8 Rockwell Automation Publication 750-AT006D-EN-P - January 2022
Chapter 1 Background
Figure 1 - Two Mass System
The following definitions are given:
J
M
= 10:900 [Motor Inertia]
J
L
= Load Inertia reflected back to the motor shaft
R = 10:901 [Load Ratio] = J
L
/J
M
J
T
= Total Inertia = J
M
+ J
L
= J
M
(R+1)
10:900 [Motor Inertia] is used to calculate the Torque Scaler, an internal parameter that compensates for the effects of inertia. This
parameter affects overall tuning. See Torque Scaler
on page 27 for more information. Four ways to determine motor inertia are given in
order from most to least effective.
• Data Sheet:
Enter a value from the motor nameplate or data sheet in units of [kg•m
2
].
-Divide [lb•ft
2
] or [WK
2
] by 23.73 to convert to [kg•m
2
].
•Measured:
Run Autotune with 10:910 [Autotune] = InertiaMotor (4) or JMtr BW Calc (7) with the load disconnected to dynamically
measure motor inertia. See Auto Tuning on page 63 for more information. This method is only an option if the load can be
disconnected to run the test.
•Estimated:
If data is not available or a motor inertia test is not possible, use the following equation to approximate 10:900 [Motor
Inertia] based on motor nameplate horsepower [Hp]:
J
M
= Hp/250*(Hp/500 +1)
– Multiply [kW] by 1.341 to convert to [Hp].
• Default Value:
This value is based on a motor power rating equal to the drive power rating.
Load Inertia that is reflected back to, and present at, the motor shaft includes coupling inertia and accounts for any gear ratio present in the
coupling mechanics. It is measured in the same units as motor inertia. Coupling mechanics between the motor and load can consist of rotary
couplings, gearboxes, belts and pulleys, shafts and gears, linkages, or any combination of them. Changing or unknown load inertias are best
compensated for by enabling the Load Observer
on page 29.
10:901 [Load Ratio] is the ratio of load inertia divided by motor inertia. It tells you how large the load is with respect to the motor driving it.
Load ratio is used to calculate the Torque Scaler, which compensates for the effects of inertia and is described in Torque Scaler
on page 27.
This parameter affects overall tuning. Load ratio is also used in calculating controller gains. Three ways to determine load ratio are given in
order from most to least effective.
• Default Value
: A value of zero is used when the load is unknown or compliant. A load ratio R = 0 initiates out-of-box tuning gain
calculations that work well for most applications. See Out-of-Box Tuning
on page 55 for more information.
•Entered
: Enter a value in [kg•m
2
] from your own calculations that are based on knowledge of the mechanical design. For a changing
inertia, enter the lowest value. A load ratio R > 0 initiates autotune gain calculations that work well on rigid loads. See Auto Tuning on
page 63 for more information.
•Measured
: Run Autotune with 10:910 [Autotune] = InertiaTotal (5) or JtotalBWCalc (8) with the load connected to dynamically measure
total inertia and load ratio. This method is only an option if motion can be initiated to rotate the load during the test. Furthermore, this
calculation assumes 10:900 [Motor Inertia] is accurate. If you get an error resulting from a negative load ratio
, the motor inertia is
incorrect and must be reduced. Where possible, perform the Autotune at the mechanical point of lowest inertia. A load ratio R > 0
initiates autotune gain calculations that work well on rigid loads. See Auto Tuning
on page 63 for more information.
Total Inertia is the sum of load inertia and motor inertia. It is the total inertia of the moving mechanical system that is driven by the drive.
Total inertia determines the maximum acceleration and deceleration of the mechanical system.
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