Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 210)

210 Rockwell Automation Publication 750-RM002B-EN-P - September 2013
Chapter 4 Motor Control
t
3
- t
2
= total time of deceleration from the rated speed to 0 speed, seconds
P
b
= peak braking power, watts (1.0HP = 746 Watts)
Compare the peak braking power to that of the rated motor power, if the peak
braking power is greater that 1.5 times that of the motor, then the deceleration
time, (t
3
- t
2
), needs to be increased so that the drive does not go into current
limit. Use 1.5 times because the drive can handle 150% current maximum for 3
seconds.
Peak power can be reduced by the losses of the motor and inverter.
Step 3 – Calculating the Maximum Dynamic Brake Resistance Value
V
d
= The value of DC bus voltage that the chopper module regulates at and is
equal to 375V DC, 750V DC, or 937.5V DC
P
b
= The peak braking power calculated in Step 2
R
db1
= The maximum allowable value for the dynamic brake resistor
The choice of the Dynamic Brake resistance value will be less than the value
calculated in Step 3. If the value is greater than the calculated value, the drive can
trip on DC bus overvoltage. Remember to account for resistor tolerances.
Step 4 – Choosing the Chopper Module
I
dl
= The minimum current flowing through the chopper module transistor
V
d
= The value of DC bus voltage chosen in Step 3
R
dbl
= The value of the dynamic brake resistor calculated in Step 3
The value of I
d1
sets the minimum value of current rating for the Chopper
Module. When the Chopper Module choice has been made, the current rating of
the Module Transistor must be greater than or equal to the calculated value for
I
d1
. See the table below for rating values.
R
db1
V
d
2
p
b
------ =
I
dl
V
d
R
dbl
---------- =