Rockwell Automation SA500 DC Bus Supply - Manual (Page 17)

Installation Guidelines 3-3
3.2 Selecting an External Braking Resistor
DC bus supplies with external braking resistors (615055-2T or 615055-2V) allow for
greater power dissipation during motor regeneration. Perform the following steps to
determine the required external braking resistor specifications.
Step 1. Using the stopping time specifications, calculate the required braking torque:
J is the combined total inertia of the motor and the machine (which can be
either calculated or measured). Alpha is the rate of deceleration of the
motor’s shaft.
Step 2. Calculate the resistance value of the external braking resistor.
Turn-on Voltage = 1.47 x V
LL
+ 5 volts, where V
LL
is the RMS input
line-to-line voltage. Turn-on voltage is 343 VDC for a nominal 230 VAC.
Torque is in lb-ft. 1 lb-ft = 0.7376 N-m.
Note that the recommended ranges of resistance values are as follows:
• 50 A DC bus supply: 8
(
minimum) to 55 ohms
• 100 A DC bus supply: 4 (minimum) to 15 ohms
A lower resistor value provides higher short-duration regenerative currents
but results in a reduction of the continuous power rating. A higher resistor
value provides for maximum continuous power operation. Over the
recommended ranges of resistance values, operation is balanced between
short duration overload current conditions and continuous power conditions.
Step 3. Determine the continuous power dissipation specifications of the external
braking resistor.
The continuous power rating is limited by both the fuse rating (0.9 x fuse
rating) and the resistor value. The regenerative duty cycle is limited by the
internal fuse. The lower value resistors will have higher peak currents and
therefore the duty cycle must be limited to prevent nuisance fuse openings.
The following equations are used to determine the typical braking resistor
specifications for each power supply, as shown in table 3.2.
Torque = J x Alpha
(lb-ft) =
(N-m) =
(lb-ft-sec 2 )
(kg-meter 2 )
x
x (radians/second 2 )
(radians/second 2 )
R =
(Turn-on Voltage) 2
Torque x RPM
x 6.33 ohms
τ
= Max Duty Cycle =
(0.9 x Fuse Rating)
(Turn-on Voltage)
seconds
2
x R
2 2
Max Continuous Braking Power watts = (0.9 x Fuse Rating)
2
x R