Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017 35
Selecting An External Resistor for PowerFlex 7-Class Drives Chapter 4
Record the Values Calculated in Section 2
Calculate Maximum Dynamic Brake Resistance Value
When using an internal Dynamic Brake Resistor, the value is fixed. However,
when choosing an external resistor, the maximum allowable Dynamic Brake
resistance value (R
db1
) must be calculated.
R
db1
= Maximum allowable value for the dynamic brake resistor (ohms)
V
d
= DC bus voltage used for calculating maximum power.
(395V DC, 790V DC, 987V DC, or 1135V DC)
P
b
= Peak breaking power calculated in Step 2 – Peak Braking Power on page 15
(watts)
Calculate Maximum Dynamic Brake Resistance:
Record Maximum Dynamic Brake Resistance:
The choice of the Dynamic Brake resistance value should be less than the value
calculated in this
step. If the value is greater, the drive can trip on DC bus overvoltage.
Calculate required joule rating (joules = watt-seconds):
Drive Efficiency = 0.975
P
b
=
P
av
=
R
db1
=
R
db1
V
d
2
P
b
--------------=
R
db1
ooooooooo
2
ooooooooo
------------------------------------
=
P
b
2
------
t
3
t
2
– watt-seconds=
watt-second losses
P
b
2
------ t
3
t
2
– 1 motor efficiency drive efficiency–=
Total watt-seconds watt-seconds watt-second losses–=
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