Shielded Cable Selection for Servo Systems in Roll Forming Machines (EMC & Noise Control Guide)

Servo systems used in flying shears, punch presses, stackers, and positioning axes are extremely sensitive to electrical noise.

Shielded Cable Selection for Servo Systems

EMC, Noise Immunity & Reliability in Roll Forming Machines

Servo systems used in flying shears, punch presses, stackers, and positioning axes are extremely sensitive to electrical noise.

Unlike standard induction motors, servo systems operate with:

High-frequency PWM switching
Closed-loop position feedback
High-speed encoder signals
Fast current response

Improper cable selection is one of the most common causes of:

Following errors
Encoder faults
Random shear misalignment
Position drift
Servo drive trips
Unstable high-speed operation

In roll forming environments with VFD-driven main motors, hydraulic solenoids, and long cable runs, electromagnetic interference (EMI) is significant.

This guide explains how to properly select, route, and terminate shielded cables for servo motor and feedback systems.

1) Why Shielded Cables Are Critical in Servo Systems

Servo drives generate high-frequency switching pulses.

These pulses:

Create electromagnetic noise
Induce voltage in nearby cables
Affect encoder feedback signals

If feedback signals are corrupted, the servo drive loses position accuracy.

Shielded cable acts as a barrier between:

Noise source → Sensitive signal conductors

Without proper shielding, system stability degrades at high speed.

2) Two Critical Servo Cable Types

Every servo axis typically requires:

Servo Motor Power Cable
Servo Feedback (Encoder) Cable

Each has different shielding requirements.

3) Servo Motor Power Cable Requirements

Power cable between drive and motor must:

Be rated for inverter duty
Include overall braid shield
Include symmetrical grounding conductor
Have low capacitance
Be oil and abrasion resistant

Word-Based Connection:

Servo Drive U/V/W → Shielded Motor Cable → Servo Motor

Shield must be terminated properly at both ends.

4) Servo Feedback Cable Requirements

Encoder cable must:

Use twisted pair for each signal
Use differential pairs (A+/A–, B+/B–, Z+/Z–)
Have foil or braid shield
Maintain signal integrity at high frequency

Feedback cable carries low-voltage signals.

Even small noise can disrupt accuracy.

5) Types of Shielding

Common shield types:

1) Foil Shield

Good for high-frequency noise
Lightweight
Less mechanically durable

2) Braided Shield

Better mechanical strength
Good overall EMI protection

3) Combination (Foil + Braid)

Best industrial protection
Recommended for flying shear systems

Combination shielding is preferred in high-speed roll forming lines.

6) Differential Signal Protection

Servo feedback systems use differential signaling.

Example:

A+ and A–
B+ and B–

Twisted pairs reduce electromagnetic pickup.

Each pair must be:

Individually twisted.

Do not untwist near terminals.

7) Shield Termination Best Practices

Servo Motor Cable:

Shield → 360° clamp at drive end
Shield → Proper motor housing bond

Servo Feedback Cable:

Shield → Bonded at drive end
Motor connector housing typically provides second bond

Avoid long pigtail grounding wires.

Long pigtails reduce shielding effectiveness.

8) Cable Routing Rules Inside Cabinet

Power cables and signal cables must be separated.

Recommended layout:

Left trunking → Power
Right trunking → Feedback and signal

Maintain minimum separation distance.

If crossing required:

Cross at 90° angle.

Never run servo feedback cable parallel to VFD motor cable.

9) Cable Routing in Machine Frame

In roll forming frames:

Avoid sharp edges
Avoid high-vibration points
Use flexible drag chain rated cable
Avoid routing near hydraulic solenoids

Mechanical stress damages shielding over time.

10) Grounding Strategy for Servo Systems

Grounding flow:

Earth Bar → Servo Drive PE → Motor Cable Shield → Motor Frame

Ensure:

Low impedance grounding
No floating shields
No ground loops

Multiple improper ground points cause interference.

11) Symptoms of Poor Shielding

In flying shear applications:

Cut length variation
Random servo following error
Encoder loss at high speed
Position jitter
Servo instability only at high RPM

If issue appears only at high speed, suspect EMI first.

12) Long Cable Run Considerations

For long runs (>15–20 meters):

Use high-quality shielded cable
Consider lower capacitance design
Ensure differential encoder signals
Verify drive supports long cable length

Excessive capacitance stresses drive output stage.

13) Environmental Factors

Roll forming environments include:

Metal dust
Oil mist
Temperature variation
Vibration

Cable jacket must be:

Oil resistant
Flexible
Rated for continuous motion if in drag chain

Low-quality cable fails prematurely.

14) Common Servo Cable Mistakes

Using standard motor cable instead of servo cable
Using unshielded feedback cable
Running feedback cable with power cables
Poor shield termination
Untwisting pairs excessively
Using incorrect connector type
No strain relief
No separation from hydraulic solenoids

These mistakes cause intermittent faults that are hard to trace.

15) Testing Cable Integrity

Testing procedure:

Inspect shield termination
Measure continuity
Inspect for mechanical damage
Monitor encoder count stability
Run at full production speed
Observe following error trend

Noise-related faults often only appear under load.

16) Shielded Cable for Multi-Axis Systems

In structural roll forming lines with:

Multiple servo axes
Flying shear
Punch system

Each axis must have:

Independent shielded cable
Separate routing if possible

Cross-coupling noise between axes can occur.

17) Export Considerations

When exporting roll forming machines:

Confirm cable rating for ambient temperature
Confirm compliance with local standards
Provide spare cable part number
Document cable specification
Verify EMC compliance

Replacing servo cable locally with generic type often causes instability.

18) Buyer Strategy (30%)

Before purchasing a servo-driven roll forming machine, verify:

Servo motor cable is shielded inverter-duty rated
Feedback cable uses twisted differential pairs
Shield termination uses 360° clamp
Power and signal routing separated
Cable rated for industrial environment
Cable length within drive specification
Proper grounding architecture implemented
Documentation includes cable specification

Red flag:

“Standard multi-core cable used for servo feedback.”

That design will fail at high speed.

6 Frequently Asked Questions

1) Can I use normal motor cable for servo motor?

No. Servo systems require inverter-duty shielded cable.

2) Why does servo fault only at high speed?

High-frequency noise increases with speed and PWM switching.

3) Should shield be grounded both ends?

Follow manufacturer recommendation. Typically bonded at drive end.

4) What is most common shielding mistake?

Using long pigtail ground instead of 360° clamp.

5) Why is twisted pair important?

Reduces electromagnetic interference on differential signals.

6) Does cheap cable cause servo problems?

Yes. Poor shielding and high capacitance reduce stability.

Final Engineering Summary

Proper shielded cable selection for servo systems in roll forming machines requires:

Inverter-duty shielded motor cable
Differential twisted-pair feedback cable
360° shield termination
Physical separation from power cables
Clean grounding architecture
Environmental durability

Incorrect cable selection causes:

Following errors
Cut length variation
Encoder instability
Random drive faults
Production downtime

In high-speed flying shear applications, cable quality and shielding discipline directly determine system accuracy and reliability.