Dynamic Braking Resistors
From Power 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 over voltage 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.
SOLUTION: DYNAMIC BREAKING (DB) RESISTORS
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.
Filnor DB Resistors are sized based on the following customer supplied information:
- Drive input voltage.
- Drive horsepower.
- Braking Torque.
- Duty Cycle.
- On time / Off time
- The maximum braking current or minimum ohmic value as specified by the drive manufacturer.
- Regeneration type:
- Deceleration braking.
- Overhauling load.
The braking torque is usually specified as 100% or 150% which is a function of the ohmic value of the resistor. Higher braking torque means lower resistance, higher braking currents and faster motor stops, but as indicated, caution should be used to not exceed the drive braking current.
The total amount of wattage actually dumped into the resistor is determined by the duty cycle and by the regeneration type. The duty cycle can be determined by dividing the 'Cycle Time' into the 'Braking Time'.
The regeneration type is a critical piece of information because as previously mentioned, an overhauling load develops about twice the energy of a normal braking cycle.
Filnor Inc. uses the following method to calculate DB resistor requirements for normal braking loads:
Calculate the motor/drive wattage:
Motor Wattage (MW) = Motor or Drive horse power (HP) x 746
Calculate the peak wattage:
Peak Wattage (PW) = MW x BTBT = Brake Torque
• Use 1.0 for 100%
• Use 1.5 for 150%
Calculate the required resistance:
Resistance = (DC bus voltage)^2 / PW
Calculate the Duty Cycle (DC)
DC = Braking Time / Cycle Time
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.
Calculate the DB resistor current:
Bi = √(PW/RESISTANCE)
Calculate the Braking Current:
DBi = √(DBrw/RESISTANCE)