whole assembly (85%+ coverage) | Primary EMI barrier against external interference and internal power-to-signal crosstalk |
| Internal metal divider | Physical metal separation between power and signal groups | In hybrid cables, physically separates the noisy power cores from sensitive signal cores |
| 360° shield termination | Shield bonded around the full circumference at the connector backshell | Poor "pigtail" grounds raise high-frequency impedance and cause intermittent feedback loss |
⚠ The termination is half the shield: A perfectly built cable with a badly terminated shield still fails. The classic error is a "pigtail" ground — a single wire from the shield to the connector instead of a 360° clamp. At high frequency, a pigtail adds impedance and lets noise through, causing the intermittent feedback faults that are so hard to diagnose. For PWM motor cables, bond the shield 360° at both ends (drive and motor) with low impedance — this is standard practice for motor cables. For encoder/feedback circuits, follow the drive manufacturer’s guidance: single-ended, capacitive, or both-ended bonding may be specified depending on the interface.
Separation Outside the Cable
The cable’s internal shielding is only part of the answer. How the cable is routed matters too:
- Keep as much parallel separation as practical between servo/encoder cables and other power cables — check your drive manufacturer’s installation guide; typical recommendations range from ~10 cm to 50 cm depending on power level
- Cross power cables at 90°, never run them parallel and touching
- Use metal partitions or separate trays between strong and weak power areas
- Don’t route servo feedback cable next to drive output or contactor wiring
💡 Encoder impedance check: For differential encoder signals, the cable should match the system’s characteristic impedance. The exact value depends on the feedback interface — commonly 120 Ω for RS-485-based encoders (e.g., EnDat, SSI), 110 Ω ± 10 Ω for Hiperface DSL — with typical capacitance around 50 pF/m. Always match the drive/encoder manufacturer’s specification. Mismatched impedance or high capacitance degrades signal rise times over long runs. Ask your cable supplier for the encoder pair’s impedance and capacitance values if your run is long or your encoder is sensitive.
Connector Compatibility: M23, M17, and the Interface That Matters
The connector is where the cable meets the motor and drive — and it’s a compatibility minefield.
| Connector | Typical Use | Rating | Notes |
|---|---|---|---|
| M23 | The dominant heavy-duty servo connector | Power up to 30 A, signal up to 8 A, power inserts up to 630 V / signal inserts typically ≤300 V, pins 6/8/9/12/17/19, IP67/68 | Larger shell gives room to isolate power and signal; 360° EMC shielding |
| M17 | Primarily for encoder and signal-level connections | Smaller, signal-level | Often used for the separate encoder cable from drive to motor |
| M12 | Compact servos, fieldbus connections | Lower power than M23 | Common for small servos and sensor/fieldbus links |
⚠ The pinout trap: “M23” describes the shell size and thread, not the pinout. Different motor brands and generations use different inserts, keying, conductor layouts, and electrical functions in the same M23 shell. You cannot assume two M23 cables are interchangeable. Always verify the original cable reference, the motor manufacturer’s connector pinout, and the drive-side interface before ordering — the connector shell alone tells you almost nothing.
Sorivo Cables for Servo and Motion Systems
Sorivo supplies the flexible cables and control wiring that feed servo and motion systems — and can provide custom assemblies to your interface.
| Application | Recommended Cable | Key Feature |
|---|---|---|
| Servo power / motor feeders | CU/XLPE/SWA/PVC | Fixed-installation armoured power cable for static motor feeders — not for flexing or drag-chain use |
| Control & signal in motion systems | CY screened control cables | Flexible screened construction for signal and control in automation |
| Motion network / fieldbus on moving axes (EtherCAT, PROFINET) | EtherCAT / PROFINET high-flex PUR | High-flex data cable for motion networks in moving axes |
| Custom ready-to-connect assemblies | Custom prefab cable assemblies (contact Sorivo) | Pre-cut, pre-terminated to your connector interface — reduces field wiring time |
| Note: For drag-chain-rated servo motor power (feeder) cable, Sorivo supplies custom flexible assemblies on request — tell us your flex/torsion requirements and connector interface. | ||
⚠ Static vs. moving axes: Standard armoured power cables (CU/XLPE/SWA) are for static feeders ONLY — do not use them in moving axes. SWA cable has a rigid steel armour that cannot flex repeatedly; in a drag chain or robot arm it will work-harden, crack, and fail quickly. For any axis that moves, use a flexible cable rated for flexing (or torsion, if the axis twists). When in doubt, tell us the axis type when you specify.
