Solar panels are mostly seen stationary and not spinning. There are several factors behind the workings of solar panels, when and how they work.
When we drive from town to town, there’s always a wind turbine insight. However, even on some windy days, the blades don’t turn as they should do.
This article covers all the aspects of;
- Why wind turbines stop
- When and why they start
- The speed they operate at
After reading this article, whenever someone comments ‘why do wind turbines stop’ you’ll be able to answer like an expert.
Why Do Wind Turbines Stop?
Wind turbines do not run all the time. Often, a wind farm will have turbines that are turning and some that are not. There are a few reasons for this.
- Low power demand.
- Scheduled Maintenance.
- Breakdown.
They Stop Due To No Demand
The power grid is in a constant state of flux. Demand rises and falls constantly. Since power plants are difficult to shut down, when demand is low the wind turbines are shut down.
The most important reason for this is to avoid an unbalanced power supply on the grid. The other is to reduce unnecessary wear on the turbines.
Low Demand
When there is low power demand on the grid, turbines will be shut down. This reduces the potential of imbalance in the grid due to an over-supply of power.
It also conserves the life of the turbine. Unproductive running time wastes the turbine’s lifetime. It also increases the need for unnecessary maintenance.
When demand increases, these turbines will be put back in service providing the required power.
Mechanical Issues
Like any mechanical device, a wind turbine requires routine maintenance. Scheduled maintenance on a turbine takes place for one day, twice a year.
These days are scheduled for low demand or low production seasons. Barring any breakdowns, a turbine should available for service 363 days a year.
An idle turbine may be undergoing routine or unscheduled maintenance. Turbines do require regular maintenance to gearboxes, power transmission devices, braking devices, and control systems.
As with any mechanical device, breakdowns occur. An idle turbine may have suffered a malfunction and is in the process of being repaired. As you can imagine, repairing such a gigantic device is quite an undertaking.
A breakdown may leave a turbine inoperative for several weeks as parts are ordered and repair schedules are arranged.
Why Do Wind Turbines Turn So Slowly?
There are two reasons that turbines APPEAR to be turning slowly.
- Scale.
- RPM and Torque.
Big things look slow
Would you believe that the tip of a wind turbine blade is actually traveling at between 110 and 200 MPH? No joke! It is their immense size that makes them appear to be moving so slowly.
Remember, the big turbines have blades over 150 feet long. That creates a diameter of 300 feet. A typical turbine rotates between 10 and 20 revolutions per minute. Some quick math shows that…
- A 300 foot diameter times Pi (3.14) gives circumference of 942 feet.
- 942 feet times 10 rotations a minute is 9420 feet per minute.
- 9420 divided by 5280 feet (one mile) is 1.78 miles per minute.
- 1.78 miles a minute times 60 minutes is 107 miles per hour.
And 10 rpm is a slow turbine. At 20 RPM the blade tip would be moving 214 MPH!
Rotation and a Gearbox
While the blade assembly does not spin at a very great rate, they do produce a lot of torque. The slow speed and great torque of a 3-megawatt turbine can produce approximately 4000 horsepower.
The gearbox within a wind turbine nacelle is a speed multiplying device. With a gear ratio of about 90:1, the gearbox can take the 20 RPM of high torque input from the turbine blade assembly and supply the generator with 1800 RPMs of power-producing speed.
Wind turbines stop and start regularly. Starting is the easy part. Stopping is a different case. Let’s look at how and why commercial turbines are slowed, stopped, and restarted.
Why do turbines not turn in slow wind speeds?
A wind turbine blade assembly can weigh over 25,000 pounds. It takes a lot of wind energy to move that much weight. Even a high-tech blade assembly takes a wind speed of 3 to 5 MPH to start the blades moving.
At such low speeds, the rotation created will not be enough to produce power. In most cases, a wind turbine has a production CUT IN speed of 5 to 10 MPH.
Some turbines will lock the blades in conditions that will not produce power.
At What Wind Speed Do Wind Turbines Stop?
