VFD Faults on Roll Forming Machines — Overcurrent, DC Bus & Braking Resistor Sizing

Introduction — Why VFD Faults Stop Roll Forming Lines

Variable Frequency Drives (VFDs) are widely used in roll forming machines to control the speed of motors such as:

• main forming drive motors
• pinch roll motors
• entry feed motors
• stacker motors
• conveyor motors.

When a VFD detects abnormal electrical or mechanical conditions, it shuts down to protect the motor and drive electronics.

The most common VFD faults in roll forming machines include:

• overcurrent faults
• DC bus overvoltage faults
• braking resistor problems.

These faults can stop production immediately and may indicate deeper mechanical or electrical issues.

Understanding the causes and solutions helps technicians restore operation quickly.

How VFDs Work in Roll Forming Machines

A VFD controls motor speed by converting incoming AC power into DC, then generating a variable-frequency AC output.

The basic internal stages include:

1 AC input rectifier
2 DC bus capacitor bank
3 inverter output stage.

The DC bus stores energy and supplies power to the motor through the inverter stage.

During normal operation, energy flows from the power supply to the motor.

However, during deceleration or load changes, energy may flow back toward the drive.

If the drive cannot dissipate this energy safely, faults occur.

Common VFD Fault Types

Several types of faults commonly occur in roll forming applications.

Typical examples include:

• overcurrent fault
• DC bus overvoltage fault
• overtemperature fault
• ground fault
• communication fault.

This guide focuses on the three most frequent issues affecting roll forming lines.

VFD Overcurrent Fault

An overcurrent fault occurs when the drive detects current exceeding its rated limits.

This usually indicates that the motor or mechanical system is experiencing excessive load.

Typical symptoms include:

• VFD trips immediately after starting
• drive faults during acceleration
• motor stalls when material enters the machine.

Common Causes of Overcurrent Faults

Several issues can trigger overcurrent faults.

Mechanical Overload

Roll forming stands may be overloaded due to:

• thick material
• incorrect roll setup
• misaligned tooling.

The motor must produce higher torque, causing current to exceed safe limits.

Sudden Load Changes

Material entering the forming rolls suddenly can create torque spikes.

If acceleration settings are too aggressive, the drive may trip.

Incorrect Motor Parameters

VFDs require correct motor parameters such as:

• rated voltage
• rated current
• motor power rating.

Incorrect settings can cause improper current control.

Short Circuits or Wiring Faults

Damaged motor cables or incorrect wiring can cause current spikes.

Technicians should inspect motor wiring if faults occur repeatedly.

Fixing Overcurrent Faults

Technicians should perform several checks.

Typical steps include:

1 inspect roll tooling alignment
2 verify material thickness within machine limits
3 check motor parameter configuration in the VFD
4 inspect motor cables and terminals
5 increase acceleration time if necessary.

These actions usually resolve overcurrent issues.

DC Bus Overvoltage Fault

A DC bus overvoltage fault occurs when the voltage inside the VFD’s DC bus rises above safe levels.

This commonly happens during motor deceleration.

When a motor slows down, it acts as a generator and feeds energy back into the drive.

If the drive cannot dissipate this energy, the DC bus voltage rises.

Once the voltage exceeds a safe threshold, the drive shuts down.

Why DC Bus Voltage Rises

When the motor decelerates quickly, its rotational energy converts into electrical energy.

This energy flows back toward the drive.

If there is no way to absorb the energy, it accumulates in the DC bus capacitors.

The voltage rises rapidly.

To protect the electronics, the drive trips.

Typical Causes of DC Bus Overvoltage

Common causes include:

• aggressive deceleration settings
• heavy rotating machinery
• missing or undersized braking resistor.

Roll forming machines with large flywheel effects may generate significant regenerative energy.

Fixing DC Bus Overvoltage Faults

Several solutions may resolve the problem.

Increase Deceleration Time

Slowing down the deceleration ramp reduces energy regeneration.

This is often the simplest solution.

