Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 209)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 209
Motor Control Chapter 4
If the line you drew lies to the left of the constant temperature power curve of the
Dynamic Brake Resistor, then there is no application problem. If any portion of
the line lies to the right of the constant temperature power curve of the Dynamic
Brake Resistor, then there is an application problem. The application problem is
that the Dynamic Brake Resistor is exceeding its rated temperature during the
interval that the transient power curve is to the right of the resistor power curve
capacity. It is prudent to parallel another Dynamic Brake Module or apply a
Brake Chopper Module with a separate Dynamic Brake Resistor.
Sizing the Chopper and Resistors
Chopper and Resistors (no longer a Rockwell Automation product)
Sizing the chopper module is the same as the dynamic brake module with a
couple of added steps. Because the chopper is separate from the resistors, an
additional calculation for current needs to be made. Additionally a calculation
for watt-seconds or joules needs to be made for resistor sizing.
Step 1 – Determine the Total Inertia
J
T
= J
m
+ GR
2
x J
L
J
T
= Total inertia reflected to the motor shaft, kilogram-meters
2
, kg•m
2
, or
pound-feet
2
, lb•ft
2
J
m
= motor inertia, kilogram-meters2, kg•m
2
, or pound-feet2, lb•ft
2
GR
2
= the gear ratio for any gear between motor and load, dimensionless
J
L
= load inertia, kilogram-meters2, kg•m
2
, or pound-feet2, lb•ft
2
(1.0 lb•ft
2
=
0.04214011 kg•m
2
)
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
P
b
J
T
ω 2 ×
t
3
t
2
–
----------------- =
Rad s
⁄
2
πN
60
---------- =