Saturday, September 13, 2008



A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator. A commutator is a common feature of direct current rotating machines. By reversing the current in the moving coil of a motor's armature, a steady rotating force torque is produced. Similarly, in a generator, reversing of the coil's connection to the external circuit produces unidirectional current in the circuit. The first commutator-type direct current machine was built by Hippolyte Pixii in 1832, based on a suggestion by Ampere.

Conventional continuous current flows from the battery. The commutator itself is the red and blue curved segments. The brushes are dark gray and contacting the commutator contacts, and the rotor winding is violet. As the motor rotates, the commutator contacts will turn through 180° and the current flowing in the winding will reverse. The reverse in coil current compensates for the fact that the coil has rotated 180° relative to the fixed magnetic field (not shown). By varying the relative angle between the two fields a torque on the coil can be produced and maintained by the commutator allowing work to be extracted from the coils rotation.


A commutator typically consists of a set of copper contacts, fixed around part of the circumference of the rotating part of the machine (the rotor), and a set of spring-loaded carbon brushes fixed to the stationary frame of the machine. The external source of current (for a motor) or electrical load (for a generator) is connected to the brushes.

Friction between the copper contacts and the brushes eventually causes wear to both surfaces. The carbon brushes, being made of a softer material, wear faster and may be designed to be replaced easily without dismantling the machine. The copper contacts on small motors (say, less than a kilowatt rating) are not designed to be repaired. On large motors the commutator may be re-surfaced with abrasives, or the rotor may be removed from the frame, mounted in a large metal lathe, and the commutator resurfaced by cutting it down to a smaller diameter.

Each conducting segment on the armature of the commutator is insulated from adjacent segments. Initially when the technology was first developed, mica was used as an insulator between commutation segments. Later materials research into polymers brought the development of plastic spacers which are more durable and less prone to cracking, and have a higher and more uniform breakdown voltage as with mica.

The segments are held onto the shaft using a dovetail shape on the edges or underside of each segment, using insulating wedges around the perimeter of each commutation segment. Due to the high cost of repairs, for small appliance and tool motors the segments are typically crimped permanently in place and cannot be removed; when the motor fails it is simply discarded and replaced. On very large industrial motors it is economical to be able to replace individual damaged segments, and so the end-wedge can be unscrewed and inidividual segments removed and replaced.

Commutator segments are connected to the coils of the armature, with the number of coils (and commutator segments) depending on the speed and voltage of the machine. Large motors may have hundreds of segments.

A practical commutator must contain more than two segments to avoid a "dead spot" where the brushes span both segments, resulting in a short-circuit between them.

Limitations and alternatives

While commutators are widely applied in direct current machines, up to several thousand kilowatts in rating, they have limitations.

Brushes and copper segments wear. On small machines the brushes may last as long as the product (small power tools, appliances, etc.) but larger machines will require regular replacement of brushes and occasional resurfacing of the commutator. Brush-type motors may not be suitable for long service on aerospace equipment where maintenance is not possible.

The efficiency of direct current machines is limited by the "brush drop" due to the resistance of the sliding contact. This may be several volts, making low-voltage direct-current machines very inefficient. The friction of the brush on the commutator also absorbs some of the energy of the machine.

Lastly, the current density in the brush is limited and the maximum voltage on each segment of the commutator is also limited. Very large direct current machines, say, more than several megawatts rating, cannot be built with commutators. The largest motors and generators, of hundreds of megawatt ratings, are all alternating-current machines.

With the widespread availability of power semiconductors, it is now economic to provide electronic switching of the current in the motor windings. These "brushless direct current" motors eliminate the commutator; these can be likened to AC machines with a built-in DC to AC inverter.

The Commutating Plane

In a dynamo, the contact point of where a pair of brushes touch the commutator is referred to as the commutating plane. In this diagram the commutating plane is shown for just one of the brushes.

Compensation for stator field distortion

In a real dynamo, the field is never perfectly uniform. Instead, as the rotor spins it induces field effects which drag and distort the magnetic lines of the outer non-rotating stator.

Exaggerated example of how the field is distorted by the rotor.

The faster the rotor spins, the further the degree of field distortion. Because the dynamo operates most efficiently with the rotor field at right angles the stator field, it is necessary to either retard or advance the brush position to put the rotor's field into the correct position to be at a right angle to the distorted field.

Centered position of the commutating plane if there were no field distortion effects.

Actual position of the commutating plane to compensate for field distortion.

These field effects are reversed when the direction of spin is reversed. It is therefore difficult to build an efficient reversible commutated dynamo, since for highest field strength it is necessary to move the brushes to the opposite side of the normal neutral plane.

The effect can be considered to be somewhat similar to timing advance in an internal combustion engine. Generally a dynamo that has been designed to run at a certain fixed speed will have its brushes permanently fixed to align the field for highest efficiency at that speed.



An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field but linear alternators are occasionally used. In principle, any AC generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines.

Early 20th century Alternator made in Budapest, Hungary, in the power generating hall of a hydroelectric station.

Theory of operation

Alternators generate electricity by the same principle as DC generators, namely, when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.

