Variable speed drives have transformed industrial motor control over the past two decades — delivering energy savings, process flexibility, and precise speed control that fixed-speed drives cannot match. But VSDs also introduce coupling challenges that were not present with DOL starters: torsional resonance at specific operating speeds, shaft current paths that can damage bearings, and the need to ensure coupling performance is acceptable across the full operating speed range rather than at a single point. This guide explains the VSD-coupling interaction in detail, with diagnostic steps for identifying speed-dependent vibration problems and engineering solutions — primarily through selection of the F-type flexible tyre coupling with appropriate stiffness grade and, where needed, the high-stiffness disc coupling for precision VSD servo applications.
The VSD Torsional Resonance Problem — Explained
Every coupling-drivetrain system has a natural torsional frequency determined by the coupling stiffness and the rotational inertias of the motor and driven machine. When a variable speed drive operates the motor across a speed range, the excitation frequencies from the motor (at running speed and harmonics) sweep through the entire range. At some operating speed — the resonant speed — the excitation frequency coincides with the system’s natural frequency, and torsional vibration amplitude is amplified by a factor of 5–15 or more.
In a fixed-speed drive, this resonance may or may not be present at the single operating speed. If it is present, it is always present; if it is not, it is never present. In a VSD drive, the resonance is almost certainly encountered every time the drive accelerates from zero to maximum speed — the question is how long the drive dwells at the resonant speed and whether the resulting vibration amplitude damages the coupling or connected machinery.
Identifying the Resonant Speed on Your VSD Drive
1
Sweep the Speed Range While Measuring VibrationProgramme the VSD for a slow ramp — 0.5 Hz/second from minimum to maximum speed. Simultaneously monitor the overall vibration velocity (mm/s RMS) at the motor bearing housing using a vibration meter with data logging. Plot vibration vs VSD output frequency. A peak in this plot at a specific frequency identifies the resonant speed.
2
Calculate the Expected Natural FrequencyUsing the formula fn = (1/2π) × √(kt / Jeq), calculate the expected natural frequency from the coupling datasheet (kt) and the motor and driven machine inertias (from motor datasheet and machine calculation). Compare the calculated value to the measured resonant speed — close agreement confirms torsional resonance as the mechanism; significant discrepancy suggests a different vibration source.
3
Determine Whether the Resonance Falls in the Required Operating RangeIf the resonant speed falls outside the required operating range (e.g. between 0 and 20% of maximum speed that the process never uses), the simplest solution is a VSD minimum speed setting that prevents operation at or below the resonant speed. If the resonance falls within the required operating range, either a skip frequency or a coupling stiffness change is required.
Three Engineering Solutions — Ranked by Preference
Change the elastomeric element hardness grade to shift the natural frequency outside the operating range. A softer element (80A vs 92A Shore) lowers the natural frequency — use when the resonance falls in the upper part of the operating range and a lower natural frequency would place it below the minimum operating speed. A harder element (98A Shore) raises the natural frequency — use when the resonance falls in the lower operating range and a higher natural frequency would place it above the maximum operating speed. This is the most durable solution — it eliminates the resonance without a process operating constraint.
Solution 2 (Effective): Set VSD Skip Frequency
Programme a skip frequency range in the VSD that causes the drive to accelerate rapidly through the resonant speed without dwelling. Most VSDs allow one or more skip frequency zones. Set the skip range to the resonant speed ±5–10% to provide adequate margin. The limitation of this approach is that the exclusion zone must not include process-critical speeds — check with the process engineer before programming the skip frequency.
Solution 3 (Targeted): Change Coupling Type
If elastomeric grade changes do not produce sufficient natural frequency shift, changing from an elastomeric coupling to a disc or diaphragm coupling — with fundamentally higher torsional stiffness — moves the natural frequency dramatically higher. For servo VSD applications where the natural frequency must be above the servo bandwidth, this is the standard solution. The trade-off is reduced vibration damping compared to an elastomeric design.
Coupling Selection Guide for VSD Applications by Speed Range
VSD Speed Range
Application
Coupling Stiffness Recommendation
Notes
0–50% rated speed (low-speed operation)
Pump, fan, conveyor at reduced speed
80A PU — lowest stiffness, best isolation
Natural frequency well above operating range — maximum isolation
0–100% rated speed (full range)
Standard VSD pump and fan drives
92A PU — medium stiffness
Check for resonance at mid-range; set skip frequency if found
Higher speed = higher centrifugal forces on element; check max RPM rating
0–100% for precision control (servo VSD)
CNC, servo positioning, test rig
Disc coupling — very high stiffness
Natural frequency above servo bandwidth; zero backlash required
Frequently Asked Questions
Why does a VSD drive produce more coupling vibration than a DOL starter?+
A DOL-started motor operates at a single fixed speed and produces torsional excitation predominantly at the electrical supply frequency (50 Hz) and its harmonics. The coupling system either avoids resonance at this fixed frequency or does not. A VSD sweeps continuously through a speed range, so the motor’s excitation frequencies sweep through the range too. At some speed within the range, the excitation frequency will pass through — or dwell near — the torsional natural frequency of the coupling-load system, producing a resonance condition that does not exist in a fixed-speed drive. This resonance crossing, if sustained, generates coupling vibration significantly above the baseline level.
Can I set a VSD skip frequency to eliminate coupling resonance?+
Yes. A VSD skip frequency (or prohibited speed range) programmes the drive to pass through a defined speed range rapidly without dwelling, preventing sustained operation at a resonant speed. The skip frequency is typically set at the identified resonant speed ±5% of that speed value. This is an effective solution when the resonance crossing is narrow and the exclusion zone does not conflict with the required process speed range. If the resonance falls in the middle of the required operating range, a coupling stiffness change (changing the elastomeric element hardness grade) is the more permanent engineering solution.
Does increasing coupling stiffness reduce VSD-induced vibration?+
Increasing coupling torsional stiffness raises the system’s torsional natural frequency. If the raised natural frequency now falls above the highest operating speed’s excitation frequency, the system operates in the sub-resonant region where vibration is attenuated — this reduces VSD-induced coupling vibration. However, a stiffer coupling also reduces vibration isolation effectiveness at frequencies below the natural frequency. The engineering objective is to place the natural frequency either well below the lowest operating excitation frequency (maximum isolation) or well above the highest (sub-resonant, stiff coupling). The middle ground — natural frequency within the operating range — is what produces vibration problems.
What vibration level is acceptable for a VSD-driven coupling system?+
ISO 10816-3 provides vibration severity limits for industrial machinery. For machines in the 15–300 kW range on rigid mounting: zone A (new machine, satisfactory) is below 2.3 mm/s RMS; zone B (acceptable for long-term operation) is 2.3–4.5 mm/s; zone C (allowable for short periods only) is 4.5–7.1 mm/s; zone D (risk of damage, shutdown) is above 7.1 mm/s. Apply these limits at the motor and driven-machine bearing housings. A VSD-driven pump showing coupling vibration above zone B threshold requires investigation and correction before the next planned maintenance window.
Why does my coupling vibrate more at some VSD speeds than others?+
Speed-selective vibration is the defining characteristic of torsional resonance. The vibration amplitude peaks at the speed where the excitation frequency matches the system’s torsional natural frequency and drops on either side of that speed. If you observe this pattern — vibration rising to a peak at one specific speed then declining as speed increases or decreases — torsional resonance is confirmed. A Campbell diagram analysis (plotting excitation frequency vs speed against the natural frequency) will identify the specific speed and excitation harmonic responsible.
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