★ When specifying a servo cable, give us: 1) the motor and drive brand/model (for pinout and connector), 2) the cable type (composite hybrid, separate power + encoder, or brake-inclusive), 3) the connector types at both ends (e.g., M23 both ends, or M23 to flying leads), 4) flex/torsion requirements if on a moving axis, and 5) the run length. With those five inputs we can match or custom-build the right assembly instead of guessing.
Sorivo Motion Cables vs. Economy-Grade
| Feature | Market Generic / Economy | Sorivo Premium Grade |
|---|---|---|
| Conductor | Coarse strands fatigue under flex, higher resistance | Class 5/6 flexible stranding per IEC 60228 — verified, consistent DC resistance |
| Shielding | Basic braid, inconsistent coverage, pigtail grounding | Screened constructions with verified coverage, correct shield termination practice |
| Jacket | Standard PVC, oil/coolant sensitive | Flexible compound / PUR options, oil and abrasion resistant for machine environments |
| Traceability | None | Metre-marked, batch traceable, full certification available |
| Custom assemblies | Not available | Pre-terminated assemblies to your connector interface on request |
Composite Hybrid or Separate Cables? A Quick Decision Matrix
Both configurations work — the right one depends on your machine layout, axis type, and how you want to handle maintenance.
| Project Scenario | Recommended Configuration | Why |
|---|---|---|
| Moving axis with tight space (robot arm, drag chain) | Composite hybrid cable | One cable, one routing path, one connector — saves space and weight where every gram counts |
| Long static run from cabinet to motor | Separate power + encoder cables | If one cable fails, you replace just that one — easier to troubleshoot and cheaper to fix |
| High-EMI environment or existing connector interfaces | Separate cables, 360° bonded at both ends | Full physical isolation between noise and signal; faults are easier to isolate |
| Retrofit on a tight budget | Separate cables | A failed hybrid cable means replacing the whole assembly; separate cables fail one at a time |
★ The decision in one line: If the axis moves and space is tight, go composite. If the run is static, long, or you expect maintenance, separate cables usually win.
Frequently Asked Questions
Can I use a standard power cable for a servo motor?
You can physically connect it, but it won't work reliably. A servo motor is driven by PWM from a servo drive, which injects high-frequency switching noise into the motor cable. A standard power cable has no shielding to contain that noise, so it couples into the encoder feedback and causes position errors, axis drift, and intermittent glitches. Servo applications need shielded motor cables with the power and signal properly separated — the shielding isn't optional, it's the thing that makes the system work.
What's the difference between separate power + encoder cables and a composite hybrid cable?
A separate arrangement uses two cables — one shielded power cable and one encoder cable — run to the motor. A composite (hybrid) cable combines power, feedback, and brake conductors in one jacket with internal shielding and separation. Composite saves space, weight, and routing on moving axes, but demands excellent internal isolation because the noise and signals share a jacket. Separate cables give physical separation by default but add a second run and connector. The right choice depends on your machine layout and whether the axis moves.
Why does my servo occasionally lose position or glitch?
Intermittent servo glitches are frequently a shielding or termination problem. Common causes: a pigtail shield termination instead of 360-degree contact, encoder cable routed parallel to a power or VFD cable, a loose shield ground, or a cable where the power and signal cores aren't properly separated. Because the fault is intermittent, it's hard to reproduce — but checking the cable shield termination, routing separation, and ground continuity is usually where the answer is. A stable, well-terminated shielded cable eliminates most of these.
Are M23 connectors interchangeable between different servo brands?
No. M23 describes the shell and thread size, not the electrical interface. Different manufacturers use different inserts, keying, pin arrangements, and functions within the same M23 shell. An M23 cable for one brand of servo will not necessarily work on another, even if the connector threads on. Always verify the original cable reference and the specific pinout for your motor and drive combination before ordering — and confirm with the motor manufacturer's documentation, not just the connector type.
What impedance should the encoder pair be?
For differential encoder signals, the cable is commonly specified at 120 ohms differential impedance for RS-485-based encoders (Hiperface DSL specifies 110 ohms ± 10 ohms). The exact value depends on the feedback interface, so always match the drive/encoder manufacturer's specification. Matching impedance preserves signal integrity, especially on longer runs. If the impedance is mismatched or the capacitance too high, the signal rise times degrade and you can get position errors. Ask your cable supplier for the encoder pair's impedance and capacitance values — a quality cable will have them documented.
Need cables for your servo and motion system?
Sorivo supplies flexible power, screened control, and high-flex industrial Ethernet cables for motion control, plus custom pre-terminated assemblies on request. Contact our team with your motor, drive, and connector details.
sale@sorivocable.com | +86 19282905529
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