Why should tower resonance be checked across the rotor-speed range?

The short answer

A repeating excitation changes frequency as rotor speed changes, so a tower mode may be approached away from rated speed. The review should distinguish a brief startup crossing from sustained operation near a mode and assess the resulting response. A favorable rated-speed comparison alone does not establish dynamic suitability over the complete operating envelope.

Map the relevant excitation across speed

Rotation frequency is rotor rpm divided by 60. A mechanism producing several repeating events per revolution creates the corresponding order of excitation. Its physical source and amplitude must be established rather than assumed from a plotted line.

Hypothetically, a once-per-revolution excitation equals a 2 Hz mode at 120 rpm. This example locates a possible crossing only; it assigns no operating speed or modal frequency to a MARSTON turbine.

Examine duration and actual response

A crossing can occur during acceleration or deceleration without the rotor remaining there. Sustained dwell provides a different opportunity for response to build. Damping, force magnitude and the rate of passage all influence the outcome.

Repeated transient passages can also matter for fatigue. Therefore, neither the presence of a crossing nor its brief duration is, by itself, a complete acceptance decision. The structural calculation needs the actual operating behavior and loading evidence.

Resolve the support decision without improvised settings

Request the assessed speed envelope, relevant modes and the response or fatigue criteria used. If a concern is identified, the turbine and tower designers should evaluate structural or control alternatives together.

Do not create a speed-exclusion band or change acceleration settings from a generic Campbell diagram. Those changes affect turbine behavior and need authorized design review. The useful outcome is demonstrated suitability of the defined assembled support across the relevant states, including startup, stopping and permitted sustained operation.