A dynamic braking chopper in a Variable Frequency Drive (VFD) is an essential electronic switch that efficiently manages excess energy generated when an electric motor rapidly decelerates or brakes. Its primary role is to connect a dynamic braking resistor to the VFD's DC bus, dissipating this regenerative energy as heat to prevent VFD over-voltage faults and enable controlled stopping.
What is a Dynamic Braking Chopper?
A dynamic braking chopper, often integrated within or externally connected to a VFD, acts as a high-speed electronic switch, typically using an Insulated Gate Bipolar Transistor (IGBT). Its function is to dissipate excess energy generated during deceleration or braking of an electric motor by routing it through a specialized resistor.
When a motor is driven by a VFD and commanded to slow down quickly, or if the load overhauls the motor (e.g., a descending hoist), the motor acts like a generator. This process, known as regenerative braking, feeds electrical energy back into the VFD's DC bus. If this energy is not managed, the DC bus voltage will rise rapidly, potentially exceeding the VFD's safe operating limits and triggering an over-voltage fault, which causes the drive to trip and the motor to coast to a stop.
This is where the dynamic braking chopper comes in. It constantly monitors the DC bus voltage. When the voltage rises above a preset threshold due to regenerative energy, the chopper automatically switches on, connecting an external or internal dynamic braking resistor (DBR) across the DC bus.
How a Dynamic Braking Chopper Works
The operation of a dynamic braking chopper is a precise, high-speed process:
- DC Bus Voltage Monitoring: The VFD's control circuit continuously monitors the voltage on its internal DC bus.
- Threshold Detection: When the motor enters a regenerative state (decelerating or being overhauling), it feeds energy back, causing the DC bus voltage to rise. The chopper's control unit is set to activate when this voltage surpasses a specific upper threshold.
- Chopper Activation: Upon detecting the over-voltage condition, the control circuit activates the IGBT (or other switching device) within the dynamic braking chopper.
- Energy Dissipation: With the chopper switched on, the dynamic braking resistor is connected directly across the DC bus. The excess electrical energy is then shunted through the resistor, converting it into heat, which is safely dissipated into the surrounding environment.
- Voltage Regulation: As the energy is dissipated, the DC bus voltage drops back down. Once it falls below a lower threshold, the chopper switches off, disconnecting the resistor. This cycle repeats rapidly as needed to maintain the DC bus voltage within safe operating limits.
- Modulation: Many choppers utilize Pulse Width Modulation (PWM) to control the amount of energy dissipated, ensuring precise voltage regulation and efficient braking.
Components of a Dynamic Braking System
A complete dynamic braking system involves two primary components working in tandem:
- Dynamic Braking Chopper: This is the intelligent electronic switch (typically an IGBT-based power transistor) that rapidly connects and disconnects the braking resistor to the VFD's DC bus. Its quick switching action is crucial for controlled energy management.
- Dynamic Braking Resistor (DBR): Also known as a dynamic brake resistor, this component is used in AC VFDs to dissipate excess energy generated during deceleration or braking of an electric motor. It is designed to handle significant amounts of power and convert the electrical energy into heat. DBRs come in various resistance and power ratings, which must be carefully matched to the motor, VFD, and application requirements.
| Component | Function |
|---|---|
| Dynamic Braking Chopper | An electronic switch (e.g., IGBT) that connects and disconnects the dynamic braking resistor to the VFD's DC bus, based on monitored voltage levels. It actively manages the flow of regenerative energy. |
| Dynamic Braking Resistor | A resistive element that converts excess electrical energy from the DC bus into heat, safely dissipating it to prevent over-voltage faults. This component is essential for dissipating excess energy generated during deceleration or braking of an electric motor. |
| VFD DC Bus | The internal DC link within the VFD where rectified AC power is stored, and regenerative energy accumulates during braking. |
| Control Circuitry | The VFD's internal logic that monitors the DC bus voltage and commands the chopper to switch on or off. |
Types and Configurations
Dynamic braking choppers can be configured in a few ways:
- Internal Choppers: Many smaller and medium-sized VFDs have a dynamic braking chopper built directly into the drive unit. These often come with specified power ratings for compatible braking resistors.
- External Choppers: For larger VFDs or applications requiring extensive braking capacity, an external dynamic braking chopper is used. This is a separate module connected to the VFD's DC bus terminals. This configuration allows for greater flexibility in resistor sizing and heat dissipation.
Benefits and Applications
Integrating a dynamic braking chopper offers several advantages for VFD-controlled systems:
- Controlled Deceleration: Enables fast, smooth, and controlled stopping of high-inertia loads, preventing motor coasting.
- Prevents VFD Faults: Protects the VFD from over-voltage trips caused by regenerative energy, ensuring continuous operation.
- Enhanced Safety: Provides reliable braking, critical for applications where quick stops are necessary for safety.
- Increased Productivity: Reduces cycle times by allowing rapid machine deceleration and stopping.
Common Applications:
- Conveyors: Especially those moving product downhill or stopping quickly.
- Centrifuges: Requiring rapid deceleration to reduce cycle times.
- Hoists and Cranes: Managing the potential energy of descending loads.
- Winders and Unwinders: Precisely controlling tension and speed.
- Machine Tools: For quick and accurate positioning.
Practical Considerations
When implementing a dynamic braking system, several factors must be considered:
- Resistor Sizing: The braking resistor must be correctly sized for both resistance (ohms) and power dissipation (watts/joules) to match the application's braking energy requirements. Incorrect sizing can lead to inadequate braking or resistor damage.
- Heat Management: Braking resistors generate significant heat. They must be mounted in well-ventilated areas, often in robust enclosures, to prevent overheating and ensure safety.
- Cable Length and Shielding: The cables connecting the chopper to the resistor should be kept as short as possible and typically shielded to minimize electromagnetic interference (EMI).
- Duty Cycle: Resistors are rated for specific duty cycles (e.g., 10% ED for intermittent use, 100% ED for continuous braking). This must align with the application's braking frequency and duration.
By effectively managing regenerative energy, the dynamic braking chopper extends the capabilities of VFDs, allowing for robust and controlled operation in demanding industrial environments.
