Rockwell Automation 1336_S_F_T Allen-Bradley Dynamic Braking - Manual (Page 5)

Heavy Duty Dynamic Braking 5
1336-5.64 — July, 2005
How to Select a Dynamic Brake
Module As a rule, a Dynamic Brake Module can be specified when regenerative
energy is dissipated 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 Module
rating to use. 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.
The peak regenerative power of the drive must be calculated in order to
determine the maximum ohmic value of the Dynamic Brake Resistor of the
Dynamic Brake Module. Once the maximum ohmic value of the Dynamic
Brake Resistor current rating is known, the required rating and number of
Dynamic Brake Modules can be determined. If a Dynamic Brake Resistance
value greater than the minimum imposed by the choice of the peak
regenerative power is made and applied, the drive can trip off due to transient
DC Bus overvoltage problems. Once the approximate ohmic value of the
Dynamic Brake Resistor is determined, the necessary power rating of the
Dynamic Brake Resistor can be calculated.
The wattage 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 of the regenerative mode must be
estimated and the wattage of the Dynamic Brake Resistor chosen to be
greater than the average regenerative power dissipation of the drive. If the
Dynamic Brake 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. 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 maximum temperature
limits during the application of pulse power to the element and could exceed
the safe temperature limits of the resistor. The resistors used in the Dynamic
Brake Modules have thermodynamic time constants of less than 5 seconds.
This means restrictions must be imposed upon the application of the
Dynamic Brake Modules.
Peak regenerative power can be calculated as:
• Horsepower (English units)
• Watts (The International System of Units, SI)
• Per Unit System (pu) which is dimensionless
The final number must be in watts of power to estimate the ohmic value of
the Dynamic Brake Resistor. The following calculations are demonstrated
in SI units.