A practical engineering guide to VFD stopping and braking methods, including Ramp Stop, Coast Stop, DC Injection Braking, Braking Resistors, and Regenerative Braking. Learn why DC-bus overvoltage occurs during deceleration and how to select the appropriate stopping method for each application.
Introduction
A Variable Frequency Drive does more than control motor speed during normal operation. It also determines how the motor and connected load decelerate and stop.
Selecting an unsuitable stopping method may result in:
- DC-bus overvoltage faults
- Excessive motor heating
- Uncontrolled or extended stopping time
- Mechanical stress on the driven equipment
- Braking resistor overheating
- Repeated VFD trips
- Unsafe machine operation
- Inconsistent process performance
The correct braking method depends on:
- Load type
- Load inertia
- Required stopping time
- Operating speed
- Number of stops per hour
- Amount of regenerative energy
- VFD braking capability
- Mechanical brake requirements
- Safety and process requirements
Common VFD stopping and braking methods include:
- Ramp Stop
- Coast Stop
- DC Injection Braking
- Dynamic Braking using a Braking Resistor
- Regenerative Braking
There is no single method suitable for every application.
What Happens When a Motor Decelerates?
During normal motoring operation, electrical energy flows in the following direction:
Power Supply → VFD → Motor → Mechanical Load
When a high-inertia load is decelerated, the load may continue rotating faster than the speed commanded by the VFD.
The motor can then temporarily operate as a generator.
The energy direction becomes:
Mechanical Load → Motor → VFD DC Bus
The generated electrical energy charges the VFD DC-bus capacitors.
If this energy cannot be dissipated or returned to the power supply, the DC-bus voltage rises. When the voltage exceeds the VFD protection threshold, the drive trips on an:
Overvoltage Fault
Why Does Overvoltage Occur During Deceleration?
The VFD attempts to reduce motor speed according to the programmed Deceleration Time.
If the required stopping time is too short for the inertia of the load, the load continues driving the motor.
The sequence is typically:
- The VFD reduces the commanded output frequency.
- The rotating load continues moving because of its stored mechanical energy.
- The motor begins generating electrical energy.
- The generated energy returns to the VFD DC Bus.
- The DC-bus voltage increases.
- The VFD trips if the voltage reaches the protection limit.
The risk increases with:
- High-inertia loads
- Short Deceleration Times
- High operating speeds
- Frequent stopping
- Vertical loads
- Overhauling loads
- Loads that drive the motor
- Missing or incorrectly selected braking equipment
Ramp Stop
What Is Ramp Stop?
During a Ramp Stop, the VFD gradually reduces the output frequency from the operating speed to zero within a programmed Deceleration Time.
For example:
- Operating frequency: 50 Hz
- Deceleration Time: 10 seconds
- The VFD reduces the output frequency progressively from 50 Hz to 0 Hz over 10 seconds
Advantages of Ramp Stop
Ramp Stop provides:
- Controlled deceleration
- Adjustable stopping time
- Reduced mechanical shock
- Improved process repeatability
- Better coordination with PLC systems
- More predictable stopping than Coast Stop
Limitations of Ramp Stop
Ramp Stop can produce a DC-bus overvoltage fault when:
- The Deceleration Time is too short
- The load has high inertia
- The load returns significant energy
- The VFD cannot absorb the regenerative energy
- A Braking Resistor is missing or incorrectly selected
- The braking system Duty Cycle is exceeded
Typical Applications
Ramp Stop is commonly used for:
- Conveyors
- Pumps
- Mixers
- Production machinery
- Process equipment
- Applications requiring a repeatable stopping time
Increasing Deceleration Time
Increasing the Deceleration Time often reduces regenerative power because the load releases its stored energy over a longer period.
This may prevent overvoltage trips without additional braking hardware.
However, increasing the stopping time is not suitable when the process requires rapid stopping. In that case, Dynamic Braking or Regenerative Braking may be required.
Coast Stop
What Is Coast Stop?
During a Coast Stop, the VFD disables its output to the motor.
The motor and load then continue rotating because of their inertia until friction and mechanical resistance bring them to a natural stop.
It is similar to disconnecting electrical power from a freely rotating motor.
