Harmonics and Their Impact on VFD Systems in Roll Forming Machines (THD, Filters & Failures)

Power Quality Engineering for Roll Forming & Coil Processing Lines

Modern roll forming factories are dominated by VFD-driven motors.

• Main forming drives
• Hydraulic pumps
• Uncoilers and recoilers
• Servo axes
• Slitting line tension systems

VFDs provide precision and efficiency — but they also introduce harmonic distortion.

Harmonics affect:

• Transformers

• Busbars

• Breakers

• Cables

• Power factor correction systems

• Sensitive control electronics

• Generator compatibility

If not engineered correctly, harmonics cause:

• Overheating without obvious overload

• Nuisance breaker trips

• VFD undervoltage faults

• PLC instability

• Capacitor bank failures

• Reduced transformer lifespan

This guide explains what harmonics are, how they form, how to measure them, and how to mitigate them in roll forming environments.

1) What Harmonics Actually Are

In an ideal power system:

Voltage and current waveforms are perfect sine waves.

But VFDs do not draw current in a smooth sine shape.

Instead:

They draw current in pulses as the rectifier section charges the DC bus.

This creates distorted current waveforms.

That distortion can be broken down into:

• Fundamental frequency (50Hz or 60Hz)

• Higher frequency components called harmonics

For 50Hz systems:

• 3rd harmonic = 150Hz
• 5th harmonic = 250Hz
• 7th harmonic = 350Hz
• etc.

For 60Hz systems:

• 3rd = 180Hz
• 5th = 300Hz
• 7th = 420Hz

These higher frequency components increase heating and stress.

2) Why VFDs Create Harmonics

A typical VFD input stage contains:

AC SUPPLY → RECTIFIER (Diodes or IGBTs) → DC BUS → INVERTER → MOTOR

The rectifier converts AC to DC in pulses.

Those pulses are not sinusoidal.

Result:
Non-linear current draw.

This non-linearity creates harmonic distortion on the supply.

The more VFDs in a factory:
The greater the harmonic distortion.

3) Key Harmonic Terms You Must Understand

3.1 THD (Total Harmonic Distortion)

THD measures distortion compared to the fundamental waveform.

THDi = current distortion
THDv = voltage distortion

Typical acceptable guidelines (general engineering practice):

• THDv ideally under ~5%

• THDi depends on system strength

Higher values indicate significant distortion.

3.2 RMS Current Increase

Harmonics increase RMS current without increasing useful kW.

This means:

• Cables heat more
• Transformers heat more
• Breakers may trip earlier
• Busbars experience extra thermal stress

Even though kW seems normal.

4) Why Roll Forming Factories Are Especially Sensitive

Roll forming loads are:

• High torque

• Long duty cycle

• Multi-drive systems

• Often retrofitted over time

When several VFDs operate simultaneously:

• Main forming motor
• Hydraulic pump VFD
• Recoiler drive
• Slitter drives
• Stacker motors

Harmonics accumulate.

Small machines may not show issues.

Large factories with 5–10 lines often do.

5) Harmonic Effects on Key Equipment

5.1 Transformers

Harmonics increase:

• Eddy current losses

• Core heating

• Insulation stress

Transformers may run hot even when not fully loaded in kW terms.

Repeated overheating reduces lifespan significantly.

5.2 Busbars and Cables

Higher RMS current causes:

• Elevated temperature rise

• Insulation degradation

• Reduced safety margin

Harmonic currents also concentrate near conductor surfaces (skin effect), increasing effective resistance.

5.3 Breakers & MCCBs

Breakers respond to RMS current.

Increased RMS due to harmonics can cause:

• Apparent overload conditions

• Nuisance tripping

• Reduced margin between operating and trip threshold

5.4 Capacitor Banks (Power Factor Correction)

This is a major risk.

Capacitors can resonate with harmonic frequencies.

This may cause:

• Amplified current at specific harmonic frequencies

• Capacitor overheating

• Explosion or failure

• System instability

Installing standard capacitors in VFD-heavy plants without harmonic review is dangerous.

