Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 215)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 215
Motor Control Chapter 4
Step 6 – Estimating the Minimum Wattage requirements for the Dynamic
Brake Resistor
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 in watts that is chosen will be equal to
or greater than the value calculated in Step 6.
Step 7 – Calculate the requires Watt-Seconds (joules) for the resistor
To be sure the resistor’s thermal capabilities are not violated, a calculation to
determine the amount of energy dissipated into the resistor is made. This
determines the amount joules the resistor must be able to absorb
P
ws
= Required watt - seconds of the resistor
t
3
- t
2
= Elapsed time to decelerate from
ω
b speed to
ω
0 speed, seconds
P
b
= Peak braking power, watts
P
av
t
3
t
2
– ( )
t
4
------------------
P
b
2
------ × =
P
ws
t
3
t
2
– ( )
P
b
2
------ Ч =