Advantages of Coast Stop
Coast Stop:
- Does not force electrical deceleration
- Reduces regenerative energy returning to the VFD
- Reduces the risk of DC-bus overvoltage
- Requires no external braking equipment in many applications
- Is simple to configure
Limitations of Coast Stop
- Stopping time is not accurately controlled
- High-inertia loads may take a long time to stop
- Stopping time changes with load and friction
- The VFD does not control the motor during coasting
- It may not meet process or safety requirements
Typical Applications
Coast Stop may be suitable for:
- Large fans
- Blowers
- Equipment where gradual natural stopping is acceptable
- High-inertia loads where rapid stopping is unnecessary
- Applications where process control during stopping is not required
When Coast Stop Is Not Suitable
Coast Stop is generally unsuitable for:
- Position-controlled conveyors
- Cutting machines
- Hoists and cranes
- Equipment requiring rapid stopping
- Machines with mechanical hazards
- Applications requiring a fixed stopping time
Coast Stop should not be treated as a safety-rated stopping function unless the complete machine safety system is designed and validated for that purpose.
DC Injection Braking
What Is DC Injection Braking?
DC Injection Braking applies direct current to the motor windings, normally after the motor has decelerated to a low speed or when the output frequency reaches a programmed value.
The DC current creates a stationary magnetic field inside the motor.
As the rotor continues rotating through this stationary field, a braking torque is produced that opposes the motion.
Typical Operating Sequence
- A stop command is issued.
- The VFD reduces the output frequency.
- At a selected low speed, the VFD applies DC current.
- Braking torque is produced.
- The motor slows further or is held briefly at zero speed.
The braking performance depends on:
- DC Injection current
- Injection duration
- Starting frequency for DC braking
- Motor resistance
- Motor size
- Load inertia
- Motor temperature
Advantages of DC Injection Braking
- Normally does not require a Braking Resistor
- Can reduce stopping time at low speed
- Helps stop residual motor rotation
- Available in many general-purpose VFDs
- Can provide brief holding torque after stopping
Limitations of DC Injection Braking
The braking energy is converted into heat inside the motor.
Excessive DC Injection may cause:
- Motor winding overheating
- Reduced insulation life
- Motor thermal overload
- Unsuitability for frequent stopping
- Limited braking performance with high-inertia loads
Typical Applications
DC Injection Braking may be suitable for:
- Small fans
- Machine tools
- Low- and medium-inertia loads
- Preventing low-speed coasting
- Applications requiring brief electrical holding after stopping
Safety Limitation
DC Injection Braking is not a mechanical safety brake.
It must not be used as the only load-holding method for:
- Hoists
- Cranes
- Elevators
- Suspended loads
- Personnel-lifting systems
- Applications where loss of power could release the load
These applications require a properly designed mechanical brake and a complete safety assessment.
Dynamic Braking
What Is Dynamic Braking?
Dynamic Braking dissipates the regenerative energy returned by the motor as heat in an external:
Braking Resistor
The resistor is connected to the VFD DC Bus through:
- An internal Braking Chopper
- Or an external Braking Unit
How Does a Braking Chopper Work?
The Braking Chopper monitors the VFD DC-bus voltage.
When the voltage rises above its operating threshold:
- The Braking Chopper switches on.
- Current flows through the Braking Resistor.
- Electrical energy is converted into heat.
- The DC-bus voltage is reduced.
- The VFD continues decelerating the motor.
The chopper switches on and off as required to control the DC-bus voltage.
Dynamic Braking System Components
A complete Dynamic Braking system may include:
- VFD
- Internal or external Braking Chopper
- Braking Resistor
- Thermal protection
- Correctly rated cables
- Ventilation
- Protective enclosure
- Thermal switch or thermostat
Advantages of Dynamic Braking
Dynamic Braking:
- Allows shorter stopping times
- Reduces DC-bus overvoltage trips
- Supports high-inertia loads
- Provides controlled deceleration
- Is less complex than many regenerative systems
- Can support repeated stops when correctly sized
Limitations of Dynamic Braking
- Regenerative energy is wasted as heat
- The resistor can reach very high temperatures
- Adequate ventilation and clearance are required
- Resistor resistance and power must be calculated correctly
- The resistor can fail if the Duty Cycle is exceeded
- It does not return energy to the power supply
Braking Resistor Selection
A Braking Resistor must not be selected based only on motor power.
The following factors should be reviewed:
- VFD model
- Motor power
- VFD rated current
- DC-bus voltage
- Minimum permitted resistance
- Braking Chopper current capacity
- Load inertia
- Operating speed
- Required stopping time
- Number of stops per hour
- Duration of each braking event
- Braking Duty Cycle
- Peak braking power
- Average thermal power
- Cooling time between braking events
Minimum Resistance Value
The selected resistance must not be lower than the minimum value permitted by the VFD or Braking Unit manufacturer.
Using a resistance below the permitted limit can cause:
- Excessive Braking Chopper current
- Braking IGBT failure
- Overcurrent faults
- Cable overheating
- Braking Unit damage
- VFD damage
- Fire risk
The manufacturer’s minimum resistance value must always be respected.
