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

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:

1. Motor power

2. Inertia of system

3. Deceleration time

4. 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:

1. Run line at production speed

2. Command rapid deceleration

3. Monitor DC bus voltage

4. Observe resistor activation (if visible)

5. Confirm no overvoltage trip

Infrared thermometer may verify heat distribution.

16) Common Installation Mistakes

1. Undersized resistor

2. Poor ventilation

3. Mounted inside sealed cabinet

4. Incorrect wiring terminals

5. No thermal protection

6. Long cable runs increasing impedance

7. Ignoring duty cycle

8. 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:

1. Brake resistor properly sized

2. Thermal rating documented

3. Mounted with proper ventilation

4. Emergency stop tested under load

5. Deceleration parameters tuned

6. DC bus voltage monitored during commissioning

7. Regenerative option considered for large systems

8. 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.