A braking resistor is a critical component in many Variable Frequency Drive (VFD) applications, essential for safely managing excess energy and enabling precise, rapid motor deceleration.
Understanding the Role of Braking Resistors
When an electric motor slows down, especially quickly, or when it is driven by an external force (like a descending load on a hoist), it acts like a generator. This process is known as regenerative braking. During regenerative braking, the motor feeds electrical energy back into the VFD's DC bus.
Without a mechanism to handle this regenerated energy, the DC bus voltage would rise excessively, potentially causing an overvoltage fault in the VFD and even damaging the drive or other components. This is precisely where the braking resistor comes into play.
The primary functions of a braking resistor include:
- Dissipating Excess Energy: The resistor is connected to the VFD's DC bus via a chopper circuit. When the DC bus voltage exceeds a predetermined safe limit due to regenerated energy, the chopper circuit switches on, diverting the excess energy through the braking resistor. The resistor then dissip dissipates this energy as heat, effectively preventing the voltage from climbing to dangerous levels.
- Enabling Fast and Controlled Deceleration: By dissipating regenerated energy, braking resistors allow the VFD to slow the motor to an exact speed set by the drive much more quickly and controllably than if the motor were allowed to coast to a stop. This is especially useful for applications requiring rapid deceleration.
- Preventing VFD Trips and Damage: Managing the DC bus voltage prevents overvoltage trips, ensuring continuous and stable operation of the VFD. While its primary role is to prevent overvoltage by dissipating this energy, it is involved in handling the excess energy that would otherwise cause an uncontrolled boost in voltage on the DC bus, ensuring stable drive operation.
How Braking Resistors Work
Here’s a simplified breakdown of the process:
- Motor Regeneration: As the motor decelerates or is overdriven, it generates electrical energy.
- Energy Feedback: This generated energy is fed back into the VFD's DC bus, causing the voltage across the DC capacitors to rise.
- Voltage Monitoring: The VFD continuously monitors the DC bus voltage.
- Chopper Activation: When the DC bus voltage reaches a preset threshold, the VFD's internal (or external) braking chopper circuit activates.
- Energy Dissipation: The activated chopper connects the braking resistor across the DC bus, diverting the excess energy through the resistor. The resistor converts this electrical energy into heat, safely dissipating it into the surrounding environment.
- Voltage Regulation: This dissipation prevents the DC bus voltage from exceeding safe limits, allowing the motor to decelerate smoothly and rapidly without tripping the VFD.
When are Braking Resistors Essential?
Braking resistors are particularly crucial in applications characterized by:
- High Inertia Loads: Equipment with significant mass that continues to spin for a long time after power is removed (e.g., centrifuges, large fans, flywheels).
- Rapid Deceleration Requirements: Applications where the motor needs to stop quickly and repeatedly (e.g., packaging machines, textile machinery, machine tools).
- Vertical or Overhauling Loads: Systems where gravity assists the load, causing the motor to be driven (e.g., hoists, elevators, cranes, conveyors moving downhill).
- Frequent Start/Stop Cycles: Applications with constant acceleration and deceleration phases.
- Precise Positioning: Where exact stopping positions are critical.
Key Benefits of Using a Braking Resistor
Implementing braking resistors offers several advantages for industrial applications:
- Enhanced Control: Provides precise control over motor deceleration rates and stopping times.
- Increased Safety: Prevents uncontrolled stops and potential hazards associated with overspeeding loads or motor coasting.
- VFD Protection: Safeguards the VFD from damaging overvoltage conditions, extending its lifespan and reducing maintenance costs.
- Improved Productivity: Allows for faster cycle times by enabling quicker stops, leading to higher throughput.
- Reduced Wear on Mechanical Brakes: In some cases, dynamic braking can reduce the reliance on or wear of mechanical brakes.
Applications Requiring Braking Resistors
The table below illustrates common applications and why braking resistors are beneficial:
| Application Sector | Specific Examples | Why Braking Resistors are Needed |
|---|---|---|
| Material Handling | Cranes, Hoists, Conveyors, Elevators | Managing regenerative energy from descending or high-inertia loads. |
| Machine Tools | Lathes, Mills, Grinders | Fast, accurate stopping for tooling changes and emergency stops. |
| Textile Industry | Weaving machines, Spinning frames, Winders | Quick and precise stops for thread breaks and production changes. |
| Pulp & Paper | Winders, Rewinders, Slitters | Controlling high-inertia rolls and rapid deceleration. |
| Food & Beverage | Mixers, Centrifuges, Packaging lines | Fast cycle times, controlled stopping of high-inertia equipment. |
| Fitness Equipment | Treadmills, Ellipticals | Managing energy generated by user motion during deceleration. |
By effectively managing regenerative energy, braking resistors ensure the reliable, safe, and efficient operation of VFD-driven systems, particularly in dynamic applications where controlled deceleration is paramount.
