Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 213)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 213
Motor Control Chapter 4
Step 2 – Calculate the Peak Braking Power
J
T
= Total inertia reflected to the motor shaft, kg•m
2
ω
= rated angular rotational speed,
N = Rated motor speed, RPM
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 drive regulates at and is equal to 375V
DC, 750V DC, or 937.5V DC depending on input voltage
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.
P
b
J
T
ω 2 ×
t
3
t
2
–
----------------- =
Rad s
⁄
2
πN
60
---------- =
R
db1
V
d
2
p
b
------ =