Example calculation – Rockwell Automation 1336_S_F_T Allen-Bradley Dynamic Braking User Manual
Page 13
Heavy Duty Dynamic Braking
13
1336-5.64 — July, 2005
Example Calculation
A 50 HP, 4 Pole, 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 and is to be decelerated to 0 speed in 6.0
seconds
•
Motor load can be considered purely as an inertia, and 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
Calculate the necessary values to choose an acceptable Dynamic Brake
Module.
Rated Power = 50 HP
× 746 = 37.3 kW
This information was given and must be known before the calculation
process begins. This can be given in HP, but must be converted to watts
before it can be used in the equations.
Rated Speed = 1785 RPM = 2
π
× 1785/60 = 186.93 Rad/s =
ω
b
This information was given and must be known before the calculation
process begins. This can be given in RPM, but must be converted to radians
per second before it can be used in the equations.
ω
o
= 0 RPM = 0 Rad/s
Total Inertia = 9.61 kg-m
2
= J
T
This value can be in lb-ft
2
or Wk
2
, but must be converted into kg-m
2
before
it can be used in the equations.
Deceleration Time = (t
3
- t
2
) = 6.0 seconds.
Period of Cycle = t
4
= 60 seconds.
V
d
= 750 Volts
This was known because the drive is rated at 460 Volts rms. If the drive
were rated 230 Volts rms, then V
d
= 375 Volts, and if the drive were rated
at 575 Volts rms, then V
d
= 937.5 Volts.
All of the preceding data and calculations were made from knowledge of
the application under consideration. The total inertia was given and did not
need further calculations as outlined in Step 1.
This is 150% rated power and is equal to the maximum drive limit of 150%
current limit. This calculation is the result of Step 2 and determines the peak
power that must be dissipated by the Dynamic Brake Resistor.
= 55.95 kW
J
T
×
ω
b
(
ω
b
-
ω
o
)
(t
3
- t
2
)
Peak Braking Power = P
b
=