Home Masterbuilder Low RPM Low Wind Velocity Aero-generator

Low RPM Low Wind Velocity Aero-generator


Avinash D. Shirode

There are various methods of generation of electrical power. Thermal, hydraulic, atomic are conventional and solar; wind, bio-mass, fuel cells etc. are non-conventional methods. Unfortunately, these sources are thought of on large scale generation (on MW scale) which are not within the reach of an individual. Also, the present day cost of solar cells & wind power generators for small requirements is quite exorbitant. The wind speed is very fluctuating, inconsistent, unreliable and varies from time to time through out the year from place to place. There are ‘wind zones’ and ‘wind maps’ earmarked for India. It is speculated that at most of the places in our country windmills will not be viable due to low wind speed. Hence, a generator, which will produce the energy even at very low speed, is required. This paper discusses the generation of electrical power (1 to 5 kW range), at a reasonably low cost, by the alternator, which is driven by wind energy.

Nowadays, electric power supply of state electricity board is very uncertain and the voltage is also unreliable. There is frequent load shading, interruptions, voltage fluctuation and shutdowns usually without any notice to consumers. This results into stoppage of working and mental agony. Also, the cost per unit is increasing day by day. In future the electrical energy is going to be more and more scarce and more and more costly. Especially, those, who own a small office with computers, have to suffer a lot due to such uncertain supply of power. In houses and small offices/ establishments, normally, generator run by petrol /diesel /kerosene is used to supply electricity when there is no grid power. The use of such appliance is not always satisfactory as every time a person has to go to generator to start it and shut it down. Also, even for 15 watt bulb, the generator has to be kept running with full consumption of fuel. It makes lot of noise and creates smoke and fumes. Thus, there is a great nuisance of noise & air pollution.

To overcome the said problem, inverter and UPS system with large capacity batteries to give sufficient backup (time & wattage) is provided. This arrangement works well for some years even without changing batteries. But, even such an arrangement fails when there is great scarcity of power. If, for example, there is no power for two full days in a week and almost 4-5 hours on other days, even a very efficient battery back up system fails as the batteries cannot get sufficiently charged due to such frequent and long duration power cuts.

It is well known that solar energy with the help of photovoltaic cells can be used. But, harnessing the solar energy is very costly. Wind energy has been harnessed in the windmill farms. These farms require large investments and at present are available mostly in MW range.

At present there is no contrivance to overcome these shortcomings of the present situation. Therefore, there is a dire need for a set of equipment, which will generate just sufficient electrical energy for domestic needs of 1 to 5 kW.

The wind energy, though not consistent, is quite cheaper to solar energy. This does not create any pollution, environment friendly, totally free of running cost, maintenance free and satisfying.


The Windmill consists of:

1) Propeller Blades 2) Tail Vane 3) Alternator 4) Supporting Tower 5) Storage Batteries 6) Inverter

1) Propeller Blades

The blades of suitable size and shape are made up of either fiber glass/ dried Dewdar wood/ FRP/ Plastic material/ aluminium sheets etc., in the aero foil shape. The shape of blades is the most important factor. The blades are to be dynamically balanced. It should sweep maximum wind to produce sufficient torque to rotate the blades even at low wind speeds.

2)Tail Vane

The direction of wind is not same or uniform all the time. The tail vane is used to bring the blades in front of the wind so that the blades get maximum available wind force to keep rotating most of the time. The design of tail vane and blades depend on each other. Additionally, the tail vane is also used to balance the weight of blades.

3)Alternator or Electrical Generator

The alternator is the main component of the wind mill. It produces electricity.

The idea of making an alternator was actually thought from the spinning toy, ‘Bhingaree’ i.e. large diameter with thin width, which the children play in villages.

