A braking resistor is essential when an electric motor system, particularly one controlled by a Variable Frequency Drive (VFD), experiences regenerative braking, leading to excess energy that needs to be dissipated. This solution becomes crucial to prevent frequent tripping of the drive due to overvoltage, mitigate reduced equipment life or high maintenance costs, and avoid motor and drive damage and/or overheating.
Understanding Regenerative Braking and its Challenges
Electric motors, especially those powered by VFDs, can act as generators when mechanical energy is fed back into them. This phenomenon is known as regenerative braking. It commonly occurs in applications where:
- The load is overhauling the motor: For instance, lowering a heavy load with a crane or an elevator descending.
- Rapid deceleration is required: Stopping a high-inertia load like a large fan, flywheel, or centrifuge quickly.
- Gravity assists the motion: Conveyors moving downhill or pumps emptying at a lower level.
During regenerative braking, the motor generates electrical energy that flows back to the VFD's DC bus. If this excess energy is not consumed or dissipated, the DC bus voltage will rise rapidly, potentially exceeding the VFD's safe operating limits. This overvoltage can lead to several critical problems:
- Frequent drive tripping: The VFD's built-in protection mechanisms will shut it down to prevent damage, causing unexpected halts in operation and production downtime.
- Reduced equipment life: Persistent overvoltage stresses internal components of the VFD and motor, accelerating wear and tear.
- High maintenance costs: Frequent tripping and component stress necessitate more frequent repairs and replacements.
- Motor and drive damage and/or overheating: Prolonged exposure to excessive voltage and current can permanently damage the VFD, motor windings, and other electrical components.
To counteract these issues, braking resistors are employed. In general, resistors consume heat and are effectively used to slow down or stop a mechanical system by converting the excess electrical energy into thermal energy, which is then dissipated into the ambient air.
Key Scenarios for Braking Resistor Application
Braking resistors are specifically designed to manage this excess regenerative energy, ensuring smooth, controlled deceleration and protecting drive components. Here are the primary scenarios where their use is highly recommended:
1. Frequent Deceleration or Stopping Cycles
Applications requiring repeated starts, stops, or changes in speed often generate significant regenerative energy.
- Examples: Conveyors, centrifuges, indexers, winding machines.
2. High Inertia Loads
When a motor drives a load with high rotational inertia, stopping it quickly requires removing a large amount of kinetic energy.
- Examples: Flywheels, large fans, saws, presses, grinding machines.
3. Vertical Movement Applications
Systems involving lifting and lowering heavy loads are prime candidates for regenerative braking.
- Examples:
- Cranes and Hoists: When lowering a load, gravity assists the movement, causing the motor to regenerate.
- Elevators: Descending elevators or those with imbalanced loads generate energy.
- Inclined Conveyors: Moving materials downhill.
4. Emergency Stops and Rapid Braking
Situations demanding quick and controlled halts to ensure safety or process integrity.
- Examples: Any industrial machinery where rapid deceleration is a safety or operational requirement.
5. Energy Management and System Protection
Beyond preventing trips, braking resistors contribute to overall system health and longevity.
- Preventing Overvoltage: Stabilizes the DC bus voltage, keeping it within safe limits for the VFD.
- Protecting Components: Shields the VFD, motor, and other electrical components from damage caused by excess energy.
- Reducing Maintenance: By mitigating stress on equipment, they extend operational life and reduce the frequency and cost of repairs.
How a Braking Resistor Works
A braking resistor connects to the VFD's DC bus via a braking chopper (also known as a braking transistor or dynamic braking unit). When the DC bus voltage rises above a predetermined threshold due to regenerative energy, the braking chopper switches on, diverting the excess energy from the DC bus through the braking resistor. The resistor then converts this electrical energy into heat, which is safely dissipated, preventing the overvoltage condition and allowing the VFD to continue operating smoothly without tripping.
For a deeper dive into how VFDs manage motor control, refer to resources like What is a VFD?.
Benefits of Implementing Braking Resistors
Integrating a braking resistor solution offers substantial advantages for industrial applications:
- Enhanced System Reliability: Prevents frequent drive trips and unexpected shutdowns, ensuring continuous operation.
- Reduced Downtime: Fewer trips mean less time spent troubleshooting and restarting equipment.
- Extended Equipment Lifespan: Protects the VFD and motor from damaging overvoltage conditions, leading to longer service life and reduced capital expenditure.
- Improved Control and Safety: Allows for precise and rapid deceleration, which is crucial for safety-critical applications and process control.
- Lower Maintenance Costs: Reduces wear and tear on components, leading to fewer repairs and replacements.
Practical Considerations for Selection
Choosing the correct braking resistor involves assessing several factors:
- Resistance Value (Ohms): Must match the VFD's specifications. Too low can cause the braking chopper to fail; too high reduces braking effectiveness.
- Power Rating (Watts): The amount of energy the resistor can dissipate continuously or during peak braking events. This depends on the application's duty cycle and the amount of regenerative energy.
- Duty Cycle: The percentage of time the resistor is active. Some applications require continuous dissipation, while others only need short bursts.
- Cooling: Resistors dissipate heat, so proper ventilation or forced-air cooling might be necessary depending on the power rating and enclosure.
The following table summarizes common scenarios and the role of braking resistors:
| Scenario | Problem Solved | Benefit of Braking Resistor |
|---|---|---|
| Frequent Deceleration | Overvoltage trips, drive damage | Stable operation, prevents tripping, protects drive |
| High Inertia Loads | Slow, uncontrolled stops; overvoltage during deceleration | Fast, controlled stopping; protects motor and drive |
| Vertical Movement | Regenerative energy feedback, overheating, trips | Safely dissipates energy, smooth lowering, extends life |
| Emergency Stops | Uncontrolled, potentially damaging rapid stops | Quick, controlled braking; safety enhancement |
| Reduced Equipment Life | Stress from recurrent overvoltage | Prolongs VFD and motor lifespan, reduces replacement needs |
| High Maintenance Costs | Frequent repairs due to stress/damage | Lowers maintenance expenses, increases uptime |
In conclusion, braking resistors are a critical component in many VFD-controlled systems, ensuring operational stability, protecting valuable equipment, and enhancing safety by effectively managing regenerative energy.
