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Rockwell Automation Publication 750-AT006D-EN-P - January 2022
Chapter 4 Auto Tuning
Inertia Tests
Autotune inertia tests are used to measure the following parameters accurately.
10:900 [Motor Inertia]
10:901 [Load Ratio]
10:900 [Motor Inertia] – Four ways to determine this value 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. This method is only an option if the load can be disconnected to run this 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.
for more information.
10:901 [Load Ratio] and 10:900 [Motor Inertia] are used to calculate the torque scaler K
J
, an internal parameter that compensates for the
effects of inertia and affects overall tuning. 10:901 [Load Ratio] is also used to calculate controller gains, however the effectiveness of this
calculation is largely based on the accuracy of 10:900 [Motor Inertia]. See
for more information.
Figure 66 - Autotune Inertia Parameters
When Autotune performs an inertia test, a momentary tuning torque profile is applied to the motor and the acceleration and deceleration
times are measured. As a result, the motor shaft moves, which ramps the speed up and down. The following parameters allow you to set
dynamic limits on the torque profiles that are used during the rotate tests.
10:911 [Autotune Psn Lim]
10:912 [Autotune Vel Lim]
10:913 [Autotune Trq Lim]
10:910 [Autotune] = InertiaMotor (4) – Enables the Autotune function to perform the Motor Inertia Test. After selecting this value, you must
issue a start command to begin the test. This test initiates momentary motor rotation to measure and update 10:900 [Motor Inertia]. Also,
10:901 [Load Ratio] is set to zero.
IMPORTANT
In legacy PowerFlex® 755 drives, total inertia required accurate measurement using autotune. In this drive, total
inertia is calculated from 10:900 [Motor Inertia], which now requires accuracy. As a result, a total inertia autotune test
is often not necessary.
IMPORTANT
Rotation during the tune process can exceed machine limits. Caution must be used when connecting and
disconnecting the load and when setting these autotune limits, so that the autotune motor rotation does not damage
the machine. Note that systems with mechanical restrictions or travel limits may not complete the autotune test. As a
result, these parameters may require readjustment.
IMPORTANT
This test is only an option if the load can be disconnected to run the test. It must be performed with the load
disconnected from the motor to generate accurate results. Otherwise, the motor inertia is calculated as the total
inertia of the motor and load together.
000
= Read Only
000
= User Entered
901
900
Load Ratio
Motor Inertia
[Autotune] =
“InertiaMotor”
“InertiaTotal”
“JMtr BW Calc”
“JTotalBWCalc”
To Gain and Kj
Calculations
000
= Read Only
000
= User Entered
901
900
Load Ratio
Motor Inertia
[Autotune] =
“InertiaMotor”
“InertiaTotal”
“JMtr BW Calc”
“JTotalBWCalc”
To Gain and Kj
Calculations
