Dynamic braking resistors are used to absorb the energy of an electric motor that has entered generator mode and to dissipate the torque of its shaft.
In general, there are three main methods for braking electric motors:
- DC current injection and creation of a locked-rotor magnetic condition
- Application of reverse current and creation of reverse rotation of the magnetic field
- Transferring the motor to generator mode and installing a resistor to dissipate the electrical energy on it
The third method is the one that provides better controllability in terms of braking requirements and also applies less stress to the shaft, axis, and mechanical assembly of the electric motor. This method is known in industry as dynamic braking, and the resistor used to absorb the regenerative energy of the electric motor is called a DBR (Dynamic Braking Resistor).
In the past, this method was only used for motors capable of operating in generator mode. However, today, due to the widespread use of drives in motor control, the application of this method has become common for most motors by installing a DBR in the DC link of the drive.
Technical Specifications
In dynamic braking, the braking power is related to the power rating of the braking resistor; the higher the resistor power rating, the stronger and more intense the braking action.
In modern methods where different braking power levels are required, dynamic braking resistors are manufactured with multiple taps. Changing the taps during the braking period enables the implementation of intelligent braking conditions.
One of the key industries with extensive use of this type of resistor is the railway industry and traction motors.
- Resistive Elements and Connections
Various common resistive elements such as wire wound, grid, and ribbon types are commonly used in DBRs. Since dynamic brakes are often installed on equipment that is subject to movement, vibration, or motion, vibration and movement considerations are taken into account in the connections and tightening of this type of resistor.
- Cooling
Due to the duty cycle of dynamic braking resistors and their installation methods—especially in traction systems—both forced air (AF) and natural air (AN) cooling methods are used for this type of resistor.
Ordering Information
The range and capacity of these resistors are determined based on the power of the electric motor, the amount of rotational energy (inertia) on the motor shaft at the moment of braking (which depends on the weight, torque, and load characteristics), and the recovery time (intervals between braking events), according to the type of application.
Therefore, the minimum information required for ordering this equipment includes:
- Drive information with full details, manufacturer, and model
- Motor information with full details, manufacturer, and model
- Application (e.g., fan, mill, pump, etc.)
- Duty cycle expressed in percentage or seconds, specifying braking frequency and the recovery times between successive braking events