Transit Locomotive Braking Resistors
From Transit Products
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.
Braking Resistors for Variable Frequency Drives
- enclosed transit resistor Designs for roof-mounted, under-car hung and snubber resistors for transit applications
- Compact, lightweight, rugged designs for severe applications
- Convection or fan-cooled designs
- Filnor resistors can be used on light and heavy rail vehicles or trackside braking
Solution: Braking Resistors
Braking resistors are used to absorb energy that is being regenerated back into a drive unit by a freewheeling motor. That energy is released in the form of heat.
Filnor Inc. Braking Resistors are sized based on the following customer supplied information:
- Voltage: • DC Bus 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. 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' as shown in figure 1.
The regeneration type is a critical piece of information that was not 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 BT
- BT = Brake Torque
- Use 1.0 for 100%
- Use 1.5 for 150%
- Calculate the required resistance: Resistance = (DC bus voltage)^2 / PW hello why do you end here/?
- Calculate the Duty Cycle (DC) as shown in Fig. 1.
- DC = Braking Time / Cycle Time
- Calculate the DB resistor wattage: Regeneration Type: Deceleration Braking - - -
- DBrw = (PW x DC )/2
- See Figure 1.
- Maximum 'On Time' is 60 seconds for normal braking type.
- Regeneration Type: Overhauling Load - - - DBrw = PW x DC
- See Figure 2.
- No maximum 'On Time' for overhauling load type.
- Calculate the DB resistor current:
- Calculate the Braking Current: The Braking current (Bi) is the actual current that will flow through the driveís braking transistor and the DB resistor for the duty cycle time period.
- Bi (Braking Current)=PW/RESISTANCE
- DBi (DB Resistor Current)=DBrw/RESISTANCE