Hello friends,

A machine runs quietly at 20 Hz, begins to shake near 25 Hz, and becomes quiet again at 30 Hz. A skip frequency function tells the drive to avoid holding a steady operating point in that troublesome range. It does not stop time, freeze the output for a set number of electrical cycles or turn the motor into a device that jumps instantly from one speed to another.

On this page: What it does · Skip vs jump · Simulation · Field setting · FAQ

How does the avoided frequency band work?

In a variable frequency drive, the skip band is defined around an identified resonance or another speed at which prolonged running is unsuitable. For example, an illustrative band of 22–28 Hz around 25 Hz makes a steady 25 Hz request unavailable. When approached from below, the output can remain at 22 Hz; when approached from above, it can remain at 28 Hz. This boundary behaviour is model dependent: confirm the particular drive manual.

When the command asks for 35 Hz, the drive still has to pass through the 22–28 Hz band on its way there. The acceleration/deceleration ramp controls that crossing; it does not automatically become six times faster. The band affects permitted steady operation, not a physical discontinuity in rotating speed. In the ABB example referenced below, 22 → 28 Hz at 5 Hz/s takes about 1.2 s. Some drive families explicitly ignore skip bands while accelerating or decelerating.

Are “skip frequency” and “jump frequency” separate functions?

Often they name the same resonance avoidance feature. Schneider Electric calls the settings Jump Frequency inside a page titled “set up a skip frequency”. Some manuals use critical speed. A manufacturer may offer several band centres and band widths, but “jump” is not a general-purpose quick-acceleration mode; that job belongs to the acceleration and deceleration ramp or a separately documented ramp-selection function.

Watch the meaning of bandwidth. On one model the parameter is the entire span, so centre 25 Hz plus width 6 Hz yields 22–28 Hz. On another it may be a half-width on either side. Copying the number “6” into a different brand without reading its diagram can make the excluded region too wide or too narrow.

Try the frequency and ramp controls

Start with the 35 Hz target and watch the output cross the red band at the selected ramp rate. Move the target to 25 Hz while the output is below 22 Hz: it settles at the lower boundary. Raise the target to 35 Hz and it crosses the band. To see the upper boundary, let the output reach 35 Hz first, then lower the target to 25 Hz. The bottom graph plots drive output frequency against time, not measured shaft rpm.

How I set it on an actual machine

First I find the operating point where vibration or noise genuinely increases, measure the repeatable range and check bearings, balance, looseness and coupling condition. Skip settings can stop the machine from lingering near resonance; they cannot repair a defective bearing or an unbalanced impeller. The vibration itself is not necessarily equal in hertz to the drive’s displayed electrical output frequency.

Next I select the smallest useful band that covers the measured troublesome region, using the exact model’s definition of centre, width and permitted number of bands. Then I try a speed request inside the band from below and from above and verify where the drive settles. Finally I command a speed across it and check actual vibration, current and transit time under load.

I check the ramp timing independently. If the machine has a demanding start/stop profile, the S-curve ramp affects acceleration shape; it is not a second name for a skip band. Low-frequency vibration and the PWM carrier frequency concern different possible sources of noise. An output-frequency exclusion band should be chosen for the observed mechanical issue.

Quick questions

Does the drive “jump over” every frequency in the band? No. It can traverse the region during a ramp. The purpose is to avoid a steady operating point there; details depend on the model.

Is skip frequency the same as jog frequency? No. Jog frequency is a commanded low-speed operating point for a jog action; a skip band restricts continuous operation at selected speeds.

Does the function save energy? No universal saving follows from enabling it. Its main purpose is to avoid sustained operation near an unsuitable speed.

Can I choose a skip band by motor nameplate alone? No. Resonance depends on the whole motor-and-machine system, the load, mounting and operating conditions.

Technical references: Schneider Electric · Skip/Jump parameter naming · ABB · Skip band and ramp behaviour · Schneider Electric · ATV212 transient behaviour.