User Manual

Rockwell Automation Publication 20B-UM002G-EN-P - July 2014 137
Application Notes Appendix C
Skip Frequency
Figure 21 - Skip Frequency
Some machinery can have a resonant operating frequency that must be avoided to
minimize the risk of equipment damage. To assure that the motor cannot
continuously operate at one or more of the points, skip frequencies are used.
Parameters 084…086, ([Skip Frequency 1-3]) are available to set the frequencies
to be avoided.
The value programmed into the skip frequency parameters sets the center point
for an entire “skip band” of frequencies. The width of the band (range of
frequency around the center point) is determined by parameter 87, [Skip Freq
Band]. The range is split, half above and half below the skip frequency parameter.
If the commanded frequency of the drive is greater than or equal to the skip
(center) frequency and less than or equal to the high value of the band (skip plus
1/2 band), the drive will set the output frequency to the high value of the band.
See (A) in Figure 21
.
If the commanded frequency is less than the skip (center) frequency and greater
than or equal to the low value of the band (skip minus 1/2 band), the drive will
set the output frequency to the low value of the band. See (B) in Figure 21
.
Acceleration and deceleration are not affected by the skip frequencies. Normal
accel/decel will proceed through the band once the commanded frequency is
greater than the skip frequency. See (A) & (B) in
Figure 21
. This function affects only continuous operation within the band.
Frequency
Time
(A)
(A)
(B)
(B)
Skip Frequency
Command
Frequency
Drive Output
Frequency
Skip + 1/2 Band
Skip – 1/2 Band
35 Hz
30 Hz
25 Hz