5.5 VFDs Themselves

High supply distortion can cause:

• DC bus instability

• Overvoltage trips

• Undervoltage faults

• Communication noise

Ironically, VFDs can damage other VFDs via shared harmonic distortion.

6) Worked Example — Multi-Line Roll Forming Factory

Factory load:

5 lines
Each line has:

• 30 kW forming VFD

• 15 kW hydraulic VFD

• 7.5 kW recoiler VFD

Total VFD load per line = 52.5 kW
Total VFD load factory-wide = 262.5 kW

If harmonic distortion increases RMS current by even 10%:

Current-related heating increases roughly by I² relationship.

10% current increase →
(1.1)² = 1.21 → 21% more heating

This is why transformers run hot unexpectedly.

7) Mitigation Strategies

7.1 Line Reactors

Installed between supply and VFD.

Benefits:

• Reduce harmonic current

• Smooth waveform

• Reduce peak charging currents

Simple and cost-effective first step.

7.2 DC Bus Chokes

Installed inside or added to VFD.

Reduces harmonic content.

Common on higher-quality drives.

7.3 Passive Harmonic Filters

Tuned to specific frequencies.

Reduce harmonic magnitude.

Must be engineered carefully to avoid resonance.

7.4 Active Harmonic Filters

Electronic devices that inject counter-waveforms to cancel harmonics.

More expensive but highly effective.

Often justified in:

• Large coil processing plants

• Facilities with utility distortion limits

• Plants with sensitive automation equipment

7.5 Multi-Pulse Drives (12-Pulse, 18-Pulse)

Higher-end approach.

Reduces harmonic generation at source.

Used in high-power industrial systems.

8) Generator Compatibility Warning

If factory operates on generator backup:

Harmonics become more severe because:

Generators typically have:

• Higher source impedance

• Lower short-circuit strength

Result:

• Greater voltage distortion

• Unstable VFD operation

• Protection nuisance trips

Generator + VFD systems must be engineered together.

9) Measuring Harmonics in a Roll Forming Factory

Proper harmonic assessment requires:

• Power quality analyzer

• Measurement at main switchboard

• Measurement during full production load

Measure:

• THDi

• THDv

• Individual harmonic components

• Transformer temperature rise

Do not assume harmonic level from drive count alone.

10) Common Harmonic-Related Symptoms

• Transformers unusually hot

• MCCB trips without overload

• Capacitor bank failure

• PLC resets

• Encoder noise

• VFD faults during peak load

• Neutral conductor overheating (in some systems)

If these occur in VFD-heavy factory, harmonics should be investigated.

11) Buyer Strategy (30%)

Before installing multiple VFD-driven roll forming lines, ask:

1. Has harmonic analysis been performed?

2. What is expected THDi at full load?

3. Are line reactors included as standard?

4. Is transformer sized for harmonic heating?

5. Is power factor correction system detuned?

6. Is facility running on generator at times?

7. Is there provision for future expansion?

Red flag:

“We’ve installed many drives — no problem.”

Harmonic issues often appear only after expansion.

6 Frequently Asked Questions

1) Do all VFDs create harmonics?

Yes. All rectifier-based VFDs create harmonic distortion to some degree.

2) Are harmonics dangerous?

They are not inherently dangerous, but they increase heating and can cause instability if unmanaged.

3) Will adding capacitors fix harmonic problems?

No. Standard capacitors can worsen harmonic resonance.

4) Do small roll forming factories need harmonic filters?

Not always. It depends on drive size, count, and supply strength.

5) Why is my transformer hot even when load seems normal?

Harmonic currents increase RMS heating beyond what kW measurements suggest.

6) What is the biggest mistake with harmonics?

Ignoring them until failures occur, or installing correction equipment without harmonic study.

Final Engineering Summary

Harmonics in VFD-driven roll forming systems:

• Increase RMS current

• Overheat transformers, cables, and busbars

• Reduce breaker margin

• Destabilize control systems

• Interact dangerously with capacitor banks

• Increase generator instability

Mitigation requires:

• Measurement

• Reactor integration

• Proper transformer sizing

• Filter selection

• Forward planning for expansion

As roll forming factories become more automated and VFD-heavy, harmonic engineering is no longer optional — it is core electrical design.