The alternator consists of a stator and a rotor. The stator and a rotor comprise of two circular shaped, concentric rings. The inner ring is rotated by a shaft, which is connected to propeller blades. Bearings are used for smooth rotation of the shaft. The outer ring is stationary and does not rotate with the shaft. Permanent magnets of equal size, shape and magnetic strength and copper wire coils of equal size, shape, turns and electrical characteristics are pasted on inner & outer rings to generate electricity. The rotating ring is dynamically balanced. This whole arrangement of the stator and rotor of the alternator is covered by aluminium /G. I. /m. s. sheet on both sides to protect the alternator from weather. The three phase A. C. current output is obtained which is converted to D. C. current through bridge rectifier and stored in batteries.

The alternator can be designed in two ways either for large outputs or for small outputs within the said kW range.

If the alternator is designed for large outputs, the magnets of large size are used. In such an eventuality, the magnets are pasted at equal distance from each other to the inner side of the outer stationery ring and coils are pasted at equal distance from each other to the outer side of the inner concentric rotating ring. This design requires slip ring & brushes and makes alternator bulky. This also requires maintenance of setting of brushes.

If the alternator is designed for small outputs, the magnets of small size are used. In such a case, the magnets are pasted to inner rotating ring and coils to outer stationery ring. Such an arrangement eliminates the need of slip ring & brushes and also helps the continued rotation by way of ‘flywheel effect’ obtained due to weight of the magnets. The disadvantage of this system is that comparatively high initial torque is required to start the rotation of inner ring due to its higher weight.

However, it is to be understood that such a distinction of arrangement with respect to placement of magnets and coils is purely arbitrary and both the said arrangements can be used with equal effectiveness in all the cases.

The assembly consists of blades on one side, the alternator in middle and tail vane on the other side. The whole assembly is erected at a reasonable height from ground level on supporting tower. The whole assembly rotates in 360 degrees as per the direction of wind guiding the tail vane. The supporting tower could be placed on top of overhead water tank or top of staircase tower of the building. Depending upon the generator capacity, it will be quite heavy and cumbersome to erect the same at such height.

Alternately, the alternator, which is quite heavy in weight (mainly due to the weight of magnets), is not mounted on top of tower along with the blades and tail vane assembly but is placed at the bottom of the tower. This facilitates easy observation and maintenance of the alternator and reduces the weight on tower and thus, relaxing the requirement of balancing of blades & tail vane system. The rotations from propeller blades to alternator shaft are transmitted at right angles with the assembly of bevel gears through flexible/ solid shaft. However, there are some losses in transmission of rotations from blades through gearbox and long flexible/ solid shaft to alternator. A pulley and belt system can also be used. The losses are negligible and far outweighed considering the advantages of placing the alternator at bottom of tower. This is also advantageous from the safety of the alternator.

The basic design concept of alternator is to generate electricity at very low wind speed, i.e. low R.P.M. of the shaft. This can be achieved by using more number of small magnets than using less number of bigger magnets. Also, the number of turns, diameter, quality and current carrying capacity of wire plays important role in design of alternator.

It is also seen that using more number of magnets, the diameter of alternator becomes very big and requires big lathe machines for turning/ machining operation and costly also. Hence series of small diameter alternators rotating on a common shaft, individually producing small voltage, to make up required system voltage can be used.

4) Supporting Tower

The wind speed normally increases at greater heights from ground level. The supporting tower is designed considering its height, loads coming on it and the wind speeds encountered at the location and is suitably fabricated out of steel material with a foundation strong enough to support the tower. The steps are welded to structural members of the tower for easy climbing up/down and working. Also, a removable pulley arrangement is made to hoist the blades and tail vane assembly in place.

5) Batteries

Since the availability of wind and the wind speed is very much fluctuating, the electricity generated needs to be stored in batteries. When the generating current is more than consumption current, the excess is stored in batteries and utilised when the reverse occurs. The number of batteries and their current capacity depends on generating capacity, wind condition and the consumption. Normally, two batteries of 12 volts are used in series to produce 24 volts of system voltage. But, even 8 batteries can be connected to produce 96 volts. More the system voltage more is the current output at the same RPM. However, the initial RPM (torque) to produce system voltage increases proportionately. Where there is sufficient wind speed almost throughout the day and all seasons, higher system voltage is recommended. More efficient, maintenance free, long life batteries, though initially little more costly, can be used for long life.

