{"id":2614,"date":"2026-11-14T06:00:00","date_gmt":"2026-11-14T06:00:00","guid":{"rendered":"https:\/\/xnhang.com\/?p=2614"},"modified":"2026-11-13T03:33:05","modified_gmt":"2026-11-13T03:33:05","slug":"vfd-skip-frequency","status":"publish","type":"post","link":"https:\/\/xnhang.com\/ru\/blog\/vfd-skip-frequency\/","title":{"rendered":"VFD Skip Frequency: Plan and Verify a Vibration Avoidance Band"},"content":{"rendered":"

A VFD skip frequency function restricts steady operation within a selected output-frequency band, commonly to avoid a troublesome mechanical speed range. It does not repair the machine or establish that passing through the band is safe. Before programming it, confirm the drive supports the required function, investigate the vibration and obtain approved operating limits. Then verify what the drive actually does when a command enters the restricted band. The correct limits depend on the complete motor, driven equipment and process; there is no universal skip frequency that can be copied into every inverter.<\/p>\n

This guide helps owners prepare a measured commissioning and handover plan. It does not specify parameter numbers for a particular firmware revision. If vibration is severe, increasing or accompanied by mechanical distress, take the equipment out of service according to site procedures and obtain qualified assessment before any further trial.<\/p>\n

What the skip function changes<\/h2>\n

Output frequency is the drive quantity used to influence motor speed. A skip function introduces a restriction around one or more selected frequency values. Depending on the implementation, the manual may describe a center and width, upper and lower boundaries, or another convention. The number of available bands and the way a command is handled must be checked in the exact drive documentation.<\/p>\n

Tony Kuphaldt’s VFD educational module<\/a> includes skip frequency among the important drive parameters and explains its use where an operating frequency causes difficulty. That principle is useful background, but the word \u201cskip\u201d alone does not describe every drive’s transition behavior or all conditions under closed-loop control.<\/p>\n

The NENA 860S general-purpose current-vector inverter<\/a> is the product reference used in this article’s images. A photograph of its keypad does not verify the available skip functions, adjustment range or firmware menus. Request the applicable manual and confirm these points before treating them as requirements in a purchase or commissioning specification.<\/p>\n

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860S front keypad and controls. The visible keypad does not establish firmware menus or supported skip parameters. AI-generated visualization based on the actual website product photograph.<\/figcaption><\/figure>\n

Investigate vibration before selecting a band<\/h2>\n

A narrow speed range associated with greater vibration can justify further investigation, but it is not enough to diagnose resonance by ear. The problem may involve the motor, coupling, driven machine, support structure or connected piping. A qualified mechanical review should determine whether there is damage or a condition that must be corrected before a controlled operating test is appropriate.<\/p>\n

NPTEL’s lesson on forced vibration<\/a> explains the relationship between excitation, response and resonance, including the influence of mass, stiffness and damping. This is the physical reason a particular speed range deserves investigation. It does not supply an allowable vibration limit for a motor-pump assembly.<\/p>\n

Agree the measurement plan with the responsible specialist. Name the locations, instruments, operating conditions and stop criteria. Keep the machine load and process state alongside the vibration record. A result obtained with a valve position or process load that never occurs in normal operation may not establish a useful operating restriction.<\/p>\n

Keep the different frequency quantities separate<\/h2>\n

A VFD can have both an output-frequency setting and a switching or carrier-frequency setting. These are different quantities with different purposes. The skip band normally concerns the operating output-frequency range, while carrier frequency concerns the drive’s switching operation. Changing a carrier setting because a machine vibrates at one operating speed can introduce a second change without resolving the original cause.<\/p>\n

Likewise, electrical output frequency and measured shaft speed are not identical labels. Motor construction, slip and control method affect their relationship. Where the mechanical assessment identifies a critical shaft-speed range, the engineer needs a supported mapping to the drive quantity used by the skip function. Retain the conversion assumptions and measured verification rather than silently relabeling rpm as hertz.<\/p>\n

For the broader operating principles, use the guide to how a VFD controls motor speed<\/a>. The separate carrier-frequency article<\/a> addresses switching-related considerations. Keeping those subjects distinct makes the change record easier to review.<\/p>\n

