Rockwell Automation AK DBU - Dynamic Braking Unit 600/690 VAC - Manual (Page 44)

C-6 Design Information
Step 3 Minimum Power 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:
P
av
= Average dynamic brake resister dissipation (watts)
t
3
– t
2
= Deceleration time from
ωb to ω
o
(seconds)
t
4
= Total cycle time or period of process (seconds)
t
4
cannot exceed 900 + (t
3
– t
2
). See Note below.
P
b
= Peak braking power (watts)
ω
b
= Rated angular rotational speed
ω
o
= Angular rotational speed,
less than rated speed down to zero
The Average Power in watts regenerated over the period t
4
is:
Calculate Average Power in watts regenerated over the period t
4
:
Record Average Power in watts regenerated over the period t
4
:
Note: Since a resistor will typically cool in 15 minutes (900 seconds),
it will not be possible to take advantage of a higher duty cycle.
P
av
=
P
b
2
-----
ω
b
ω
o
+
( )
ω
b
------------------------ ×
Rad
s
---------
Rad
s
---------
P
av
t
3
t
2
– ( )
t
4
------------------
P
b
2
-----
ω
b
ω
o
+ ( )
ω
b
------------------------ =
P
av
oooooo oooooo – ( )
oooooo
[ ]
-----------------------------------------------
oooooo [ ]
2
----------------------- ×
oooooo oooooo + ( )
oooooo
[ ]
----------------------------------------------- Ч
=