6) Inverter

An inverter of a suitable required capacity is used to convert the DC power from the batteries to usable 230 volts AC power.

7) Lighting System

Since the electrical power generated by wind energy is very precious and hence has to be utilized very meticulously. The CFL tubes consume very little power and its illumination is equally high compared to conventional lighting system available in the market. These energy efficient/ energy saving bulbs & tubes are initially costly but by way of saving power they turn out more economical. The power appliances like washing machine, air conditioner, refrigerator, electrical motors etc. require 3-4 times initial starting current and brings over load on batteries and hence not easily used. The ‘soft start’ technology can be effectively applied to start and use these appliances.

Normally, in most of the affordable houses, the battery back up system (inverter/ UPS) and wiring is already done where the batteries are charged by grid power by investing money. The cost of windmill, even if taken only as an alternative charging media, is quite reasonable. However, with effective working of windmill system, one may not require grid power at all and can say GOODBYE to State Electricity Board. The costing can be substantially reduced if it is commercially manufactured on large scale and can be brought within a reach of common man if government gives some subsidy on such products. This windmill can be erected at any normally windy place. This can be used at remote areas where grid power has not reached. It is best suited for residential colonies, apartments, schools, hospitals etc. The investment can be recovered within 3-4 years as the running cost is almost zero. This gives pollution free, noise free, maintenance free constant power at constant voltage which safeguards electrical appliances too.

8) Patent

Patent has been granted for 20 years from 18th June 2002 by Govt. of India for Alternator.

Future Work

Different types of alternators, using different materials to make it light weight, and maintenance free, for inner & outer rings, like aluminium casting, can be used. Instead of blades to rotate shaft, the cylindrical impeller with pelton wheel type louvers can be used. For large diameter alternators, the spokes of inner rotating ring can be made in blade shape which will support the ring as well as work as blades, thus making it compact, light weight and cheap. Even the blades can be made of aluminium casting which is light and rust proof. Small alternators with self rotating inner spoke blades can be erected on street poles to light 100-200 watt bulbs with a small dry battery. It can be used on car top carrier or in front/ sides keeping set of batteries in dickey to use for car A.C. and other gadgets. Energy can be created by rotating a fly wheel with the alternator ring by battery power input just to start the rotation and keeping the flywheel rotating by inertia effect.


The components of the windmill to produce about 2500 Watts of power have been designed and got fabricated locally and erected on the terrace of the house and is working smoothly and efficiently since August 2002. Experiments were carried out for different alternators (magnets on stationary ring / magnets on rotating ring), different size of magnets, flexible and solid shafts, wooden and fiber blades, alternator on top of tower attached to blade & tail vane assembly and detached from assembly (at bottom of tower) etc. Light weight and proper profile blades, perfectly dynamically balanced rotating ring of alternator, smooth working high speed bearings and balancing tail vane will make the wind mill work very efficiently to produce electricity at low wind speeds. The cost will depend on the capacity of alternator, number of batteries and height of supporting tower but is definitely within the reach of needy persons.

Author’s Bio

Born in 1948, Mr. Avinash Shirode graduated as a Civil Engineer in 1970 and completed his post-graduation in Structural Engineering from the Indian Institute of Science, Bangalore in 1972. While he is an aerospace engineer by employment in the Vikram Sarabhai Space Center and has been involved in several industrial World Bank projects as an architectural engineer, Mr. Avinash Shirode’s interests and engineering skills run across different engineering disciplines including software, electrical and mechanical engineering, agricultural engineering and even pharmaceutical engineering. He has been awarded several patents by the Govt. of India and is a winner of several prestigious national and international awards and felicitations. An active member of several technical, educational and social institutions as office bearer, Mr.Shirode has also published several technical papers & reports in national and international journals. He is also a poet, writer, thinker, traveller, visionary, orator, spiritual meditator and donor.


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