Executive Overview:
This technical white paper explains how dynamic braking works, when a VFD application may require a braking resistor, and the main installation, protection, and safety checks that must be completed before implementation.
Manufacturer Limits Govern Selection
Always refer to the VFD manufacturer’s manual to confirm:
- The approved braking resistor range
- The braking transistor capability
- The permitted braking duty
- The required wiring method
- Compatible braking accessories
Key principle: Never select a resistance value below the minimum resistance specified for the VFD or braking unit. Pulse-energy, thermal, and environmental ratings must also be confirmed before commissioning.
How Dynamic Braking Works
During rapid deceleration, or when the driven load forces the motor to rotate, the motor operates as a generator.
The regenerated electrical energy flows back into the VFD DC bus, causing the DC-bus voltage to rise.
Energy Flow During Braking
Mechanical energy from the motor and load is converted into regenerated electrical energy by the inverter stage.
The energy then flows to the DC bus, increasing its voltage. When the voltage reaches a specific threshold, the brake chopper switches the braking resistor into the circuit.
The resistor converts the regenerated electrical energy into heat.
Without a Braking Path
Without a suitable braking path, the DC-bus voltage may continue to rise until the VFD trips on an overvoltage fault.
When the application allows a longer stopping time, increasing the deceleration time can reduce the required braking power and may prevent the overvoltage trip.
With Dynamic Braking
The braking transistor connects the braking resistor to the DC bus for controlled intervals.
The resistor absorbs the regenerated energy as heat, allowing the motor to stop in a shorter and more controlled time.
Important: A braking resistor cannot hold a stopped load. It does not replace a mechanical holding brake, emergency brake, or any safety-rated stopping device.
When Is a Braking Resistor Required?
A braking resistor may be required when the application generates significant energy during deceleration or when a short stopping time is necessary.
Common Applications
- High-inertia loads requiring short deceleration times
- Centrifuges
- Large fans
- Flywheels
- Heavy rotating machinery
- Conveyors with frequent start-stop cycles
- Unwinders
- Test benches
- Machines where the load can overrun the motor
- Hoists and vertical axes during controlled lowering, when permitted by the complete safety design
- Applications that repeatedly trip on DC-bus overvoltage during deceleration
A Braking Resistor May Not Be Required When
- The load can safely coast to a stop
- A longer deceleration time is acceptable
- Mechanical losses naturally absorb most of the regenerated energy
- Stopping events are rare and remain within the VFD’s built-in braking capability
- A regenerative front end or regenerative drive returns energy to the electrical supply
- A common DC-bus system shares braking energy with other drives operating in motoring mode
First Engineering Question
Before adding braking hardware, determine whether the required stopping performance can be safely achieved by increasing the VFD deceleration time.
When a longer stopping time is acceptable, adjusting the VFD parameters may be the simplest solution.
Braking Duty Considerations
Occasional Stopping
For occasional braking events, the resistor’s pulse-energy rating may be more important than its continuous wattage rating.
Frequent Braking Cycles
For applications with frequent start-stop cycles, average thermal power, ventilation, and cooling become critical.
Continuous Regeneration
For applications with continuous or high-energy regeneration, a braking resistor may be inefficient or unsuitable.
A regenerative drive, regenerative front end, or common DC-bus solution should be evaluated.
Installation, Protection, and Safety
Braking resistors can reach very high temperatures and are connected to potentially lethal DC-bus voltages. Correct installation and protection are therefore essential.
Heat and Fire Risk
- Mount the braking resistor on a non-combustible surface
- Maintain the clearances specified by the manufacturer
- Keep the resistor away from cables, plastic components, and temperature-sensitive equipment
- Provide sufficient ventilation
- Do not install the resistor inside an enclosure without completing a thermal calculation
- Use a thermal switch when specified by the manufacturer
- Clearly label the resistor as a hot surface
Hazardous Voltage
The braking resistor circuit is connected to the VFD DC bus and can carry lethal voltage.
Before working on the circuit:
- Isolate the electrical supply
- Follow the required lockout and tagout procedures
- Wait for the discharge time specified by the VFD manufacturer
- Verify that the voltage has reached a safe level before touching any terminals
- Use cables, insulation, glands, terminals, and enclosures with the correct voltage and temperature ratings
Wiring Practice
- Keep the braking resistor cables as short as permitted by the manufacturer
- Route braking resistor cables away from low-level control and communication cables
- Use the recommended cable type and conductor size
- Follow the manufacturer’s terminal and torque requirements
- Do not connect the braking resistor directly to ordinary VFD output terminals
The braking resistor must only be connected to the terminals specifically designated by the manufacturer for braking-resistor or braking-unit connections.
Protection and Interlocks
- Use the specified external braking unit when the VFD does not include an internal braking chopper
- Connect the resistor’s thermal protection to stop the drive safely if the resistor overheats
- Confirm the operation of braking-unit fault contacts
- Define the required system response for every braking fault
- Never bypass protective trips or thermal protection
Safety Limitation
Dynamic braking is an operating function and is not a safety-rated stopping method.
The machine risk assessment and applicable safety requirements determine whether mechanical holding brakes, emergency brakes, or other safety-rated stopping systems are required.
Final Engineering Checklist
Before purchasing, installing, or commissioning a braking resistor, confirm the following:
- A shorter stopping time is genuinely required
- The exact VFD model and voltage class are known
- The VFD includes a suitable braking chopper, or the correct external braking unit has been selected
- The selected resistor is within the manufacturer-approved resistance range
- The resistance value is not below the specified minimum resistance
- The permitted braking current has been checked
- The required braking duty cycle has been calculated
- The resistor pulse-energy rating is sufficient
- The resistor average thermal power rating is sufficient
- Ambient-temperature derating has been applied
- Altitude derating has been applied when required
- Enclosure and ventilation conditions have been considered
- Cable insulation and conductor ratings are suitable
- Terminal ratings and tightening torque are correct
- Safe clearances are provided
- Hot-surface warning labels are installed
- Thermal protection and system interlocks are connected when required
- Commissioning will be performed under controlled conditions
- DC-bus voltage, stopping performance, resistor temperature, and fault operation will be monitored during testing
- The selected resistor, VFD parameters, calculations, and test results will be documented
Selection Guidance
When a Longer Deceleration Time Is Acceptable
Increase the VFD deceleration time and test the machine under normal and worst-case operating conditions.
When a Short Stop Is Required with Intermittent Regeneration
Use a manufacturer-approved braking resistor and braking-chopper solution.
When Regeneration Is Continuous or High-Energy
Evaluate a regenerative drive, regenerative front end, or common DC-bus system instead of relying only on a braking resistor.
Technical Conclusion
A reliable braking resistor application starts with confirming the actual stopping requirement and reviewing the limits specified by the VFD manufacturer.
The braking duty, minimum resistance, pulse-energy capacity, thermal rating, ventilation, wiring, protection, and environmental conditions must all be verified before implementation.
Selecting a braking resistor based only on the motor power in kilowatts can lead to:
- DC-bus overvoltage trips
- Braking resistor overheating
- Insufficient braking performance
- Damage to the braking transistor
- Fire and electrical safety risks
Final verification: Always use the documentation for the exact VFD model, braking unit, and braking resistor before procurement, installation, or commissioning.