The Servo That Tripped on Its Own Cable: Motor Cable Shielding and Grounding
A servo drive that faulted with an overcurrent error every time the motor moved fast. The motor cable was unshielded. The drive’s PWM (20 kHz, high dv/dt) radiated noise that coupled back into the drive’s encoder input. The drive saw a false position error and tripped. This is about motor cable shielding, not about the drive being defective.
The dv/dt problem
A servo drive switches the motor voltage at 4-16 kHz. The switching edges are fast (dv/dt up to 5000 V/μs). This high-frequency noise radiates from the motor cable. If the cable is unshielded, the noise couples onto nearby cables (encoder, limit switches) and back into the drive. The drive’s internal circuits see the noise as a fault.
The motor cable requirements
| Requirement | Why |
|---|---|
| Shielded (braid coverage 80%+) | Contains the PWM noise |
| Shield grounded at both ends | Provides a return path for the noise current |
| Twisted power wires (U,V,W) | Reduces magnetic radiation |
| Separate from signal cables | Noise doesn’t couple onto encoder cables |
| Brake wire shielded separately | Brake switching creates additional noise |
What we did
1. Replaced the motor cable. The original was a 3-conductor unshielded cable. We replaced it with a servo-rated shielded cable (4 conductors: U, V, W, PE, plus brake wires, all shielded). The shield grounded at both ends (drive and motor). The noise stopped radiating.
2. Moved the encoder cable. The encoder cable ran parallel to the motor cable. We moved it to a separate tray, 200 mm away. The remaining noise (from the shielded motor cable) was negligible.
3. Added a ferrite bead. We clipped a ferrite ring on the motor cable at the drive end. It absorbs high-frequency common-mode noise that the shield doesn’t contain. The drive stopped tripping.
The grounding
For motor cables, the shield grounds at both ends (unlike signal cables, which ground at one end). The motor cable carries high-frequency noise current that needs a return path. Grounding at both ends provides that path. The shield isn’t an antenna — it’s a conductor. Both ends grounded = noise current flows on the shield, not radiating.
The servo cable rule: Use a shielded motor cable, grounded at both ends. Separate it from encoder and signal cables. If the drive faults on fast moves, check the motor cable shielding first. An unshielded servo motor cable radiates noise that the drive itself picks up. The drive isn’t tripping on a fault — it’s tripping on its own EMI.
