Rockwell Automation AK DBU - Dynamic Braking Unit 600/690 VAC - Manual (Page 39)

Appendix
C
Design Information
This appendix provides the design information which is necessary for
calculating and selecting an external brake resistor for connecting to the
Dynamic Braking Unit.
How to Determine Dynamic Brake 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 is used when the need to dissipate
regenerative energy occurs 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 Brake Resistor value is needed.
The Peak Regenerative Power and Average Regenerative Power
required for the application must be calculated in order to determine the
brake resistor value, and to verify the suitability of the DBU.
The power rating of the Dynamic Brake 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 Brake Resistor chosen to be greater
than that average. If the Dynamic Brake Resistor has a large thermo-
dynamic 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. Thermal time constants in
the order of 50 seconds and higher satisfy the criteria of large heat
capacities for these applications. If a resistor has a small heat capacity
(defined as thermal time constants less than 5 seconds) the temperature
of the resistor element could exceed its maximum.
For information on… See page
Determining Dynamic Brake Requirements C-1
Determine Values of Equation Variables C-4
Selecting the Resistor C-7
Example Calculation C-10
Determining Dynamic Brake Requirements