Detailed Description
Why a Single Core? The Case Against Multi-Core Motor Cables
In servo and VFD motor power applications, the conventional approach is a 4-core shielded cable (U, V, W phases + PE ground) in a single jacket. This works — until one of these constraints forces a different solution:
Constraint 1: Carrier Space
A 4 × 16 mm² shielded motor cable has an outer diameter of approximately 22–25 mm. In a compact machine with a carrier internal height of 40 mm, that single cable occupies 55–62% of the height — leaving minimal room for encoder feedback, sensor, and control cables. Routing each phase as a separate 16 mm² single-core cable (OD ≈ 8–10 mm each) allows a much flatter arrangement.
Constraint 2: Minimum Bend Radius
Multi-core cables have a larger OD, which directly drives a larger minimum bend radius. A 25 mm OD cable at 7.5×D requires a 188 mm bend radius — too large for many compact machine designs. The same cross-section in single cores at 10 mm OD each requires only 75 mm at 7.5×D.
Constraint 3: VFD Reflected Wave Voltage
In PWM-driven motor circuits, the fast voltage rise time (dv/dt) interacts with cable capacitance to create reflected wave overvoltage at the motor terminals — potentially 2× the DC bus voltage. Single-core cables with PP insulation have inherently lower capacitance to ground than multi-core cables, reducing dv/dt stress on motor winding insulation. This is why leading VFD manufacturers increasingly recommend individually routed single cores for long motor cable runs (> 15–20 m).
Single Core vs Multi-Core — Decision Framework
| Criterion | Multi-Core Motor Cable | Single Core PUR |
|---|---|---|
| Carrier height utilization | Single large cable, limited flexibility | Multiple small cables, flat arrangement possible |
| Bend radius for equivalent mm² | 7.5 × 22–25 mm = 165–188 mm | 7.5 × 8–10 mm each = 60–75 mm |
| Capacitance to ground | Higher (cores bundled) | Lower (individual, spaced) |
| VFD reflected wave risk | Higher — longer runs may need output reactor | Lower — reduced capacitance reduces overshoot |
| Heat dissipation | Cores heat each other at high continuous current | Each core cools independently |
| Replacement | Replace entire cable | Replace single faulty phase |
| Installation complexity | Lower — one cable to route and terminate | Higher — three cables to route, label, and terminate |
| Cost | Lower material cost | Slightly higher — but potentially reduced carrier size cost |
Typical Cross-Section Selection Guide
| Cross-Section | OD (approx) | Continuous Current* | Typical Motor Power (400V) | Common Application |
|---|---|---|---|---|
| 1.5 mm² | 3.5 mm | 16 A | Up to 5.5 kW | Small servo, control circuit |
| 2.5 mm² | 4.2 mm | 21 A | Up to 7.5 kW | Compact servo drive |
| 4 mm² | 5.1 mm | 28 A | Up to 11 kW | Mid-size servo |
| 6 mm² | 5.8 mm | 36 A | Up to 15 kW | Standard automation servo |
| 10 mm² | 7.2 mm | 50 A | Up to 22 kW | Large servo, spindle motor |
| 16 mm² | 8.5 mm | 66 A | Up to 30 kW | High-power spindle |
| 25 mm² | 10.8 mm | 89 A | Up to 45 kW | Heavy spindle, linear motor |
| 35 mm² | 12.9 mm | 110 A | Up to 55 kW | Large linear motor, DC bus |
| 50 mm² | 15.5 mm | 137 A | Up to 75 kW | Multi-axis power distribution |
| 70 mm² | 18.0 mm | 170 A | Up to 90 kW | Portal mill, gantry crane |
| 95 mm² | 21.0 mm | 206 A | Up to 110 kW | Heavy gantry, DC link |
| 120 mm² | 23.5 mm | 238 A | Up to 132 kW | Large port crane |
| 150 mm² | 26.5 mm | 273 A | Up to 160 kW | Ship-to-shore crane |
| 185 mm² | 29.5 mm | 312 A | Up to 200 kW | Rolling mill, steel plant |
*Continuous current at 30°C ambient, single cable in free air. Reduce for bundles or elevated ambient. Use appropriate derating factors per IEC 60364-5-52.
Installation — Routing Single Cores in Drag Chains
Phase Identification
Individually routed single cores require clear identification. Standard practice:
- U / L1 phase: Black
- V / L2 phase: Red (standard) or blue
- W / L3 phase: Blue (standard) or red
- PE / Ground: Green-yellow (mandatory per IEC 60445)
- DC+ / DC-: Red / black (for DC link applications)
Physical Arrangement
- Route the three phases adjacent to each other in a single layer inside the carrier — not stacked vertically
- Maintain equal spacing (±2 mm) between the three cables for the entire carrier length
- This equal-spacing arrangement minimizes inductance imbalance between phases, which reduces common-mode noise
- Bundle the three single cores with PUR-compatible spiral wrap at 200–300 mm intervals to prevent individual cables from migrating into different carrier lanes
- At the carrier entry and exit points, use a single 3-hole rubber-lined clamp plate — not three individual clamps — to ensure equal strain relief per phase
Shield Considerations
Single-core motor cables for VFD applications may be ordered with or without shield:
- Unshielded: Suitable for short runs (< 5 m) where EMI is not a concern and a separate shielded encoder cable carries the sensitive signals
- Shielded (85%+ braid): Recommended for runs > 5 m or where the single-core power phases run in the same carrier as encoder or sensor cables. Each phase has its own braid; all three shields should be commonly grounded at both ends through a low-impedance EMC busbar
Why Choose Yichi Cable PUR Single Core?
- Full cross-section range: 1.5–185 mm² from standard stock; larger sections on request — no minimum order quantity for standard sizes
- VFD-optimized insulation: PP or TPE core insulation with verified low capacitance reduces reflected wave stress on your motor windings
- UL/CSA certified: North American equipment builders can specify our single-core PUR cables without additional certification
- Color and marking options: Standard colors + custom RAL + laser-printed phase markers — built into the production run, no post-processing
- Application review: For VFD motor circuits over 20 m or PWM carrier frequencies above 8 kHz, our engineering team will review your specific dv/dt, cable capacitance, and motor insulation class — we may recommend output reactors or different insulation, not because we want to upsell, but because we want your machine to work