Siemens SINAMICS G120D - Page 157

Advanced commissioning
7.5 Motor control
Converter with control units CU250D-2
Operating Instructions, 04/2015, FW V4.7.3, A5E34261542B AB
157
Calculating the load torque
At low speeds, the inverter calculates the load torque
M
L
from the actual motor torque.
The calculation takes place under the following co
n-
ditions:
•
Speed ≥ p1226
•
Acceleration setpoint < 8 1/s
2
(≙ speed change
480 rpm per s)
•
Acceleration × moment of inertia (r1493) < 0.9 ×
p1560
Calculating the moment of inertia
F
or larger changes, the inverter initially calcu-
lates the accelerating torque M
B
as difference
between the motor torque M
M
, load torque M
L
and frictional torque M
R
:
M
B
= M
M
- M
L
- M
R
The moment of inertia J of the motor and load
is then obtained from the ac
celerating torque
M
B
– and the angular acceleration α (α = rate
of change of speed):
J
= M
B
/ α
If all of the following conditions are met, the inverter calculates the moment of inertia:
•
① The rated accelerating torque M
B
must satisfy the following two conditions:
– The sign of M
B
is the same as the direction of the actual acceleration
– M
B
> p1560 × rated motor torque (r0333)
•
② speed > p1755
•
The inverter has calculated the load torque in at least one direction of rotation.
•
Acceleration setpoint > 8 1/s
2
(≙ speed change 480 rpm per s)
③
The inverter calculates the load torque again after acceleration.