Braking Resistor Power Rating
Selecting the resistance value in ohms is not sufficient.
The resistor must also withstand:
- Instantaneous braking power
- Energy produced during each stop
- Average power over the complete operating cycle
- Number of braking events
- Available cooling time
- Ambient temperature
- Enclosure conditions
A resistor may withstand a high power level for a few seconds but may not be capable of dissipating the same power continuously.
Peak power and average thermal power must be checked separately.
Critical Braking Resistor Connection Warning
The Braking Resistor must only be connected to the braking terminals specified in the VFD or Braking Unit manual.
Terminal names may include:
- P and PB
- P+ and PB
- DC+ and BR
- B1 and B2
The exact terminal designation varies by manufacturer.
Never Connect the Braking Resistor to Motor Output Terminals
Do not connect the Braking Resistor to U, V, and W.
It must also not be connected directly between the VFD input phases or output phases.
Incorrect connection may cause:
- Immediate VFD failure
- Short circuit
- Braking Resistor damage
- Fire
- Serious electrical injury
Always follow the manufacturer’s approved wiring diagram.
Braking Resistor Installation
The Braking Resistor should be installed in a location that is:
- Well ventilated
- Away from combustible materials
- Away from heat-sensitive equipment
- Protected against accidental contact
- Suitable for the required protection rating
- Able to dissipate the generated heat
A high-power Braking Resistor should not be installed inside a small, sealed control panel without a thermal assessment.
The installation may require:
- Metal enclosure
- Protective barrier
- Thermal switch
- Thermostat
- Forced ventilation
- High-temperature cables
- Adequate spacing
The resistor surface may remain hot after the motor has stopped.
Regenerative Braking
What Is Regenerative Braking?
Regenerative Braking returns the energy generated during motor deceleration to the incoming electrical supply instead of converting it into heat.
The energy direction becomes:
Mechanical Load → Motor → Regenerative Drive or Unit → Power Supply
Typical Applications
Regenerative Braking may be suitable for:
- Frequent braking cycles
- Continuous regenerative operation
- Overhauling loads
- Large centrifuges
- Test benches
- Unwinders
- High-energy production machinery
- Properly designed lifting systems
- Applications where energy recovery is economically valuable
Advantages
- Returns energy to the power supply
- Reduces wasted heat
- Suitable for frequent or continuous braking
- Reduces the need for large Braking Resistors
- Can improve system energy efficiency
Limitations
- Higher initial cost
- More complex design
- Requires compatibility with the power network
- May require reactors or harmonic filters
- Requires suitable protection and control
- Harmonic performance must be reviewed
- Installation and commissioning are more demanding
Dynamic Braking vs Regenerative Braking
Dynamic Braking
The regenerative energy is converted into heat in a Braking Resistor.
It is generally suitable when:
- Braking is intermittent
- The amount of energy is moderate
- A simpler system is preferred
- Energy recovery is not economically important
Regenerative Braking
The energy is returned to the electrical supply.
It is generally suitable when:
- Braking is frequent or continuous
- Regenerative energy is significant
- Heat dissipation is difficult
- Energy recovery can justify the higher system cost
Comparison of VFD Stopping Methods
Ramp Stop
Stopping principle:
The VFD gradually reduces output frequency.
Stopping-time control:
Good.
Regenerative energy:
Possible.
Additional equipment:
A Braking Resistor may be required.
Typical use:
General applications requiring controlled deceleration.
Coast Stop
Stopping principle:
The VFD output is disabled and the load stops naturally.
Stopping-time control:
Limited.
Regenerative energy returned to the VFD:
Normally low.
Additional equipment:
Usually not required.
Typical use:
Applications where slow and uncontrolled stopping is acceptable.
DC Injection Braking
Stopping principle:
DC current is applied to the motor windings.
Stopping-time control:
Moderate.
Energy dissipation:
Inside the motor.
Additional equipment:
Normally not required.
Typical use:
Low-speed braking and low- to medium-inertia loads.
Dynamic Braking
Stopping principle:
Regenerative energy is dissipated in a Braking Resistor.
Stopping-time control:
Very good.
Energy dissipation:
External Braking Resistor.
Additional equipment:
Braking Chopper and Braking Resistor.
Typical use:
Rapid stopping and high-inertia loads.
Regenerative Braking
Stopping principle:
Regenerative energy is returned to the power supply.
Stopping-time control:
Excellent when correctly designed.
Energy destination:
Incoming electrical supply.
Additional equipment:
Regenerative Drive, Active Front End, or Regenerative Unit.
