Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C Frame) - Classic Control - Manual (Page 300)

Commissioning 4-95
7000 “C” Frame 7000L-UM300I-EN-P – Juen 2013
Calculating Total Inertia
If the system inertia cannot be measured, it can be calculated if the
moment of inertia of the motor and load are known. The value of
parameter "Total inertia" is defined as the time required to accelerate
the motor and load to rated speed when rated torque is applied. It can
be calculated using the following formula:
Total inertia = total inertia of motor & load in kg-m 2 X ( rated speed in rad/ sec) 2
rated power in watts
or
Total inertia = 6.21 x 10
-7
total inertia of motor and load in lb-ft
2
X ( rated speed in rpm)
2
rated power in hp
If there is a gearbox between the motor and load, the inertia of the load
must be referred to the motor side of the gearbox.
Running the Load Motor Starting Torque
When starting without a tachometer or encoder, the drive operates in
an open loop mode below approx. 3 Hz at which point the drive
switches to closed loop speed control. The starting currents are set
by three parameters; Torque Commands 0 (P86), 1 (P87), and
Minimum (P101). Torque Command 0 sets the breakaway torque
and Torque Command 1 is the torque at the transition point from
open to closed loop. Torque Command Minimum operates in
conjunction with Torque Command1 to minimize speed overshoot at
the transition point. If you are starting an uncoupled motor, or
simply autotuning, the default starting torque values may be
sufficient to run the motor. But the defaults are generally not high
enough to start a loaded motor.
Be prepared to have to increase starting torque and endure Motor
Stall faults during initial operation.
Reaching Specific Load Points
Verify the drive can reach rated speed and load. Monitor the Torque
Reference (P291) and the displayed value of the motor current. If
you are running into a torque limit the Torque Reference will be
running near the Torque Limit Motoring (P84) limit. If you are not
realizing rated motor current, you may increase the Torque Limit
Motoring slightly. If increasing the Torque Limit Motoring does not
help to increase the motor amps and speed then the drive is most
likely running out of input voltage.