If the blade assembly of a turbine is not able to achieve a specified speed it will not produce electricity. A rotational speed of 12 to 15 RPM is required for a turbine to start producing power.
It takes a wind speed of 6 to 9 MPH to achieve the energy required for a turbine to start producing power. At speeds below that the turbine will not move. On the opposite side of the spectrum is too much wind.
At wind speeds over 50 or 55 MPH, the stresses on the components of the turbine become too great and it will shut down.
Optimum Speed
Every turbine has an optimum operating speed. Most turbines only produce their rated power at wind speeds of 15 to 30 MPH.
At wind speeds above or below the optimum range, a turbine may turn very slowly or even stop. There are practical and safety reasons for this.
- Low wind power.
- Cut In Speed.
- Power Curve.
- High Wind Shut Down.
Why Do Wind Turbines Stop At High Speeds?
Slowing the blades
The power curve of most commercial wind turbines reaches its maximum rated output at around 30 MPH. At speeds above the maximum output, the unit is in danger of damaging the power-producing equipment.
An anemometer in the tail of the nacelle measures wind speed. As speeds approach the maximum limit the turbine control systems may feather the blades or furl the blade assembly.
This will reduce the force of the wind on the blades and slow the device.
Stopping the blades
In severe winds, it may become necessary to stop the turbine assembly completely. At wind speeds over 50 MPH not only is the power-producing equipment in danger of damage, but the blades, hub, and shaft assemblies are in danger.
The blade assembly will be slowed using furling, feathering, and electromagnetic braking to slow the assembly.
Finally, the mechanical braking system will bring the assembly to a complete stop. When wind speeds return to a safe condition the turbine will be restarted.
How Do Wind Turbines Stop?
There are three methods of slowing and stopping a wind turbine. In most cases, a combination of methods is employed to slow and stop a turbine. Due to the massive scale of modern turbines, stopping is a slow process.
- Blade pitch adjustment.
- Electromagnetic braking.
- Mechanical braking.
Blade Pitch
The pitch of a wind turbine blade is the angle of the blade relative to the plane of rotation. By adjusting the pitch angle, the force of the wind on the blade can be increased or decreased.
In some cases, the blade angle can be almost parallel to the direction of the wind. This is referred to as feathering the blades. At such an angle the wind will produce little or no rotation of the turbine.
Feathering is usually only used in high wind, emergency situations, or during a shut-down.
Electromagnetic braking
By increasing the resistance of the generator in the turbine’s nacelle, the speed of the blades can be slowed. This is known as Electromagnetic Braking.
While this method of braking may not be able to stop the blades, it can help slow them significantly.
In the case of small, domestic wind turbines, this is the main method of slowing the blades.
Mechanical braking
This means of stopping a turbine is similar to the brakes on your car. Large braking discs and pads are used to slow either the low-speed turbine shaft or the high-speed power generation shaft.
Stopping the low-speed shaft is difficult due to the enormous torque produced by the rotating blade assembly. Braking at the high-speed shaft requires handling less torque, but risks damaging the gearbox if done too abruptly.
Both locations require dissipating the same amount of energy.
How Does a Wind Turbine Start?
Starting a wind turbine is not as complex as stopping it. Before a turbine is put into service there are a few conditions that must be met.
- No maintenance is being performed.
- All systems are in operating order.
- There is a demand for power.
- Wind conditions are appropriate for operation.
When all conditions have been met, the braking systems will be released. The furling system will then face the blade assembly into the wind. The blade pitch system will slowly adjust the blades to make use of existing wind.
As motion begins, blades will continually be adjusted until the rotation speed reached production speed. It is a simple, but time-consuming process.
Do Wind Turbines Work Without Wind?
Every electricity-generating device needs an engine to provide power. With a portable generator, it is a gasoline engine. In a power plant, it is gas or coal-powered steam turbines. For a wind turbine, it is wind.
The wind provides linear kinetic energy that the turbine converts to rotational energy to drive an electric generator. Without wind, the turbine has no fuel and will not run.
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