2-6 Determining Dynamic Brake Requirements
For the purposes of this document, it is assumed that the motor used in
the application is capable of producing the required regenerative torque
and power.
Step 4 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 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 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
:
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[]
-----------------------------------------------
×=
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