Rockwell Automation Publication 20A-UM001N-EN-P - July 2013 107
Application Notes Appendix C
Skip Frequency
Figure 2 shows the skip frequency band parameters.
Figure 2 - 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 sets the output frequency to the high value of the band. See
(A) in Figure 2
.
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 sets
the output frequency to the low value of the band. See (B) in Figure 2
.
Acceleration and deceleration are not affected by the skip frequencies. Normal
accel/decel proceeds through the band once the commanded frequency is greater
than the skip frequency. See (A) and (B) in Figure 2
. 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
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