Selecting An External Resistor 4-3
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 Section 2: Step 3
(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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