Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 207)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 207
Motor Control Chapter 4
Step 3. If the parallel combination of Dynamic Brake Modules becomes too
complicated for the application, consider using a Brake Chopper Module with a
separately specified Dynamic Brake Resistor.
Step 5 – Estimate average power
It is assumed that the application exhibits a periodic function of acceleration and
deceleration. If (t
3
- t
2
) = the time in seconds necessary for deceleration from
rated speed to 0 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 0 after (t
3
- t
2
) seconds have elapsed. The average power regenerated over the
interval of (t
3
- t
2
) seconds is Pb/2. The average power in watts regenerated over
the period t
4
is:
P
av
= Average dynamic brake resistor dissipation, in watts
t
3
- t
2
= Elapsed time to decelerate from rated speed to 0 speed, in seconds
t
4
= Total cycle time or period of process, in seconds
P
b
= Peak braking power, in watts
The Dynamic Brake Resistor power rating of the Dynamic Brake Module (singly
or two in parallel) that is chosen must be greater than the value calculated in Step
5. If it is not, then a Brake Chopper Module with the suitable Dynamic Brake
Resistor must be specified for the application.
Step 6 – Calculate Percent Average Load
The calculation of AL is the Dynamic Brake Resistor load expressed as a percent.
Pdb is the sum of the Dynamic Brake Module dissipation capacity and is
obtained from the table in Step 4. This gives a data point for a line to be drawn on
the curve in Figure 3. The number calculated for AL must be less than 100%. If
AL is greater than 100%, an error was made in a calculation or the wrong
Dynamic Brake Module was selected.
AL = Average load in percent of Dynamic Brake Resistor
P
av
= Average dynamic brake resistor dissipation calculated in Step 5 (Watts)
P
av
t
3
t
2
– ( )
t
4
------------------
P
b
2
------ × =
AL
P
av
P
db
-------- 100 Ч =