Give a drive the freedom to run a motor at any speed and it will eventually be asked to hold a speed that shakes the whole machine. Most rotating installations have a narrow band of speeds where the driving frequency lines up with a natural resonance of the shaft, the piping, or the structure, and running there sets up damaging vibration. A skip frequency, also called a jump frequency or avoidance band, is a configured window of speeds the drive is told never to dwell at, so it steps across that band instead of settling inside it. This guide explains how you find those resonance points, how the skip band and its width are set, and how the drive handles a setpoint that lands in a forbidden zone.
Skip Frequency in one line: A skip frequency is a configurable frequency band that a VFD refuses to run at in steady state because that speed excites a mechanical or structural resonance - a critical speed - that causes excessive vibration. You set a center frequency and a band width, and if a speed command falls inside the band the drive accelerates through it and settles at the nearest edge instead of dwelling in it. It lets a drive cover its full speed range while never parking at a speed that would shake the machine apart.
Every mechanical assembly has natural frequencies at which it prefers to vibrate - a shaft has bending modes, a length of piping has its own modes, a skid or support structure resonates at particular frequencies. When a rotating machine turns at a speed whose rotational frequency, or a multiple of it, coincides with one of these natural frequencies, the small unavoidable imbalance in the rotor drives the structure at resonance and the vibration amplitude grows far beyond what the same imbalance would cause at any other speed. The speed at which this happens is called a critical speed.
A fixed-speed motor started direct-on-line spends only a moment passing through any critical speed on its way up to running speed, so resonance rarely gets a chance to build. A variable frequency drive changes that, because it can be commanded to hold any speed indefinitely, including one that sits right on a resonance. Parked at a critical speed, the machine vibrates continuously, and sustained resonant vibration fatigues shafts and welds, loosens fasteners, wrecks bearings and seals, and cracks piping - the kind of damage that a brief pass-through would never cause.
The purpose of a skip frequency is therefore not to stop the machine from ever moving through a resonant speed, which is unavoidable when accelerating and decelerating, but to stop it from staying there. The drive is allowed to sweep through the band quickly on its way to a different speed, but it is forbidden from selecting the band as a steady-state operating point, which is what turns a momentary vibration into a destructive one.
You cannot set a skip band until you know where the resonance is, and the usual way to find it is a slow speed sweep during commissioning. With vibration monitored - by a mounted sensor or simply by feel and sound on smaller machines - the drive is walked slowly across its speed range and the speeds where vibration spikes are noted. Those speeds are the resonance points, and each one becomes the center of a skip band. On critical or large machinery the resonant speeds may be predicted in advance from the machine's design, but a physical sweep confirms where they actually fall on the installed assembly.
Each skip band is defined by two things: a center frequency at the identified resonance, and a width or dead-band around it that spans the range of speeds where vibration is unacceptable. The width matters. Too narrow and the drive can still settle close enough to the resonance to vibrate; too wide and you needlessly forbid a large chunk of useful operating range and force the machine to jump a bigger gap. The band is sized to comfortably straddle the vibration peak with a margin on each side, and most drives allow several independent skip bands so a machine with more than one critical speed can have each one bracketed.
Because resonance depends on the whole installed system, the bands belong to the installation, not just the motor. Changing the piping, the mounting, or the load can move a critical speed, so a skip band set at commissioning may need rechecking after a mechanical modification. Documenting the identified resonances and the bands set to avoid them, and revisiting them after any change to the machine or its supports, keeps the avoidance honest over the life of the installation.
Once a skip band is configured, the drive's behaviour at a forbidden setpoint is straightforward but specific. The drive will still accelerate and decelerate straight through the band - it must, to reach speeds on the far side - but it will not hold there. If a speed command falls inside the band, the drive snaps the operating point to the nearest edge of the band instead: a command just below the center settles at the lower edge, a command just above it settles at the upper edge. The result is a small discontinuity in achievable speeds, a gap the drive simply cannot dwell in, which is the intended effect.
This matters the moment a process control loop is choosing the speed. On a pump or fan whose speed is trimmed automatically to hold a pressure or flow, the loop can call for any speed, including one inside a skip band. The skip logic quietly overrides that request, holding at the band edge rather than the exact commanded value, which the control loop then works around. Operators and engineers should know the bands exist, because a pump that seems unable to hold a particular flow, or that hunts around a certain output, may simply be bouncing off a skip band rather than misbehaving.
In a cloud SCADA platform such as Merobix, the connection runs both ways. The telemetry a drive reports - output frequency alongside vibration or motor current where instrumented - can reveal a resonance that was never characterized: a speed at which vibration or current consistently rises is a candidate for a new skip band. And when an operator sets a speed setpoint from a remote screen, understanding that the drive may refuse to sit exactly there explains why the reported running frequency can differ slightly from the commanded one. Trending frequency and vibration together across the fleet lets an engineer spot machines that are being run near resonance and add or widen a skip band before the vibration does damage, all without standing next to the machine.
A critical speed is a rotational speed at which the machine's turning frequency coincides with a natural resonance of the shaft, piping, or structure, so the rotor's small imbalance drives the assembly at resonance and vibration grows sharply. A skip frequency is a band configured around that critical speed which the drive refuses to run at in steady state. The drive can pass through the band while changing speed, but it will not park there, preventing sustained resonant vibration.
Run a slow speed sweep at commissioning with vibration monitored, and note the speeds where vibration spikes - those are the resonance points. Center a skip band on each one and set its width to comfortably straddle the vibration peak with a margin on each side. On large or critical machines the resonant speeds may be predicted from the design, but a physical sweep confirms where they actually fall on the installed assembly, since mounting and piping affect them.
The drive will not hold inside the band; instead it settles at the nearest edge. A command just below the band's center snaps to the lower edge and one just above it snaps to the upper edge, leaving a small gap of speeds the drive cannot dwell at. It will still accelerate and decelerate straight through the band on its way to other speeds. This is why a reported running frequency can differ slightly from a commanded one near a skip band.
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