Rockwell Automation 1336_S_F_T Allen-Bradley Dynamic Braking User Manual
Page 11
Heavy Duty Dynamic Braking
11
1336-5.64 — July, 2005
Step 7 — Calculate the Percent Peak Load of the Dynamic Brake Resistor
The calculation of PL in percent gives the percentage of the instantaneous
power dissipated by the Dynamic Brake Resistors relative to the steady state
power dissipation capacity of the resistors. This will give a data point to be
drawn on the curve of Figure 3. The number calculated for PL will
commonly fall between 300% and 600%. A calculated number for PL of
less than 100% indicates that the Dynamic Brake Resistor has a higher
steady state power dissipation capacity than is necessary.
Step 8 — Plot the Steady State and Transient Power Curves on Figure 3
Draw a horizontal line equal to the value of AL (Average Load) in percent
as calculated in Step 6. This value must be less than 100%.
Pick a point on the vertical axis equal to the value of PL (Peak Load) in
percent as calculated in Step 7. This value should be greater the 100%.
Draw a vertical line at (t
3
- t
2
) seconds such that the line intersects the AL
line at right angles. Label the intersection point “Point 1”.
Draw a straight line from PL on the vertical axis to Point 1 on the AL line.
This line is the power curve described by the motor as it decelerates to
minimum speed.
Figure 3
Plot Your Power Curve
PL =
PL
= Peak load in percent of Dynamic Brake Resistor
P
b
= Peak braking power calculated in Step 2 (watts)
P
db
= Steady state power dissipation capacity of dynamic brake
resistors obtained from Table 1a, 2a, or 3a (watts)
P
b
P
db
✕
100
PL =
[
]
[
]
✕
100
PL = __________ %
t
(time in seconds)
100
200
300
400
500
600
1
0
2
3
4
5
6
7
8
9
10
P
o
wer
(%)
KA, KB, KC Transient Power Capacity