Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 208)

208 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4 Motor Control
P
db
= Steady state power dissipation capacity of resistors obtained from the table
in Step 4 (Watts)
Step 7 – Calculate Percent Peak Load
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 gives a data point to be drawn on the
curve of Figure 3. The number calculated for PL commonly falls between 300%
and 600% for the Dynamic Brake Modules. 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.
PL = Peak load in percent of Dynamic Brake Resistor
P
av
= Peak braking power calculated in Step 2 (Watts)
P
db
= Steady state power dissipation capacity of resistors obtained from the table
in Step 4 (Watts)
Step 8 – Plot PL and AL on Curve
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 will be greater than 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 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.
PL
P
b
P
db
-------- 100 × =
0 5 10 15 20
0
100
200
300
400
500
600
KA, KB, KC Transient Power Capacity
Time (Seconds)
Po
w
er
(%
)