Diagnosing Electrical Noise Problems in Roll Forming Machines (EMI & Grounding Guide)

Diagnosing Electrical Noise Problems

EMI, Ground Loops & Signal Interference in Roll Forming Lines

Electrical noise (EMI – Electromagnetic Interference) is one of the most misunderstood causes of:

• Random PLC input flicker

• Encoder instability

• False sensor triggers

• Unexpected machine stops

• VFD communication faults

• Analog signal drift

• Length mismeasurement

Roll forming machines are high-noise environments because they contain:

• Large VFD-driven motors

• High switching frequencies

• Long motor cables

• High current switching

• Inductive loads

• Large steel frames acting as antennas

If grounding and shielding are not engineered correctly, electrical noise will compromise reliability.

This guide provides a systematic engineering method for diagnosing electrical noise problems in roll forming and coil processing systems.

1) What Is Electrical Noise?

Electrical noise is unwanted voltage or current that interferes with control signals.

It can be:

• Conducted noise (through wiring)

• Radiated noise (through air)

• Ground loop noise

• Common-mode noise

• Differential noise

Noise becomes a problem when:

Signal amplitude ≈ Noise amplitude.

Low-voltage signals (24VDC, 5V encoder pulses, 4–20mA analog) are especially vulnerable.

2) Common Symptoms of Electrical Noise

Noise issues often appear as:

• Random PLC input activation

• Encoder miscounts

• Shear mistiming

• Analog value fluctuating

• VFD communication errors

• Safety circuit intermittent trip

• Machine stops without clear cause

Noise problems are often intermittent.

3) Primary Noise Sources in Roll Forming Machines

Major sources include:

• VFD switching frequency (PWM)

• Long motor cables

• Poor grounding

• Inductive coil switching

• Relay contact arcing

• Improper shield termination

• Shared cable trays

VFD output cables are the most common noise generator.

4) Step 1 – Identify When Noise Occurs

Observe:

Does fault occur:

• Only when motor runs?

• Only at high speed?

• Only during shear activation?

• Only during deceleration?

If issue correlates with motor operation, VFD noise likely.

Pattern recognition is critical.

5) Check Cable Routing

Power and signal cables must not share the same path.

Incorrect:

Motor cable bundled with encoder cable.

Correct:

Separate cable trays for:

• Power cables

• Control cables

• Encoder cables

Maintain physical separation.

Minimum recommended separation: 200–300mm where possible.

6) Shielding Practices

For encoder and analog cables:

Use twisted pair, shielded cable.

Correct shield grounding method:

Ground shield at ONE end only (typically panel end).

Incorrect grounding at both ends creates ground loop.

Ground loops amplify noise.

7) Grounding System Inspection

Verify:

• Single-point grounding scheme

• Cabinet bonded to earth

• Machine frame bonded

• Low earth resistance

• Shield drains properly terminated

Poor grounding causes floating references.

Floating reference creates unstable PLC inputs.

8) Check 0V Reference Stability

In 24V systems:

Measure voltage between 0V reference and earth.

Large fluctuation indicates:

• Ground loop

• Shared return path

• High current on 0V conductor

Control 0V must be stable.

9) VFD Output Cable Considerations

VFD motor cables generate high dv/dt switching.

Mitigation methods:

• Use shielded motor cable

• Keep motor cables short

• Use line reactors

• Install output filters

• Avoid parallel routing with signal cables

Unshielded motor cable is common root cause.

10) Encoder Noise Diagnosis

Symptoms:

• Length error increases at high speed

• Pulse count inconsistent

• Shear misfires

Testing:

Monitor encoder pulses on oscilloscope.

Look for:

• Distorted waveform

• Spikes

• Dropouts

Use differential encoder signals where possible (A+, A-).

11) Analog Signal Instability

For 4–20mA pressure sensors:

If PLC reading fluctuates ±5% randomly:

Likely noise interference.

