The Toshiba H7 installation’s vibration rises from about 0.2 in/sec to above 0.8 in/sec after several minutes at roughly 650–750 rpm, while coast-down through that band shows no spike. That pattern points first to a speed-dependent mechanical response that builds with time, not a drive fault established by the reported data. Verify the actual motor speed and whether the programmed jump-frequency band is active; then test dwell time and vibration at repeatable operating conditions.
Read the vibration trend against speed and dwell time
The most useful discriminator is vibration versus actual shaft speed over time—not the drive’s displayed speed alone. The reported increase from 0.2 to more than 0.8 in/sec is a substantial trend, but the evidence does not specify whether those readings are peak, RMS, or another measurement convention, nor the sensor location or direction. Keep those conditions consistent before comparing runs.
| Observation | What it suggests | Where to read or verify |
|---|---|---|
| Vibration rises while held at 650–750 rpm | A response that accumulates during dwell, such as a mechanical resonance; a developing rub remains a competing possibility. | Trend vibration, actual motor rpm, and elapsed time at a fixed sensor point. |
| No spike while coasting down through 750–650 rpm | A brief passage may not excite a response that builds during a sustained hold. This does not rule out resonance. | Compare a controlled dwell with a repeatable ramp through the same speed range. |
| VFD indicated speed differs from motor speed by about ±20 rpm | The displayed value may be a command or estimate, not a direct shaft-speed measurement. The mismatch can shift where a speed exclusion must be set. | Measure motor shaft rpm with an independent tachometer and compare it with the drive display and commanded frequency. |
| Vibration occurs in a narrow speed band | A speed-related mechanical mode is more likely than a control oscillation that persists across a broad range. | Log the onset and decay speeds during both acceleration and deceleration. |
Do not treat 0.8 in/sec as an allowable or trip threshold from these observations. Compare measurements with the applicable site limit and the same instrument convention, mounting point, axis, and operating condition.
Understand why a mechanical mode can grow while held
A rotating fan train, motor, shaft, gearbox, and support structure form a coupled mechanical system. Its natural response can become large when operating speed excites a mode. The response may take time to approach its steady level because energy accumulates in the structure while damping dissipates it. A dwell of several minutes can therefore produce a rising vibration, while a short coast through the band may not show a peak.
The installation includes a 24-foot fan, right-angle gear, 100-inch composite shaft, horizontal TEFC motor, and tall fiberglass structure. Those components provide several possible contributors to a system mode; the vibration measurement at the motor end does not by itself identify which component is moving or what force excites it. Use measurements at multiple points to distinguish motor vibration from motion transmitted through the drive train and support.
A localized resonance should be expected to concentrate around a speed region. By contrast, a controller oscillation would generally not be confined to a narrow mechanical resonance band. The reported frequency-selective vibration and dwell-time increase make mechanical response the first branch to investigate. A developing rub is another possible explanation for vibration that grows over time, so listen and inspect for contact, temperature rise, and changing mechanical condition rather than assuming every time-dependent increase is resonance.
Separate heat-related progression from control behavior
Vibration growth over minutes can result from increasing mechanical response, changing clearances, or a developing contact condition. The rate alone does not diagnose a thermal fault. Track temperature at relevant bearing, gearbox, motor, and structural locations using the site’s established instruments and limits; correlate those readings with vibration and actual speed.
Likewise, harmonics or motor-voltage waveform features should not be treated as a vibration diagnosis without a matching mechanical or electrical measurement. Load and line reactors are reported as installed, but their presence does not establish that the motor, cable, or drive waveform is the cause—or exclude every power-quality issue. If electrical behavior remains suspect, record drive output frequency/current and motor terminal voltage using suitable measurement equipment and a qualified procedure; do not infer torque ripple from a displayed rpm difference alone.
The stated ±20 rpm difference needs a measurement-based explanation. In an open-loop induction-motor drive, rotor speed differs from synchronous speed because of slip, and drive displays may report commanded or calculated speed rather than measured shaft rpm. Establish what the H7 display represents from its manual and compare it with a tachometer. Do not convert the 650–750 rpm interval into a frequency skip setting using a guessed pole count or slip value.
Test the speed band and configure the H7 jump frequencies
Use the manufacturer’s H7 manual to locate the Jump Frequencies function and determine how the drive defines its band and response. The reported drive is a Toshiba H7 VT130H7U415KB. Do not assume a generic VFD menu name or parameter number applies to this model.
