Example calculation – Rockwell Automation 1336_F_E_T_S SERIES A CHOPPER BRAKE MODULE User Manual
Page 10
Heavy Duty Dynamic Braking
10
1336-5.65 — March, 2007
Step 7 — Choose the Dynamic Brake Resistance Value
Use to Table 1a, 2a, or 3a to choose the correct table based on the Chopper
Module’s regulating voltage.
1. Find the column that lists the value of Dynamic Brake Resistance for
the various Dynamic Brake Resistor assemblies.
2. Choose the resistor value that lies between R
db1
and R
db2
.
Preferred resistance values are as close R
db1
as possible.
Step 8 — Estimate the Minimum Wattage Requirements for the Dynamic
Brake Resistor
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 0 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 0 after (t
3
- t
2
) seconds have elapsed.
The average power regenerated over the interval of (t
3
- t
2
) seconds is P
b
/2.
The average power in watts regenerated over the period t
4
is:
The Dynamic Brake Resistor power rating, in watts, that is chosen should
be equal to or greater than the value calculated in Step 8.
Example Calculation
Application Information
A 100 HP, 460 Volt motor and drive is accelerating and decelerating as
depicted in Figure 2.
•
Cycle period (t
4
) is 60 seconds
•
Rated speed is 1785 RPM
•
Deceleration time from rated speed to 0 speed is 6.0 seconds
•
Motor load can be considered purely as an inertia
•
All power expended or absorbed by the motor is absorbed by the
motor and load inertia
•
Load inertia is directly coupled to the motor
•
Motor inertia plus load inertia is given as 9.61 kg-m
2
P
av
=
P
av
= Average dynamic brake resister dissipation (watts)
t
3
- t
2
= Deceleration time from
ω
b
to
ω
o
(seconds)
t
4
= Total cycle time or period of process (seconds)
P
b
= Peak braking power (watts)
ω
b
= Rated motor speed (Rad / s)
ω
o
= A lower motor speed (Rad / s)
[t
3
- t
2
]
t
4
✕
P
av
=
[
–
]
[
]
✕
[
]
2
P
av
= _________ watts
P
b
2
ω
b
+
ω
o
ω
b
(
)