Cut-in and cut-out speeds are the boundaries of the wind band a turbine is designed to operate in, and both surprise people. Below cut-in there is not enough energy in the wind to overcome the machine’s own losses, so the rotor turns slowly and produces nothing useful. Above cut-out the loads on the structure grow faster than the useful power does, so the machine deliberately shuts itself down to survive the storm.
Why the power in wind changes so steeply
The power available in moving air rises with the cube of wind speed. Doubling the wind speed makes roughly eight times the power available. That relationship dominates everything about turbine design and about siting. It means a modest improvement in average wind speed produces a large improvement in energy, which is why tower height and clean exposure matter so much. It also means the structural loads at high wind become enormous, and a machine sized to extract power from a gale would be absurdly overbuilt for the conditions it experiences most of the time. The economic design point sits far below the maximum wind the site will ever see, and protection handles the rest.
What happens below cut-in
At low wind the rotor may turn, but the torque available is less than what is needed to overcome friction, generator losses and the power electronics’ own consumption. Some machines idle freely in this region, some hold the rotor still. Either way, no useful energy is produced, and a grid-connected turbine may actually consume a small amount of power for its controls. Owners often watch a turbine spinning gently on a pleasant breezy day and assume it is generating. Usually it is not. The hours that matter for annual energy are the windy ones, and a site whose wind sits mostly below cut-in will produce very little regardless of how often the blades move.
The rated region and what rated power means
Between cut-in and rated wind speed, output climbs steeply with wind. At rated speed the machine reaches its nameplate output, and above that the control system limits power to protect the generator and drivetrain, holding output roughly flat as wind continues to rise. This is why nameplate power is a poor guide to annual energy: two turbines with the same rated output can produce very differently depending on the wind speed at which they reach it and how they behave in between. The meaningful comparison is the power curve across the whole range, combined with the wind speed distribution at your actual site and hub height.
Cut-out and how machines protect themselves
Above cut-out the machine stops producing and secures itself. Different designs do this differently: pitching blades to spill the wind, yawing or furling the rotor out of the wind, applying a mechanical or electrical brake, or some combination. The transition is deliberate and the control system usually waits for wind to fall and stay down before restarting, to avoid cycling in gusty conditions. Owners who see a turbine parked during the windiest hour of the month sometimes conclude it is broken. In most cases it is doing exactly what it was designed to do, and the energy given up in those hours is a small share of the annual total because such conditions are rare.
Why site data beats intuition
Whether a turbine will produce usefully at your location depends on the distribution of wind speeds at hub height, not on how windy the site feels. Ground-level wind is slowed and made turbulent by terrain, trees and buildings, and the difference between wind at ten feet and wind at eighty can be dramatic. Serious small-wind projects measure wind at the proposed hub height for a period before committing, or use modeled wind resource data as a screening step and accept the uncertainty. Skipping this step is the most common reason small turbines disappoint, and no amount of turbine quality compensates for a site that lacks wind.
What to check before you act
- Compare full power curves, not nameplate ratings.
- Get wind data at the proposed hub height, not at ground level.
- Expect zero output in light breezes and deliberate shutdown in storms.
- Ask how the machine protects itself above cut-out and how it restarts.
Common questions
My turbine is spinning but producing nothing. Is it faulty?
Probably not. Below cut-in the rotor can turn without producing useful power. Check the monitoring against the measured wind speed before assuming a fault.
Does a higher cut-out speed mean a better turbine?
Not necessarily. It reflects a design choice about loads and survival. Annual energy depends far more on the power curve in the common wind speeds at your site.
The practical takeaway
Judge a turbine by its power curve against your site’s measured wind distribution at hub height, and expect it to sit idle in light air and shut down in storms. Any work on a tower, rotor or generator is for qualified personnel with the machine secured.
References and further reading
- Department of Energy: consumer energy resources covering renewable generation options
- Department of Energy: estimating household energy use to size any generation source
- NFPA: the National Electrical Code and its provisions for interconnected generation
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