Install or Upgrade Braking Resistor

Braking resistors dissipate excess energy as heat.

If the resistor is too small, it cannot absorb all the energy.

Upgrading to a larger resistor may solve the problem.

Check Braking Circuit Wiring

Loose or damaged braking resistor connections may prevent proper energy dissipation.

Inspect wiring and terminals carefully.

Braking Resistor Function

A braking resistor connects to the VFD through a braking transistor circuit.

When DC bus voltage rises, the drive switches the resistor on.

Excess energy flows into the resistor and is converted into heat.

This prevents DC bus voltage from exceeding safe limits.

Braking Resistor Sizing

Incorrect resistor sizing is a frequent cause of braking problems.

Resistor sizing depends on:

• motor power
• inertia of rotating components
• deceleration rate
• duty cycle.

If the resistor is too small, it may overheat or fail to absorb enough energy.

Example Braking Resistor Problem

Example scenario:

Roll forming line running at high speed.

Operator stops machine quickly.

Motor inertia regenerates large energy.

Braking resistor rated too low.

Result:

DC bus voltage rises → drive trips.

Upgrading the resistor or increasing deceleration time fixes the problem.

Thermal Protection of Braking Resistors

Braking resistors generate significant heat.

To prevent overheating, resistors often include:

• thermal switches
• temperature sensors.

If the resistor overheats, the drive may generate a braking fault.

Proper ventilation around resistors is important.

Checking VFD Fault Codes

Most VFDs display fault codes that indicate the exact problem.

Examples include:

• OC — overcurrent
• OV — overvoltage
• OH — overtemperature.

Technicians should consult the drive manual to interpret these codes correctly.

Drive Parameter Configuration

Incorrect drive parameters may cause frequent faults.

Important parameters include:

• acceleration time
• deceleration time
• motor current limits
• braking resistor settings.

Engineers should verify these parameters during commissioning.

Mechanical Causes of VFD Faults

Electrical faults are sometimes triggered by mechanical problems.

Possible mechanical issues include:

• seized bearings
• misaligned rolls
• excessive material thickness.

Mechanical inspection is often necessary when electrical faults occur repeatedly.

Electrical Noise and Grounding

Poor grounding can also cause VFD fault conditions.

Electrical noise may interfere with:

• drive control signals
• encoder feedback systems
• communication networks.

Proper grounding and shielding reduce these problems.

Preventative Maintenance for VFD Systems

Regular maintenance reduces the risk of VFD faults.

Recommended practices include:

• cleaning cooling fans
• checking cable connections
• inspecting braking resistors
• monitoring drive temperature.

Maintaining the drive environment helps ensure reliable operation.

Commissioning Best Practices

During machine commissioning, engineers should verify VFD behavior under various conditions.

Typical tests include:

1 running machine at maximum speed
2 testing rapid stops
3 verifying braking resistor temperature
4 monitoring DC bus voltage.

These tests confirm that the drive system can handle real production loads.

Benefits of Correct VFD Configuration

Properly configured VFD systems provide several advantages.

These include:

• stable machine acceleration
• reliable deceleration
• reduced mechanical stress
• improved motor protection.

For roll forming machines operating continuously in industrial environments, reliable drive systems are essential.

FAQ — VFD Faults in Roll Forming Machines

What causes overcurrent faults in roll forming machines?

Common causes include mechanical overload, incorrect motor parameters, aggressive acceleration, or wiring faults.

Why does DC bus voltage increase during deceleration?

When the motor slows down, it generates electrical energy that flows back into the drive.

What is the purpose of a braking resistor?

A braking resistor dissipates excess regenerative energy during deceleration.

How can I fix DC bus overvoltage faults?

Possible solutions include increasing deceleration time or installing a larger braking resistor.

Can mechanical issues cause VFD faults?

Yes. Misaligned tooling or excessive material thickness can overload the motor.

How can engineers prevent drive faults?

Proper drive configuration, regular maintenance, and correct braking resistor sizing help prevent VFD faults.