Brake Resistors & Regenerative Braking in Roll Forming Machines (VFD & Servo Guide)

In roll forming machines, deceleration events are just as important as acceleration.

Brake Resistors & Regenerative Braking

DC Bus Control in Roll Forming & Flying Shear Systems

In roll forming machines, deceleration events are just as important as acceleration.

Typical braking events include:

Main drive slowing from production speed
Emergency stop deceleration
Flying shear carriage return
Servo-driven positioning systems
Stacker motor rapid stop

When a motor decelerates, it becomes a generator.

Energy flows back into the drive’s DC bus.

If that energy is not controlled, the result is:

DC bus overvoltage trips
Drive shutdown during deceleration
Nuisance faults
Mechanical shock
Production stoppage

Brake resistors and regenerative braking systems manage this excess energy.

This guide explains how they work and how to design them correctly in roll forming applications.

1) What Happens During Motor Deceleration?

During deceleration:

Mechanical inertia → Converts to electrical energy
Electrical energy → Flows into DC bus of drive

If the DC bus voltage rises too high:

Drive trips on overvoltage fault.

This is extremely common in:

High-speed roofing lines
Heavy structural lines
Flying shear servo systems

Without braking management, production reliability suffers.

2) Two Methods of Managing Regenerative Energy

1) Brake Resistor (Dynamic Braking)

Excess energy is diverted into a resistor and dissipated as heat.

2) Regenerative Drive System

Excess energy is fed back into the electrical supply grid.

Brake resistors are most common in roll forming.

Regenerative systems are used in high-power or multi-axis systems.

3) Word-Based Brake Resistor Operation

Motor Decelerates → DC Bus Voltage Rises → Brake Chopper Activates → Energy Diverted → Resistor Dissipates Heat

Brake chopper monitors DC bus voltage.

When voltage exceeds threshold, resistor engages automatically.

4) When Is a Brake Resistor Required?

Brake resistors are typically required when:

Deceleration time is short
Machine inertia is high
Frequent stop/start cycles occur
Servo axes return quickly
Emergency stop must be rapid

If deceleration time is long and gradual, braking resistor may not be necessary.

But production environments usually require controlled stopping.

5) Sizing a Brake Resistor (Engineering Principles)

Resistor must be sized based on:

Motor power
Inertia of system
Deceleration time
Duty cycle

If resistor too small:

Overheats
Burns out
Causes drive fault

If resistor too large:

Unnecessary cost
Oversized installation

Correct sizing requires energy calculation:

Braking Energy = ½ × Inertia × (Speed² Difference)

Engineering verification is critical for structural roll forming lines.

6) Wiring a Brake Resistor (Word-Based)

Drive Braking Terminals (P+, PB or B1/B2 depending on model) → Brake Resistor

Resistor must be:

Correctly rated
Mounted with ventilation
Away from heat-sensitive components
Properly grounded

Never connect resistor directly across supply.

It must connect only to designated drive braking terminals.

7) Ventilation & Heat Dissipation

Brake resistors convert energy into heat.

Improper installation causes:

Cabinet overheating
Premature failure
Fire hazard

Best practices:

Install outside main control cabinet
Ensure airflow
Avoid enclosed tight compartments
Respect clearance distances

Thermal management is essential.

8) Regenerative Drive Systems

Instead of wasting energy as heat, regenerative drives:

Feed energy back into the grid.

Advantages:

Energy savings
Reduced heat
Improved efficiency

Common in:

Large structural roll forming lines
Multi-servo systems
High-power applications

More complex and expensive than brake resistors.

9) Flying Shear Applications

Flying shear servo systems often require:

Rapid acceleration
Rapid deceleration
Frequent cycles

Without braking resistor:

Servo drive trips on DC bus overvoltage during return motion.

Proper brake resistor sizing is critical in shear systems.

10) Symptoms of Inadequate Braking Design

Common field faults:

“DC Bus Overvoltage”
“Overvoltage Fault During Deceleration”
Drive trips only when stopping
Trips during emergency stop
Trips when shear returns quickly

If fault only occurs on stop, braking design is suspect.

11) Deceleration Time vs Braking Requirement

If deceleration time too short:

DC bus voltage rises rapidly.

If deceleration time extended:

Less energy surge → No resistor required.

However, too long deceleration may:

Affect productivity
Delay cycle time
Cause positioning errors

Balance between speed and reliability required.

12) Emergency Stop Considerations

Emergency stop may:

Disable drive instantly
Or ramp down with controlled deceleration

If system uses ramp stop:

Brake resistor must handle full inertia energy.

Improper design causes nuisance trips during safety events.

Safety stops must be tested under full load.

13) Multi-Motor Roll Forming Lines

In systems with multiple drives:

Main drive
Stacker drive
Servo shear

Each drive may require independent braking resistor.

Or shared regenerative system may be installed.

Energy interactions must be calculated carefully.

14) Grounding & EMC Considerations

Brake resistor wiring must:

Use proper conductor size
Be short and direct
Be separated from signal wiring

Resistor wiring carries high current pulses.

Improper routing can induce electrical noise.

15) Testing Brake Resistor Function

Testing procedure:

Run line at production speed
Command rapid deceleration
Monitor DC bus voltage
Observe resistor activation (if visible)
Confirm no overvoltage trip

Infrared thermometer may verify heat distribution.

16) Common Installation Mistakes

Undersized resistor
Poor ventilation
Mounted inside sealed cabinet
Incorrect wiring terminals
No thermal protection
Long cable runs increasing impedance
Ignoring duty cycle
Using generic resistor not matched to drive

Professional design prevents these errors.

17) Export Considerations

When exporting roll forming machines:

Confirm resistor voltage rating
Confirm environmental temperature
Confirm ventilation availability
Provide resistor documentation
Provide spare resistor part number

In hot climates, thermal margin must be increased.

18) Buyer Strategy (30%)

Before purchasing a roll forming machine with VFD or servo drives, verify:

Brake resistor properly sized
Thermal rating documented
Mounted with proper ventilation
Emergency stop tested under load
Deceleration parameters tuned
DC bus voltage monitored during commissioning
Regenerative option considered for large systems
Spare resistor available

Red flag:

“Drive trips during stopping but operator just slows down manually.”

That indicates improper braking design.

6 Frequently Asked Questions

1) Why does my drive trip only when stopping?

Likely DC bus overvoltage due to insufficient braking capacity.

2) Can I fix overvoltage by increasing deceleration time?

Yes, but it may affect production speed.

3) Should brake resistor be installed inside cabinet?

Preferably outside or well ventilated.

4) Is regenerative drive better than resistor?

For large systems, yes. For most roll formers, resistor is sufficient.

5) Why does servo shear fault during return?

Rapid deceleration without adequate braking resistor.

6) What is most common braking mistake?

Undersizing resistor or ignoring duty cycle.

Final Engineering Summary

Brake resistors and regenerative braking systems in roll forming machines must:

Control DC bus voltage during deceleration
Be properly sized based on inertia and duty cycle
Be correctly wired to designated terminals
Be installed with proper ventilation
Be tested under real production conditions

Improper braking design causes:

Overvoltage trips
Production downtime
Mechanical stress
Thermal hazards

In high-speed roll forming and flying shear systems, deceleration control is just as critical as acceleration control.