You can e-mail us at [email protected] or call us at 785-737-8188
.5kW to 5kW Tower or Roof Mounted Units
Due to size, componets, and wieght these units are far more flexible
Why More Blades?
Having lots of blades really gives you the best of both worlds. In low winds speeds, systems with 6 to 12 blades create more leverage without increasing the length keeping the unit small enough to fit the roof of a building. They also have an advantage in high winds since they can't spin as fast so they end up actually protecting themselves by going slower than 3 blade systems would that either self-destruct or become very noisy. In high load conditions wind turbines experience cascade failure, The slower a blade spins the more likely it is to spin even slower and so on until it can no longer adequately catch wind as way holes form in the propellers diameter matrix area. Way holes are potential power producing area of a wind turbine propellers diameter that is allowing air to pass through in-between blade. This means that the slower the rotor spins under load the MORE likely it is to spin even slower and so on, Cascade Failure, until the blades are moving so slow they can't catch enough wind to be effective anymore because of excessive "Way-Holes". This is why two and three blade systems need a computer to keep the blades rev'd up to speed. The computer removes the load by making fast adjustments and this keeps the blades moving quickly eliminating cascade failure. However a computer is not necessary when you have lots of blades working for you since cascade failure is impossible with all of that surface area. So what we are dealing with is the need to increase costs and size to accompany the computer controlls and braking systems or simply put on more blades. This works to an extent, but on large turbines (such as our own larger units) that require far greater torque loads to turn them too many blades would create to much rotational mass and simply tear apart the generator head so the blade count is reduced to decrease this loading.
PMA
A PMA is the generator head of this smaller unit. Due to size and wieght this is the only way to make a unit this small function reliably. Permanent magnet alternators have a big advantage over a regular field-wound alternator. Permanent Magnet Alternators do NOT require a battery or grid connection to generate power. The permanent magnet alternator has a big advantage over a DC motor because the alternator has no brushes to wear out. Also, a standard field-wound alternator requires 40 watts to energize the field, whereas the permanent magnet alternator is just that, permanent so it does not require any wind spinning the unit to create a magnetic field to start generating so there is no wasted power.
Inversion
An inverter is used in these systems to convert direct current coming from the turbine head, through the charge controller, and batteries to alternating current. The function of an inverter is to change the voltage and frequency through the use of appropriate transformers, switching, and control circuits. There is some power loss in this process, slightly under 10%.
5kW+ Tower Mounted Units
The above mentioned system works well for small units but simply is impractical for larger system so we use what is described below
Introduction to Asynchronous Induction Generators
Most wind turbines in the world (even the massive 100 kilowatt commercial turbines and larger) use a so-called three phase asynchronous (cage wound) generator, also called an induction generator to generate alternating current. This type of generator is not widely used outside the wind turbine industry, and in small hydropower units, but the world has a lot of experience in dealing with it anyways.
The curious thing about this type of generator is that it was really originally designed as an electric motor. In fact, one third of the world's electricity consumption is used for running induction motors driving machinery in factories, pumps, fans, compressors, elevators, and other applications where you need to convert electrical energy to mechanical energy.
One reason for choosing this type of generator is that it is very reliable, and tends to be comparatively inexpensive. The generator also has some mechanical properties which are useful for wind turbines. (Generator slip and a certain overload capability).
The Cage Rotor
The key component of the asynchronous generator is the cage rotor. (It used to be called a squirrel cage rotor but after it became politically incorrect to exercise your domestic rodents in a treadmill, we only have this less captivating name).
It is the rotor that makes the asynchronous generator different from the synchronous generator. The rotor consists of a number of copper or aluminum bars which are connected electrically by aluminum end rings.
The rotor is provided with an "iron" core, using a stack of thin insulated steel laminations, with holes punched for the conducting aluminum bars. The rotor is placed in the middle of the stator, which in this case, once again, is a 4-pole stator which is directly connected to the three phases of the electrical grid.
Motor Operation
When the current is connected, the machine will start turning like a motor at a speed which is just slightly below the synchronous speed of the rotating magnetic field from the stator. Now, what is happening?
If we look at the rotor bars from above we have a magnetic field which moves relative to the rotor. This induces a very strong current in the rotor bars which offer very little resistance to the current, since they are short circuited by the end rings.
The rotor then develops its own magnetic poles, which in turn become dragged along by the electromagnetic force from the rotating magnetic field in the stator. We have a controller built in that detects when rpm of the turbine reaches above the motor speed rpm and it then allows current to flow to the motor which magnetizes the motor and since it is rotating above motor speed already the following occurs.
Generator Operation
Now, what happens if we manually crank this rotor around at exactly the synchronous speed of the generator, e.g. 1500 rpm (revolutions per minute)? The answer is: Nothing. Since the magnetic field rotates at exactly the same speed as the rotor, we see no induction phenomena in the rotor, and it will not interact with the stator.
But what if we increase speed above 1500 rpm? In that case the rotor moves faster than the rotating magnetic field from the stator, which means that once again the stator induces a strong current in the rotor. The harder you crank the rotor, the more power will be transferred as an electromagnetic force to the stator, and in turn converted to electricity which is fed into the electrical grid.
