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STABILIZATION OF HIGH INERTIA LOADS
The normal positioning drive’s compensation control range is adequate to provide normal
operation of loads whose inertias are as much as ten times greater than the drive motor. A
guideline of 10 times is acceptable to velocities of 2/3 maximum. At maximum velocity
performance with excess inertia is further deteriorated by lack of P.C. BUS voltage. The D.C.
BUS is derived from the AC input and low AC input lines would also deteriorate performance
with high inertia.
Inertia testing is accomplished at the factory on every drive. Inertia disks of 10 times greater
than the test motors rotor are used. 230 VAC amplifiers are tested at 2400 RPM, and the 115
VAC models are tested at 3000 RPM. Over-riding the drive’s normal values is not normal.
Generally, abnormal operation of the drive is a result of incorrect programming. Programming
ramp times or velocities beyond the drive’s physical capability is a major cause of incorrect
operation. The drive will make every effort to complete its commanded motion profile. An
inability to accomplish this smoothly is almost always diagnosed at some kind of a “servo gain
problem” instead of a programming problem.
Instability of a servo system is usually correctly noted if at STOP or at specific velocities, loping
occurs. Loping would be noticed oscillation. This could be caused by excess loop gain beyond
our present capability to correct. If this is determined, then using PCX and the dumb terminal
mode of operation, other values for loop and auto gain could be tested. In the dumb terminal
mode “EG” is loop gain and “VG3” is used by the auto gain algorithm. If an axis identifier is
used, it must precede the command. A question mark “?” after the command allows for
interrogation of the value and an equal sign after the command would allow for a change of the
value. The values for either loop gain “EG” or auto gain “VG3” are hexadecimal. You must
have cap locks on in the dumb terminal mode.
If instability in the loop is determined, then reducing loop gain “EG” and increasing auto gain
“VG3” may affect operation. Values in the following table can be tried.
Table XII
Stabilization Of High Inertia Loads
Test
“EG”
“VG3”
Alternative “VG3”
Example
1
∅∅
20
1
F
∅
25
(0 – 7F)HEX
2
E
∅
30
(0 – 7F)HEX
3
D
∅
35
(0 – 7F)HEX
4
C
∅
40
(0 – 7F)HEX
5
B
∅
45
(0 – 7F)HEX
6
A
∅
50
(0 – 7F)HEX
7
9
∅
55
(0 – 7F)HEX
8
8
∅
60
(0 – 7F)HEX
9
7
∅
65
(0 – 7F)HEX
10
6
∅
70
(0 – 7F)HEX
11
5
∅
(0 – 7F)HEX
Instability in the form of cyclic oscillation is generally improved by decreasing gain (EG).
“VG3” alters the velocity feedback gain as a function of command velocity. For example, at
lower velocities, a larger value in “VG3” increases the velocity feedback. The alternative for
“VG3” means that because of load condition for any value of “EG” a value of (
∅
to 7F) HEX
could work. Values out of the range listed could cause mis-operation and should not be tried
without consulting EMERSON EMC Service department.
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