What is DBR?
For several drive applications, the motor can go into the regenerative mode (braking mode) for certain operating states.
Examples for these types of applications include:
- Traversing drives,
- Conveyor belts, where the load on the belt moves the belt downwards,
- Dynamic traversing drives, generally, drives with high moments of inertia, if the speed set point must be quickly reduced or the load braked.
When the motor is in the regenerative mode, the braking energy of the motor is fed back into the drive converter DC link via the inverter. This means that the DC link voltage increases. If a braking chopper and braking resistor are used, from a specific value of the DC link voltage, the braking energy is pulsed into the braking resistor. This therefore limits the DC link voltage.
Determining Dynamic Braking Requirements
When a drive is consistently operating in the regenerative mode of operation, serious consideration should be given to equipment that will transform the electrical energy back to the fixed frequency utility grid. As a general rule, dynamic braking can be used when the need to dissipate regenerative energy is on an occasional or periodic basis. In general, the motor power rating, speed, torque, and details regarding the regenerative mode of operation will be needed in order to estimate what dynamic braking resistor value is needed. The peak regenerative power of the drive must be calculated in order to determine the maximum resistance value of the dynamic braking resistor.
The power rating of the dynamic braking resistor is estimated by applying what is known about the drive’s motoring and regenerating modes of operation. The average power dissipation must be estimated and the power rating of the dynamic braking resistor chosen to be greater than that average. If the dynamic braking resistor has a large thermodynamic heat capacity, then the resistor element will be able to absorb a large amount of energy without the temperature of the resistor element exceeding the operational temperature rating.
To compute the DBR value gathering the following information are necessary:
- Power rating from motor nameplate in watts, kilowatts, or horsepower
- Speed rating from motor nameplate in rpm or rps (radians per second)
- Motor inertia and load inertia in kg•m2
- Motor shaft speed, torque, and power profile of the drive application
For any DBR’s a duty cycle is usually defined. This time determines what time a DBR must be work under load. This time is important for thermal computations. In the metro applications, start to stop times are divided to 4 intervals:
- Traction time
- Motion with uniform velocity
- Braking time
- Stopping time
Resistances used in the metro, act as both starting and braking resistors.