Wind · Controls and shutdown
Every other generator on a property stops when you take the load away. A wind turbine speeds up. The electrical system is not attached to the machine to collect what it makes. It is attached to hold the machine back.
That single inversion explains the dump load, the second overspeed protection the code insists on, and why the off switch on a wind system is a short circuit rather than an open one.
What this page is
The other pages in this section give you arithmetic you can run at a kitchen table. This one does not, and the omission is deliberate.
Electrical requirements for wind systems in the United States derive from the National Electrical Code, principally Article 694, which covers wind electric systems and the generators, inverters, controllers and associated equipment that make them up. The code is adopted state by state, frequently amended locally, and different editions are in force in different places at the same time. There is no single national answer to publish.
So this page explains what the control system does and why it exists, well enough that you can hold a real conversation with an installer and recognise a design that has left something out. It gives no wire sizes, no fuse selections, no dump load resistance values and no shutdown sequence, because the correct ones belong to your equipment, your distances and your jurisdiction.
Self-reliance is knowing which jobs are yours. A household that can assess a wind resource, choose a tower, and explain why its turbine needs a second independent overspeed protection is not being passive by bringing in a qualified installer for the connections. It is being the kind of client who gets good work and can tell when they have not.
The equivalent page on the solar side, wiring, disconnects and code, covers the parts common to any property-scale electrical system. What follows here is what wind adds on top, and it is genuinely different rather than a variation on the same theme.
The inversion
Turning a generator that is delivering current into something is hard work. The current flowing in its windings creates a magnetic field that opposes the rotation producing it, and that opposition is felt at the shaft as resisting torque. The more current the generator delivers, the harder it is to turn.
This is the whole trick, and it runs in both directions. A wind turbine in steady operation is in a balance between the wind trying to accelerate the rotor and the electrical load trying to slow it. Take the load away and only one side of that balance is left.
A disconnected rotor in strong wind is not idling. It is accelerating, with almost nothing resisting it except its own friction and the air it moves through.
The outward force trying to pull a blade off its hub rises with the square of rotational speed. A rotor turning at twice its intended speed is not experiencing twice the load at the blade root. It is experiencing four times. The margin a designer built in gets consumed very quickly.
Everything else in the machine was designed around a working speed range: bearing loads, blade flex, the balance of the rotor. Once outside that range, each of those is operating somewhere it was never characterised, and nothing in the system is acting to bring it back.
Every other piece of equipment on a property is made safe by disconnecting it. That habit is correct almost everywhere, and it is exactly wrong here. Opening the circuit on a wind turbine in wind does not stop it. It removes the only thing that was holding it.
The horizontal-axis guide introduced this as the connected danger of an unloaded rotor. This page is the promised treatment of it, and the practical consequence is that every design decision downstream exists to guarantee the machine is never left without a load.
Where the power goes
This is the cleanest way to see the difference, and it matters because many households arrive at wind already fluent in solar.
When the bank is full, the controller simply stops drawing from the array. The panels sit in the sun at open-circuit voltage, producing nothing, entirely content. Doing nothing is a stable state for a photovoltaic module.
When the bank is full, the controller cannot stop drawing, because the drawing is what holds the rotor back. Doing nothing is not a stable state. The energy has to keep going somewhere, whether or not anything useful wants it.
So a wind charge controller is not really a charge controller in the solar sense. It is a traffic director. Its job is to decide, moment to moment, which of several places the turbine's output goes to, and to make certain that at every instant it is going somewhere.
When the bank wants charge, the bank is the load. When the bank is full, a diversion load takes over. The controller switches between them, and the rotor never notices the difference, which is precisely the point.
On a grid-interactive system the grid is the load, and it is an effectively bottomless one. That works beautifully until the grid goes down.
A grid-interactive inverter is required to stop exporting the moment it loses the utility, because energising a dead line puts line workers at risk. For solar that is simply a pause. For wind it means the load disappears at the exact moment nobody is present to notice, and often during the kind of weather that took the grid down in the first place.
A competently designed grid-tied wind system has an answer for that moment. Asking what the answer is, specifically, is one of the more revealing questions a household can put to an installer.
The solar section covers the conversion hardware itself in charge controllers and inverters, and the storage it feeds in battery banks. Both apply here. What does not carry over is the assumption that the source can be safely told to stop.
The diversion load
The dump load, also called a diversion load, is usually the least impressive object in the whole installation. A resistive heating element, in air or in water, wired so the controller can switch the turbine's output into it whenever the batteries do not want it.
It is easy to look at that and see waste. That reading has the purpose backwards. The dump load is not there to use energy. It is there to make sure the generator always has somewhere to push current, because a generator pushing current is a rotor being held back.