Solutions:

• Use shielded twisted pair

• Separate analog and motor cables

• Use proper analog module grounding

• Add signal filtering

12) Inductive Kickback from Coils

Solenoids and contactors generate voltage spikes.

If flyback diodes or snubbers not installed:

Spikes propagate through control wiring.

Install:

Flyback diode across DC coil
RC snubber across AC coil

Suppression reduces noise.

13) Check for Ground Loops

Ground loop occurs when:

Shield connected at both ends
Multiple earth reference paths exist

Test:

Disconnect shield at one end temporarily.

If problem resolves → ground loop confirmed.

Ground loops are common in retrofits.

14) Cabinet Layout Impact

Inside panel:

Keep:

• VFD section separated

• PLC section isolated

• Analog modules away from drive cables

Poor panel layout creates internal noise.

15) High-Frequency Switching & Harmonics

High switching frequency in VFD increases EMI.

If persistent noise:

Reduce switching frequency slightly (within manufacturer limits).

Install line reactor or EMI filter.

Balance performance with noise reduction.

16) Communication Cable Noise

If system uses:

• Ethernet

• Profinet

• Modbus

Ensure:

• Shielded industrial cable
• Proper grounding
• Avoid routing near VFD cables

Communication dropout can appear as random stops.

17) Use of Ferrite Cores

Ferrite cores around signal cables can reduce high-frequency noise.

Commonly used on:

• Encoder cables

• Communication cables

• Control supply lines

Ferrites reduce EMI propagation.

18) Diagnostic Flow Summary

• Step 1: Identify when issue occurs
• Step 2: Inspect cable routing
• Step 3: Verify shielding practice
• Step 4: Inspect grounding scheme
• Step 5: Measure 24V stability
• Step 6: Inspect motor cable type
• Step 7: Check coil suppression
• Step 8: Monitor signals with oscilloscope

Never assume PLC is faulty first.

19) Most Common Noise Causes in Roll Forming

1. Motor cable bundled with encoder

2. Shield grounded at both ends

3. No flyback diode on solenoid

4. Poor cabinet grounding

5. High switching frequency VFD

6. Shared 0V return path

7. Improper analog wiring

Installation quality determines noise immunity.

20) Preventive Design Practices

• Use shielded motor cable

• Separate cable trays

• Use single-point grounding

• Install suppression on inductive loads

• Maintain clean panel layout

• Verify phase balance

• Install line reactors when required

Noise prevention is easier than troubleshooting.

21) Buyer Strategy (30%)

When purchasing a roll forming machine, verify:

1. Shielded motor cables specified

2. Encoder cables twisted pair and shielded

3. Single-point grounding scheme documented

4. Flyback diodes installed

5. Panel layout separates VFD and PLC

6. Line reactors included (if long cable runs)

7. Commissioning report includes noise testing

8. Cable routing guidelines provided

Red flags:

• “Motor and signal cables in same tray.”
• “Shields grounded both ends without reason.”
• “No suppression devices installed.”

Proper electrical design prevents future downtime.

6 Frequently Asked Questions

1) What is most common noise source?

VFD motor cable interference.

2) Should shields be grounded both ends?

Typically no — one end only.

3) Can noise cause random machine stops?

Yes, especially false PLC inputs.

4) How do I confirm noise?

Use oscilloscope or monitor flickering input.

5) Do line reactors help?

Yes, reduce conducted EMI.

6) Why does problem worsen at high speed?

Higher switching frequency and motor load increase EMI.

Final Engineering Summary

Diagnosing electrical noise problems in roll forming machines requires systematic evaluation of:

• Cable routing

• Shielding practice

• Grounding integrity

• VFD motor cable type

• Inductive suppression

• 24V reference stability

• Encoder signal quality

• Communication wiring

Noise-related faults are often intermittent but entirely preventable with correct engineering design and installation discipline.

In high-speed roll forming environments, proper EMI control is essential for stable production, accurate length control, and reliable safety performance.