Typical use:
Frequent braking, continuous regeneration, and high-energy applications.
Application Examples
Fans and Blowers
Fans and blowers often have high inertia and long natural stopping times.
Possible stopping methods include:
- Coast Stop when a slow stop is acceptable
- Ramp Stop with a long Deceleration Time
- Dynamic Braking when a faster stop is required
Rapid stopping may require significant braking energy and should be reviewed carefully.
Pumps
Ramp Stop is often used with pumps to reduce sudden hydraulic changes.
An excessively short stopping time may cause:
- Water hammer
- Pressure surges
- Pipe stress
- Valve stress
- Unstable process pressure
The Deceleration Time should be selected according to the hydraulic system, not only the motor.
Conveyors
Conveyors commonly require:
- Controlled Ramp Stop
- Repeatable stopping time
- Product stability
- A Braking Resistor for rapid stopping or high load
The design should consider belt slip, product movement, and the possibility that the load may drive the motor.
Centrifuges
Centrifuges normally have very high inertia.
They may require:
- Large Braking Resistor
- External Braking Unit
- Regenerative system
- Accurate braking-energy calculations
Increasing the Deceleration Time may be the simplest solution when a long stopping time is acceptable.
Hoists and Lifting Applications
Lifting systems require a specialised design that may include:
- Vector Control
- Encoder feedback
- Mechanical brake
- Braking Resistor or Regenerative Unit
- Brake-control sequence
- Load-holding assessment
- Safety-rated functions
- Emergency stopping system
The VFD must not be used as the only device holding a suspended load.
Common Mistakes
Reducing Deceleration Time Without Reviewing the Load
This may cause repeated overvoltage faults or braking-system damage.
Repeatedly Resetting an Overvoltage Fault
Resetting the fault does not remove the regenerative-energy problem.
The following should be reviewed:
- Deceleration Time
- Load inertia
- Braking Resistor
- Braking Chopper
- Incoming voltage
- Regenerative operating conditions
Selecting the Resistor by Wattage Only
Resistance, peak power, average power, and Duty Cycle must all be checked.
Using a Resistance Below the Permitted Limit
This may damage the Braking Chopper or VFD.
Installing the Resistor in an Unventilated Panel
The resistor may overheat the VFD and nearby control components.
Excessive DC Injection Braking
Long or repeated DC Injection can overheat the motor.
Treating Coast Stop as a Safe Stop
The motor and load may continue rotating for a long time after the VFD output has been disabled.
Connecting the Braking Resistor to U, V, and W
This is an incorrect and dangerous connection that may destroy the VFD.
Engineering Checklist
Before selecting the stopping or braking method, confirm:
- Load type
- Motor power
- Operating speed
- Load inertia
- Required stopping time
- Number of stops per hour
- Whether the load can drive the motor
- Whether overvoltage occurs during deceleration
- VFD DC-bus voltage
- VFD braking capability
- Availability of an internal Braking Chopper
- Minimum permitted resistance
- Braking Duty Cycle
- Peak and average Braking Resistor power
- Resistor installation location
- Ventilation requirements
- Ambient temperature
- Mechanical brake requirements
- Potential value of energy recovery
- Whether stopping is a process function or a safety function
Safety Considerations
VFD stopping functions do not automatically provide electrical isolation or a safety-rated stop.
Before installation or maintenance:
- Isolate all incoming power sources
- Apply lockout and tagout procedures
- Wait for the specified DC-bus discharge time
- Verify that the DC-bus voltage is at a safe level
- Confirm that the load cannot move unexpectedly
- Use qualified personnel
- Follow the VFD and braking-equipment manufacturer instructions
A Braking Resistor may remain dangerously hot after operation.
Technical Conclusion
The motor stopping method is as important as the starting and speed-control method.
Ramp Stop provides controlled deceleration, but it may raise the VFD DC-bus voltage when a high-inertia load is stopped too quickly.
Coast Stop allows the load to stop naturally, but it does not provide accurate stopping-time control.
DC Injection Braking can improve low-speed stopping, but the braking energy is converted into heat inside the motor.
Dynamic Braking uses a Braking Resistor to dissipate regenerative energy and allows faster, more controlled stopping.
Regenerative Braking returns the generated energy to the power supply and is suitable for frequent or continuous regenerative operation.
The correct solution depends on:
- Load inertia
- Stored mechanical energy
- Required stopping time
- Braking frequency
- VFD capability
- Mechanical brake requirements
- Thermal conditions
- Safety requirements
The VFD and braking-equipment manuals must always be reviewed before selecting or connecting a Braking Resistor or Regenerative Unit.