Rockwell Automation 1336_S_F_T Allen-Bradley Dynamic Braking - Manual (Page 9)

Heavy Duty Dynamic Braking 9
1336-5.64 — July, 2005
Step 3 — Calculate the Maximum Dynamic Brake Resistance Value
The choice of the Dynamic Brake resistance value should be less than the
value calculated in Step 3. If the resistance value is greater than the value
calculated in Step 3, the drive can trip on DC Bus overvoltage. Do not reduce
P
b
by any ratio because of estimated losses in the motor and inverter. This
has been accounted for by an offsetting increase in the manufacturing
tolerance of the resistance value and the increase in resistance value due to
the temperature coefficient of resistor element.
Step 4 — Choose the Correct Dynamic Brake Module
Go to Table 1a, 2a, or 3a in this publication and choose the correct Dynamic
Brake Module based upon the resistance value being less than the maximum
value of resistance calculated in Step 3. If the Dynamic Brake Resistor value
of one Dynamic Brake Module is not sufficiently low, consider using up to
three Dynamic Brake Modules in parallel, such that the parallel Dynamic
Brake resistance is less than R
db1
calculated in Step 3. If the parallel
combination of Dynamic Brake Modules becomes too complicated for the
application, consider using a Brake Chopper Module with a separately
specified Dynamic Brake Resistor.
Step 5 — Estimate the Minimum Wattage Requirements for the Dynamic
Brake Resistors
It is assumed that the application exhibits a periodic function of acceleration
and deceleration. If (t
3
- t
2
) equals the time in seconds necessary for
deceleration from rated speed to
ω
o
speed, and t
4
is the time in seconds
before the process repeats itself, then the average duty cycle is (t
3
- t
2
)/t
4
.
The power as a function of time is a linearly decreasing function from a
value equal to the peak regenerative power to some lesser value after (t
3
-
t
2
) seconds have elapsed. The average power regenerated over the interval
of (t
3
- t
2
) seconds is:
R
db1
=
V
d
= DC Bus voltage the chopper module regulates to
(375V DC, 750V DC, or 937.5V DC)
P
b
= Peak braking power calculated in Step 2 (watts)
R
db1
= Maximum allowable value for the dynamic brake
resistor (ohms)
0.9 ✕ V
d
2
P
b
R
db1
=
[ ✕ ]
[ ]
R
db1
= _________ ohms
P
b
2
ω
b
+
ω
o
ω
b
( )