Generator Slip
The speed of the asynchronous generator will vary with the turning force (movement, or torque) applied to it. In practice, the difference between the rotational speed at peak power and at idle is very small, about 1 per cent. This difference in per cent of the synchronous speed is called the generator's slip. Thus a 4-pole generator will run idle at 1500 rpm if it is attached to a grid with a 50 Hz current. If the generator is producing at its maximum power, it will be running at 1515 rpm.
It is a very useful mechanical property that the generator will increase or decrease its speed slightly if the torque varies. This means that there will be less tear and wear on the gearbox. (Lower peak torque). This is one of the most important reasons for using an asynchronous generator rather than a synchronous generator on a wind turbine which is directly connected to the electrical grid.
Automatic Pole Adjustment of the Rotor
Did you notice that we did not specify the number of poles in the stator when we described the rotor? The clever thing about the cage rotor is that it adapts itself to the number of poles in the stator automatically. The same rotor can therefore be used with a wide variety of pole numbers.
Grid Connection Required
An asynchronous generator, because it requires the stator to be magnetized from the grid before it works has very interesting uses as a generator. It means no power flowing in on the lines, no power generated whichmakes it a fail safe system, and the motors we use are inverter duty crane motors which means that once power goes out the motors electric lap brake locks the motor so it can not turn (the brakes are made to be able to dead hold 20 plus tons) creating a double fault system that is permanently built in to it with another set of back ups to shut the turbine down in our electronic control enclosure.
The curious thing about this type of generator is that it was really originally designed as an electric motor. In fact, one third of the world's electricity consumption is used for running induction motors driving machinery in factories, pumps, fans, compressors, elevators, and other applications where you need to convert electrical energy to mechanical energy.
One reason for choosing this type of generator is that it is very reliable, and tends to be comparatively inexpensive. The generator also has some mechanical properties which are useful for wind turbines. (Generator slip and a certain overload capability).
The Cage Rotor
The key component of the asynchronous generator is the cage rotor. (It used to be called a squirrel cage rotor but after it became politically incorrect to exercise your domestic rodents in a treadmill, we only have this less captivating name).
It is the rotor that makes the asynchronous generator different from the synchronous generator. The rotor consists of a number of copper or aluminum bars which are connected electrically by aluminum end rings.
The rotor is provided with an "iron" core, using a stack of thin insulated steel laminations, with holes punched for the conducting aluminum bars. The rotor is placed in the middle of the stator, which in this case, once again, is a 4-pole stator which is directly connected to the three phases of the electrical grid.
Motor Operation
When the current is connected, the machine will start turning like a motor at a speed which is just slightly below the synchronous speed of the rotating magnetic field from the stator. Now, what is happening?
If we look at the rotor bars from above we have a magnetic field which moves relative to the rotor. This induces a very strong current in the rotor bars which offer very little resistance to the current, since they are short circuited by the end rings.
The rotor then develops its own magnetic poles, which in turn become dragged along by the electromagnetic force from the rotating magnetic field in the stator. We have a controller built in that detects when rpm of the turbine reaches above the motor speed rpm and it then allows current to flow to the motor which magnetizes the motor and since it is rotating above motor speed already the following occurs.
Generator Operation
Now, what happens if we manually crank this rotor around at exactly the synchronous speed of the generator, e.g. 1500 rpm (revolutions per minute)? The answer is: Nothing. Since the magnetic field rotates at exactly the same speed as the rotor, we see no induction phenomena in the rotor, and it will not interact with the stator.
But what if we increase speed above 1500 rpm? In that case the rotor moves faster than the rotating magnetic field from the stator, which means that once again the stator induces a strong current in the rotor. The harder you crank the rotor, the more power will be transferred as an electromagnetic force to the stator, and in turn converted to electricity which is fed into the electrical grid.
Generator Slip
The speed of the asynchronous generator will vary with the turning force (movement, or torque) applied to it. In practice, the difference between the rotational speed at peak power and at idle is very small, about 1 per cent. This difference in per cent of the synchronous speed is called the generator's slip. Thus a 4-pole generator will run idle at 1500 rpm if it is attached to a grid with a 50 Hz current. If the generator is producing at its maximum power, it will be running at 1515 rpm.
It is a very useful mechanical property that the generator will increase or decrease its speed slightly if the torque varies. This means that there will be less tear and wear on the gearbox. (Lower peak torque). This is one of the most important reasons for using an asynchronous generator rather than a synchronous generator on a wind turbine which is directly connected to the electrical grid.
Automatic Pole Adjustment of the Rotor
Did you notice that we did not specify the number of poles in the stator when we described the rotor? The clever thing about the cage rotor is that it adapts itself to the number of poles in the stator automatically. The same rotor can therefore be used with a wide variety of pole numbers.
Grid Connection Required
An asynchronous generator, because it requires the stator to be magnetized from the grid before it works has very interesting uses as a generator. It means no power flowing in on the lines, no power generated whichmakes it a fail safe system, and the motors we use are inverter duty crane motors which means that once power goes out the motors electric lap brake locks the motor so it can not turn (the brakes are made to be able to dead hold 20 plus tons) creating a double fault system that is permanently built in to it with another set of back ups to shut the turbine down in our electronic control enclosure.
You can e-mail us at [email protected] or call us at 785-737-8188