- Record the existing frequency command, displayed speed, actual shaft rpm, output current, vibration, sensor point, and elapsed time. Record the measurement convention and operating load for each reading.
- With the equipment in a controlled test condition, sweep through the suspected band in both directions and log where vibration begins and ends. Repeat at a consistent ramp rate. Separately test a sustained hold only where operating procedures permit.
- Convert the empirically identified shaft-speed band to the H7’s frequency units using the motor and drive data specified in the manual. Confirm motor pole count, rated frequency, and speed relationship from the motor nameplate and drive configuration; account for measured slip rather than guessing.
- Check the existing
Jump Frequenciesentries, limits, and enabled behavior against the manual. The installation report says the motor speeds were programmed out, yet the vibration was still observed in the band. Resolve whether those entries are active, whether the command can dwell inside the band, and whether the test occurred before or after configuration. - Configure a frequency exclusion band that covers the measured resonant region, following the H7 instructions. The described drive behavior may hold the command below the threshold or jump past it; choose the mode that fits the process requirement and confirm transitions do not create an unacceptable operating response.
- Save the change and repeat the test with the same load, speed sweep, sensor location, and dwell conditions. Keep a record of the prior and revised settings so the result is attributable to the change.
A jump frequency is an operating avoidance feature, not a repair for a damaged bearing, loose mounting, shaft problem, gear defect, or structural weakness. If vibration remains high outside the excluded band, or mechanical condition is changing, investigate the machine train rather than widening the skip range until the symptom disappears.
Verify the fix under repeatable operating conditions
Verification requires demonstrating both that the drive avoids the problematic region and that the vibration remains acceptable through permitted speeds. Compare the before-and-after trend at the same sensor locations and axes, using the same instrument and measurement convention. Confirm actual rpm independently, since the drive’s indicated speed differs from motor speed by about ±20 rpm in the reported installation.
Test acceleration, steady operation above and below the band, and deceleration. Verify the configured jump response does not allow a stable command to sit inside the empirically identified range. Record vibration through the transition as well as after the machine settles. If the machine must routinely operate at a speed within the band, a skip setting may not meet the process need; resolve the underlying mechanical response with a qualified vibration analysis and equipment review.
Compare the vibration and temperature trends with site acceptance limits and the applicable equipment documentation. Because no acceptance limit or vibration measurement convention is specified here, the reported 0.2 and 0.8 in/sec values are baseline observations, not pass/fail criteria.
Check the recurrent misdiagnoses before changing drive controls
Do not change control tuning simply because vibration appears while a VFD is operating. First determine whether the H7 configuration actually uses a closed-loop control mode and feedback device. The installation evidence identifies a speed range and mechanical train but does not identify a feedback loop or an active control instability. In a scalar, open-loop arrangement, there may be no speed-feedback loop to tune.
Long motor cables can produce high-frequency voltage reflections. Those waveform effects occur at a frequency scale unrelated to the low-speed mechanical vibration described here and should not be mistaken for a mechanical oscillation. If waveform distortion is measured, assess it with proper instruments and relate it to a demonstrated motor or drive problem before making parameter changes.
Also avoid treating installed reactors as proof that all vibration mechanisms have been eliminated. Reactors address electrical waveform characteristics; a mechanical natural frequency can still be excited at a particular speed. Conversely, a jump-frequency setting can mask operation in the band but cannot establish whether the source is the fan, gearing, shaft, motor, or support structure.
Frequently asked questions about Toshiba H7 vibration
Can a Toshiba H7 jump frequency stop vibration at 650–750 rpm?
It can keep the commanded frequency from dwelling in an empirically identified resonant band when configured as the H7 manual specifies. Confirm actual shaft rpm, active settings, and vibration after the change; a skip band does not repair a mechanical defect.
Does no vibration spike during coast-down rule out resonance?
No. A short passage may not provide enough dwell to build the vibration response. Compare a controlled hold with repeatable acceleration and deceleration sweeps.
Can the VFD’s speed display be off by 20 rpm?
The installation reports about ±20 rpm difference between indicated and actual speed. Determine whether the H7 display is command, calculated speed, or measured speed from its manual, then compare it with a shaft tachometer.
When should I stop operating and contact Toshiba support?
Stop the test if vibration continues to rise, a rub or mechanical damage appears, or the machine exceeds site operating limits. Escalate to Toshiba support with the VT130H7U415KB model, motor nameplate data, active jump-frequency settings, actual rpm, drive records, and vibration and temperature trends.