A diversion load is sized to absorb the machine's full rated output continuously, not a convenient fraction of it. The moment it is needed is the moment the wind is strongest and the batteries are already full, which is the least forgiving combination the system will ever see. An undersized element is a component that fails precisely when it is being asked to do the one job it exists for.
Every watt diverted becomes heat, and that heat appears wherever the element is. Put it in a water tank and it becomes hot water, which is the closest this arrangement comes to being useful. Put it in air and it warms a space, helpfully in January and less so in July. Either way it needs to be somewhere that can accept sustained heat safely and where nothing flammable is nearby. This is a siting decision, not an afterthought.
A tempting idea is to divert surplus into something the household wants running anyway. The difficulty is that a dump load has to be an unconditional load. Anything with a thermostat, a switch, a control board or a duty cycle can decide to stop accepting power, and it will make that decision without reference to what the rotor is doing. A diversion load is resistive, permanently connected to its controller, and free of any logic of its own.
Less than people expect. Diversion only happens when the bank is full and the wind is blowing, which on most properties is a modest share of the year. A well-matched system spends the great majority of its time putting energy into storage or into the house.
Which is the right way to think about the cost of it. A brake that is rarely used is not wasted equipment. It is a brake.
The requirement worth naming
Everything described so far depends on one controller and one dump load working correctly. If the controller loses its mind, or a relay welds, or the heating element burns open, the turbine is unloaded. In a gale. Probably at night.
The National Electrical Code does not treat that as an unlikely misfortune to be hoped against. Article 694 requires that a wind system relying on a diversion load controller also include a second and independent means of preventing overspeed and overvoltage.
Independent is the operative word. A backup that shares the controller, the relay, the wiring or the element with the primary is not a second means. It is the same single point of failure wearing a second label.
Practitioners in small wind have observed for years that the requirement exists in the code and that few installations actually carry it. The reasoning offered is usually that modern controllers are reliable and that somebody keeps an eye on the system.
Both of those things can be true and neither is a substitute. Controllers are reliable in the sense that they fail rarely, not in the sense that they fail conveniently. Keeping an eye on the system works during the hours somebody is home and awake, which is not when the strongest winds of the year arrive.
If one question from this page is worth carrying into a conversation with an installer, it is this one. Ask what the second independent overspeed protection is, ask to be shown where it is, and ask what happens to it if the primary controller loses power entirely.
The specific arrangement belongs to the equipment, and manufacturers approach it differently. What a household can usefully evaluate is whether the backup depends on anything the primary also depends on. Three questions get most of the way there.
Does it need the primary controller to be working? If the answer involves the controller detecting a fault and then acting, the two are not independent. A mechanism that engages passively when something is absent is stronger than one that engages when something is detected.
Does it share the dump load? A single element serving both the routine diversion duty and the emergency backup gives one component the ability to disable both protections by failing once.
Is it tested, and how often? A protection that has never operated is a protection nobody has confirmed works. Whatever the manufacturer specifies for exercising it belongs on the same maintenance schedule as the tower inspections.
The aerodynamic protections described in the horizontal-axis guide, furling and pitching among them, are part of this picture rather than separate from it. A machine that limits its own speed mechanically is less dependent on its electronics behaving, which is one of the better reasons to prefer a design that does so.
Bringing it to a stop
Limiting speed in a gale and stopping a machine so somebody can work on it are related but separate problems. Between them they are handled by three mechanisms, and a well-designed system carries more than one.
Automatic, continuous, needs no power or decision
Furling the rotor out of the wind, changing blade pitch, or designing blades that stall as wind speed rises. These act on the machine's ability to capture energy in the first place, which is the most reliable place to act because it requires no electronics, no signal and no functioning controller. It is the first line of defence and, on many small machines, the main one.
The off switch that closes rather than opens
Connecting the generator's output terminals to each other. Shorting the windings together makes the generator extremely hard to turn, producing strong resisting torque and slowing the rotor down, often to a standstill in light and moderate wind.
This is unusual enough that the code accommodates it explicitly. Article 694 permits a shorting switch or plug to serve as the disconnecting means for systems that control turbine speed through their output circuit. In ordinary electrical work a short circuit is the failure. Here it is the brake, and that inversion is the clearest single indication that wind systems do not follow the usual rules.
The final hold, and the one maintenance depends on
A physical brake, sometimes operated by a lever or release at the base of the tower. Its role is less about slowing a spinning rotor than about keeping a stopped one stopped while somebody is near it. Electrical braking holds a machine against the wind. A mechanical lock holds it against everything, including the electrical system being worked on.
