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

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:

1. Servo Motor Power Cable

2. 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

1. Using standard motor cable instead of servo cable

2. Using unshielded feedback cable

3. Running feedback cable with power cables

4. Poor shield termination

5. Untwisting pairs excessively

6. Using incorrect connector type

7. No strain relief

8. No separation from hydraulic solenoids

These mistakes cause intermittent faults that are hard to trace.

15) Testing Cable Integrity

Testing procedure:

1. Inspect shield termination

2. Measure continuity

3. Inspect for mechanical damage

4. Monitor encoder count stability

5. Run at full production speed

6. 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:

1. Servo motor cable is shielded inverter-duty rated

2. Feedback cable uses twisted differential pairs

3. Shield termination uses 360° clamp

4. Power and signal routing separated

5. Cable rated for industrial environment

6. Cable length within drive specification

7. Proper grounding architecture implemented

8. 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.