Dynamic Braking Resistors

When a drive unit is attempting to rapidly brake a motor “Deceleration Braking Cycle” or when an “overhauling load” condition exists, the spinning motor acts as a generator.

This freewheeling condition will force some voltage back into the drive unit (regeneration) which, depending upon the amount of regeneration, may cause an overvoltage condition if the energy is not “dumped” somewhere else.

Fifteen to twenty percent of this regenerated energy will be absorbed by the drive itself and natural mechanical / motor losses which leaves about eighty percent of the energy to be absorbed by some other means.

Dynamic braking resistors are used to absorb energy that is being regenerated back into a drive unit by a freewheeling motor and will release that energy in the form of heat.

1. Calculate the motor/drive wattage:

MOTOR WATTAGE (MW) = MOTOR OR DRIVE HORSE POWER (HP) X 746

2. Calculate the peak wattage:

PEAK WATTAGE (PW) = MW X BT

BT = BRAKE TORQUE
USE 1.0 FOR 100%
USE 1.5 FOR 150%

3. Calculate the required resistance:

RESISTANCE = (DC BUS VOLTAGE)^2 / PW

4. Calculate the Duty Cycle (DC)

DC = BRAKING TIME / CYCLE TIME

5. Calculate the DB resistor wattage:

REGENERATION TYPE: NORMAL BRAKING – – – DBRW = (PW X DC )/2
MAXIMUM “ON TIME” IS 60 SECONDS FOR NORMAL BRAKING TYPE.
REGENERATION TYPE: OVERHAULING LOAD – – – DBRW = PW X DC
NO MAXIMUM “ON TIME” FOR OVERHAULING LOAD TYPE.

6. Calculate the DB resistor current:

BI = √(PW/RESISTANCE)

7. Calculate the Braking Current:

DBI = √(DBRW/RESISTANCE)