Rockwell Automation 1336_F_E_T_S SERIES A CHOPPER BRAKE MODULE - Manual (Page 5)

Heavy Duty Dynamic Braking 5
1336-5.65 — March, 2007
How to Select a Chopper
Module and Dynamic Brake
Resistor As a rule, a Chopper 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 Chopper Module rating
and Dynamic Brake Resistor value to use. If 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.
In order to select the appropriate Chopper Module and Dynamic Brake
Resistor for your application, the following data must be calculated.
Peak Regenerative Power of the Drive (Expressed in watts of power.)
This value is used to determine:
• The minimum current rating of the Chopper Module
Choose the actual current rating from the selection tables.
• The estimated maximum ohmic value of the Dynamic Brake Resistor
If this value is greater than the maximum imposed by the peak
regenerative power of the drive, the drive can trip off due to transient
DC Bus overvoltage problems.
Minimum Dynamic Brake Resistance
If a Dynamic Brake Resistance value that is less than the minimum imposed
by the choice of the Chopper Module is applied, damage can occur to the
Chopper Transistor.
Dynamic Brake Resistor’s Allowable Ohmic Value Range
(Use the Chopper Module current rating to determine this range.)
These values range between the minimum value set by the Chopper
Transistor current rating and the maximum value set by the peak
regenerative power developed by the drive in order to decelerate or satisfy
other regenerative applications.
Wattage Rating of the Dynamic Brake Resistor
This rating 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.
Dynamic Brake Resistors with large thermodynamic heat capacities,
defined as thermal time constants less than 5 seconds, are 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, the temperature
of the resistor element could exceed maximum temperature limits during
the application of pulse power to the element.