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Technology / Technical guide
Wind Generator Shaft Critical Speed & Whirl
A generator shaft can spin while its lateral position also moves. That whirl motion and the shaft’s rotation are different quantities. Critical speed belongs to the defined rotor-bearing dynamics, not to a power rating or one unexplained vibration peak.
Compare excitation with modes in a compatible frame.
Rotational speed creates a possible excitation frequency, while bearing support and rotating-system dynamics shape the modal response. Their relationship must be defined before a crossing is interpreted.
01 / Spin and whirl
A rotating cross-section can move laterally.
Spin describes rotation about the shaft’s own axis.
Spin describes rotation about the shaft’s own axis. Whirl describes lateral motion of that axis or centreline around a reference position. Forward and backward whirl compare the orbit direction with the stated spin direction.
A shaft can therefore have a motion frequency different from its spin frequency. A plotted orbit or lateral mode is not simply another view of the torque that drives axial rotation.
02 / System boundary
Support properties are part of the dynamic model.
The model includes the shaft, represented mounted masses and bearing/support properties.
The model includes the shaft, represented mounted masses and bearing/support properties. Mass distribution, stiffness and damping enter the motion description. A bare shaft and the same shaft in its generator supports are different systems.
COMSOL and Ansys distinguish rotor-bearing behaviour and critical-speed mapping. A bearing catalogue load rating is not the same quantity as the dynamic support coefficients used in such a model.
03 / Critical speed
Resonance depends on excitation and state.
An excitation can coincide with a relevant modal frequency at a particular rotational state.
An excitation can coincide with a relevant modal frequency at a particular rotational state. For a synchronous one-per-revolution component, its frequency is n/60 Hz. The applicable mode and the frame used for its frequency must be identified.
A hypothetical constant modal frequency of 5 Hz would intersect that ideal one-per-revolution line at 300 rpm. This is arithmetic for a simplified illustration, not a generator rating or an allowable operating speed.
04 / Modal branches
The stationary spectrum need not remain unchanged.
Rotation and gyroscopic coupling can alter modal frequencies and split directional behaviour.
Rotation and gyroscopic coupling can alter modal frequencies and split directional behaviour. A stationary natural-frequency calculation is therefore not automatically a complete critical-speed map over changing spin speed.
A Campbell diagram represents modes and excitation lines over a stated range. A line crossing identifies a modelled condition for interpretation; it does not independently specify the resulting amplitude or safe separation margin.
05 / Response & stability
Forced resonance differs from growing self-excited motion.
A resonant forced response depends on forcing, damping and the relevant mode.
A resonant forced response depends on forcing, damping and the relevant mode. Dynamic instability is a different question: the represented system can sustain growing motion through its internal interactions. The labels should not be used interchangeably.
Ansys’ terminology separates critical speed and stability. The gyroscopic guide explains one directional coupling principle, without prescribing bearing or shaft changes.
06 / Scope
A simplified model is not an actual machine limit.
Assumptions about support, geometry, damping and the observation frame constrain the predicted result.
Assumptions about support, geometry, damping and the observation frame constrain the predicted result. A frequency peak measured elsewhere on the turbine does not by itself identify a generator shaft critical speed.
No actual MARSTON shaft speed, mode or limit is given here, and no procedure for passing through resonance, changing a support or testing a rotating shaft is provided. This topic is distinct from the vibration-record guide.
Continue the review with Wind Rotor Gyroscopic Effects During Yaw.
Continue the review with Wind Blade Mode Shapes & Natural Frequencies.
Continue the review with Wind Turbine Assembly Datums & Tolerance Stack-Up.
Review two-inertia torsional vibration as a separate shaft-system dynamics topic.
General background: COMSOL: Rotor-bearing critical-speed concepts · Ansys: Whirl, critical speed and stability distinctions. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Spin | Rotation about the shaft axis | Not the same as lateral whirl. |
| Whirl | Lateral orbit of the represented centreline | Direction is relative to defined spin. |
| Support coefficients | Dynamic stiffness/damping representation | Different from catalogue load capacity. |
| Critical speed | Excitation/modal coincidence in the defined model | Not automatically an operating limit. |
| Stability | Growth/decay behaviour of motion | Distinct from one forced resonance crossing. |
Engineering questions.
Is whirl frequency always spin frequency?
No. Lateral motion and axial spin are different quantities.
Can a stationary shaft mode identify all critical speeds?
Not generally; rotation can change the modal behaviour.
Does bearing load capacity give dynamic stiffness?
No. Those quantities belong to different descriptions.
Is the 300 rpm example an actual limit?
No. It is a simplified hypothetical frequency intersection.
Does a crossing alone give vibration amplitude?
No. Forcing, damping and the relevant mode are also needed.
Are critical speed and instability identical?
No. Forced resonance and growth of self-excited motion are distinct questions.
Technical enquiry
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