Prepare a setting proposal that another engineer can check<\/h2>\n
\n\n\n\n\n\n\n\n\n\n\n
Record item<\/th>\nRequired information<\/th>\nWhy it matters<\/th>\n<\/tr>\n<\/thead>\n
Equipment identity<\/td>\nDrive, firmware, motor and driven-machine identifiers<\/td>\nPrevents using another revision’s menu or limits<\/td>\n<\/tr>\n
Problem range<\/td>\nMeasured speed, output frequency, load and vibration locations<\/td>\nConnects the proposed band to an observed response<\/td>\n<\/tr>\n
Band convention<\/td>\nCenter\/width or lower\/upper limits as defined by the manual<\/td>\nAvoids accidentally doubling or halving the exclusion<\/td>\n<\/tr>\n
Process coverage<\/td>\nRequired operating points and response to unavailable commands<\/td>\nChecks whether useful capacity is lost<\/td>\n<\/tr>\n
Transition review<\/td>\nApproved ramp behavior and test stop criteria<\/td>\nAddresses passage through the restricted region<\/td>\n<\/tr>\n
Acceptance evidence<\/td>\nBefore\/after measurements and responsible approval<\/td>\nProvides a reproducible handover<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

The table is a documentation checklist rather than a setting schedule. Each numerical limit should have an identified engineering basis. A convenient round number or a copied factory example is not a substitute for that basis.<\/p>\n

Check whether the process can tolerate the exclusion<\/h2>\n

A restricted band removes some steady operating choices. A fan or pump process may request those choices when demand changes. Determine how the controller behaves if the requested operating point lies within the band: observe the accepted command, actual output and resulting process variable. Do not assume the display of the original demand means the motor is running at that value.<\/p>\n

Closed-loop systems deserve particular attention. A process controller may continue increasing or reducing its command while the drive operates at a permitted boundary. Test the agreed demand cases and retain the response trend. If stable operation requires a forbidden point, the solution may require a mechanical or process change rather than a wider exclusion.<\/p>\n

Review minimum motor cooling, driven-machine operating limits and any independent protection with the relevant suppliers. A skip band should not be used to justify an otherwise prohibited operating point. The minimum-speed discussion<\/a> covers a different boundary: how low the motor and application may operate.<\/p>\n

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860S enclosure display view. A product photograph does not verify an application-specific vibration band. AI-generated visualization based on the actual website product photograph.<\/figcaption><\/figure>\n

Validate transitions under an approved test plan<\/h2>\n

Before changing settings, save the current configuration and identify the person authorized to restore it. Record existing ramp, speed-limit and control-source settings so that the skip change can be evaluated in context. Only the qualified commissioning team should perform the operating trial, with access and guarding controlled for the complete machine.<\/p>\n

Do not shorten a ramp merely because the word \u201cskip\u201d suggests moving faster. The acceptable transition depends on available torque, driven inertia, process effects and the mechanical assessment. Check both increasing and decreasing commands, using the approved limits. Stop the trial if the predetermined criteria are met, even if the drive itself has not tripped.<\/p>\n

Work involving the cabinet or electrical connections requires a separate safe-work arrangement. The HSE electrical safe-working guidance<\/a> supports planning, isolation and verification. A successful software setting change or a stopped motor does not establish a safe condition for internal inspection.<\/p>\n

Make the commissioning record useful for future maintenance<\/h2>\n

Record the final approved band values exactly as displayed and explain the manual’s interpretation. Attach the configuration backup, measurement points, instrument details and test conditions. Include the commands tested at each boundary and the resulting process behavior. Identify the person approving the mechanical assessment separately from the person entering drive parameters.<\/p>\n

If a coupling, impeller, base, motor or piping arrangement later changes, review whether the recorded evidence still applies. Do not assume an old exclusion remains appropriate after mechanical modifications. Preserve the earlier record so the next reviewer can distinguish a changed machine from a changed measurement method.<\/p>\n

For procurement, request a written response covering supported skip functions, usable frequency resolution, behavior under each control source and backup\/restore instructions. State that final bands will be supplied or approved for the application. This makes the supplier discussion specific without inventing capabilities for the product shown in the photographs.<\/p>\n

Check the control source used in daily operation<\/h2>\n

A commissioning trial from the local keypad may not exercise the same command path used during production. Identify whether normal demand comes from an analog signal, communications, preset speeds or another controller. Include the active control source in every saved test record. Confirm that the final restriction applies in the intended operating modes, according to the exact drive documentation.<\/p>\n

Review how maintenance personnel will recognize the restriction later. The operating instructions should explain why a commanded point may be unavailable and whom to contact if process demand repeatedly reaches that region. This prevents a future operator from removing the band merely because the commanded and displayed values differ. Retain the approval reference in the configuration record so the restriction has a traceable purpose, rather than becoming an unexplained inherited setting.<\/p>\n

Further learning: forced-vibration response<\/h2>\n

The NPTEL lecture below explains forced vibration and provides background for the mechanical review. It is included to help readers understand why machine response can change with excitation frequency. It is not a drive programming demonstration or an acceptance standard for the installation.<\/p>\n