The order in which these are applied, the wind conditions under which each is safe to use, and how long a machine must be held before anyone approaches it are all specific to the equipment. Applying electrical braking to a rotor running fast is not a neutral act, and manufacturers state limits on it for good reason.
The shutdown procedure for a wind turbine lives in that turbine's manual and belongs to someone trained on it. A page that offered a generic version would be describing a machine nobody owns.
What the code expects
Article 694 was added to the National Electrical Code in 2011 for small wind systems, and in 2014 the word small was removed from its title so that it applies to wind electric systems of any size. What follows is not a compliance checklist. It is the handful of provisions that explain why a competent installation looks the way it does.
Equipment such as inverters, controllers and batteries needs a means of disconnection, and it has to be readily accessible. A switch inside a nacelle a hundred feet up plainly is not, which is why the code specifically allows the disconnecting means to sit on or adjacent to the tower, on the outside of a building, or immediately inside where the conductors enter. Equipment within the nacelle itself is excepted from the requirement, for the same practical reason.
Where equipment is fed from more than one source, the disconnects are grouped and marked. This is a provision written for the person who arrives at an emergency and needs to make something safe without discovering a second live supply by accident. On a property with wind and solar and storage, it is the difference between a system somebody else can work on and one only its owner understands.
Direct current turbine output circuits run inside a building are required to be in metal raceway or metal enclosures, from the point they penetrate the structure to the first readily accessible disconnect. Conductors carrying power in from an outdoor source are live before any protective device inside the house has a say, and the metal is there to contain a fault in that stretch.
Wind turbines and their associated equipment are required to be listed or field labelled for the application, and the work is expected to be carried out by qualified persons. The relevant product standard for small wind turbine systems is UL 6142, and certification against the national small wind performance and safety standard is separately searchable, as noted in the guide to horizontal-axis machines.
Surge protective devices are required, and the reason is standing in the field. A wind tower is a tall conductive structure in open ground, usually the tallest thing on the property, connected by wire to the house. Grounding, bonding and surge protection are what keep a strike or a nearby surge from arriving at the electronics indoors. The specifics are an engineering matter, and they are one of the strongest arguments for the installer being someone who does this regularly.
Which edition of the code is in force, what the local authority having jurisdiction requires on top of it, and what the utility demands before anything is connected to its network all vary by place. None of it can be settled from a page. The provisions above are here so that the conversation with the people who do settle it is a conversation between equals.
The conversation
None of these requires electrical training to ask, and the quality of the answers tells you most of what you need to know. Ask them before money changes hands, and prefer written answers to confident ones.
What is the second independent means of overspeed protection, and where is it? The one question from this page that matters most. A good answer names a specific mechanism and can point at it. A poor answer explains why the controller is reliable.
Where does the diverted heat go, and what is the element rated for? Confirms both that the dump load can take full output and that somebody has thought about where several kilowatts of heat will end up on a windy August afternoon.
What holds the rotor if the grid or the inverter drops out? On a grid-tied system this is the scenario where the load vanishes without warning. There should be a specific answer rather than a reassurance.
How is the machine stopped for service, and who does it? This connects directly to the tower decision. A stop procedure that requires somebody at height is a different proposition from one carried out at ground level.
Is the equipment listed for this application, and to what? Listing is a code requirement rather than a preference, and it is a question with a documentary answer.
Which code edition applies here, and has the local authority seen a system like this before? An installer who has been through the process locally is worth a great deal more than one who has not, and this question surfaces that quickly.
Not to install anything. To be able to look at a proposed system and notice that the second overspeed protection is missing, or that the dump load is undersized, or that nobody has explained what happens when the grid fails during a storm.
Those are the omissions that turn a working installation into a damaged one, and they are all visible to a household that understands why the parts are there. That is the whole purpose of this page.
Next
Control is the part that keeps everything else safe. It sits alongside the decisions about what the machine is and what holds it up, and it connects to the storage and conversion hardware shared with the rest of the property.
How the machine is reached, which decides what a shutdown for service actually costs each time.
Read the guideFurling and overspeed protection at the machine itself, and how to read a specification sheet.
Read the guideThe electrical groundwork common to any property-scale system, on the solar side of the section.
Read the guideIf the wind resource itself is still an open question, start at assessing a wind resource, and read why most residential wind disappoints before committing to anything on this page.
Sources
No wire size, overcurrent device, dump load resistance, component rating or shutdown sequence is published on this page. Those depend on the specific equipment, the installation, and the code edition and amendments in force locally, and the correct values come from the manufacturer's documentation and from the qualified person doing the work. The code is adopted and amended jurisdiction by jurisdiction, so the authority having jurisdiction is the final word rather than any summary of the model code, including this one.