The rotor magnetic field may be produced by induction (in a "brushless" alternator), by permanent magnets (in very small machines), or by a rotor winding energized with direct current through slip rings and brushes. The rotor magnetic field may even be provided by stationary field winding, with moving poles in the rotor. Automotive alternators invariably use a rotor winding, which allows control of the alternator generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor, but are restricted in size, owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines than those used in automotive applications.

A rotating magnetic field is a magnetic field which periodically changes direction. This is a key principle to the operation of alternating-current motor.Symmetric rotating magnetic field can be produced with as little as three coils. Three coils will have to be driven by a symmetric 3-phase AC sine current system, thus each phase will be shifted 120 degrees in phase from the others. For the purpose of this example, magnetic field is taken to be the linear function of coil's current.

Result of adding three 120-degrees phased sine waves on the axis of the motor is a single rotating vector. Rotor (having a constant magnetic field driven by DC current or a permanent magnet) will attempt to take such position that N pole of the rotor is adjusted to S pole of the stator's magnetic field, and vice versa. This magneto-mechanical force will drive rotor to follow rotating magnetic field in a synchronous manner.

A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect was utilised in early alternating current electric motors. A rotating magnetic field can be constructed using two orthogonal coils with 90 degrees phase difference in their AC currents. However, in practice such a system would be supplied through a three-wire arrangement with unequal currents. This inequality would cause serious problems in standardization of the conductor size and in order to overcome it, three-phase systems are used where the three currents are equal in magnitude and have 120 degrees phase difference. Three similar coils having mutual geometrical angles of 120 degrees will create the rotating magnetic field in this case.The ability of the three phase system to create a rotating field utilized in electric motors is one of the main reasons why three phase systems dominated in the world electric power supply systems. Because magnets degrade with time, synchronous motors and induction motors use short-circuited rotors (instead of a magnet) following rotating magnetic field of multicoiled stator. (Short circuited turns of rotor develop eddy currents in rotating field of stator which (currents) in turn move the rotor by Lorentz force).

Note that the rotating magnetic field can actually be produced by two coils, with phases shifted about 90 degrees, but such field would not be symmetric due to difference between magnetic susceptibility of ferromagnetic materials of pole and air. In case two phases of sine current are only available, four poles are commonly used.

Automotive alternators

Alternators are used in automobiles to charge the battery and to power a car's electric system when its engine is running. Alternators have the great advantage over direct-current generators of not using a commutator, which makes them simpler, lighter, less costly, and more rugged than a DC generator. The stronger construction of automotive alternators allows them to use a smaller pulley so as to turn twice as fast as the engine, improving output when the engine is idling. The availability of low-cost solid-state diodes from about 1960 allowed auto manufacturers to substitute alternators for DC generators. Automotive alternators use a set of rectifiers (diode bridge) to convert AC to DC. To provide direct current with low ripple, automotive alternators have a three-phase winding.

Typical passenger vehicle and light truck alternators use Lundell or claw-pole field construction, where the field north and south poles are all energized by a single winding, with the poles looking rather like fingers of two hands interlocked with each other. Larger vehicles may have salient-pole alternators similar to larger machines. The automotive alternator is usually belt driven at 2-3 times the engine crankshaft speed.

Modern automotive alternators have a voltage regulator built into them. The voltage regulator operates by modulating the small field current in order to produce a constant voltage at the stator output. The field current is much smaller than the output current of the alternator; for example, a 70-amp alternator may need only 2 amps of field current.

Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges; at part load, efficiency is between 50-62% depending on the size of alternator, and varies with alternator speed.In comparison, the best permanent magnet generators, such as those used for bicycle lighting systems, achieve an efficiency of around only 60%.

A typical automotive alternator mounted in a spacious pickup truck engine bay.

The field windings are initially supplied via the ignition switch and charge warning light, which is why the light glows when the ignition is on but the engine is not running. Once the engine is running and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which goes out. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning light fails or the "exciter" wire is disconnected, no priming current reaches the alternator field windings and so the alternator will not generate any power. However, some alternators will self-excite when the engine is revved to a certain speed. The driver may check for a faulty exciter-circuit by ensuring that the warning light is glowing with the engine stopped.

Very large automotive alternators used on buses, heavy equipments or emergency vehicles may produce 300 amperes. Very old automobiles with minimal lighting and electronic devices may have only a 30 ampere alternator. Typical passenger car and light truck alternators are rated around 70 amperes, though higher ratings are becoming more common. Very large automotive alternators may be water-cooled or oil-cooled.

Many alternators are also linked to the vehicle's on board computer system, and in recent years many other factors including air flow are considered in adjusting the battery charging voltage supplied by the alternator.

Marine alternators

Marine alternators as used in yachts are normally versions of automotive alternators, with appropriate adaptations to the salt-water environment. They may be 12 or 24 volt depending on the type of system installed. Larger marine diesels may have two or more alternators to cope with the heavy electrical demand of a modern yacht. On single alternator circuits the power is split between the engine starting battery and the domestic battery (or batteries) by use of a split-charge diode or a mechanical switch. Because the alternator only produces power when running engine control panels are typically fed directly from the alternator by means of an auxiliary terminal. Other typical connections are for charge control circuits.

Brushless Alternators


The stationary part of a motor or alternator is called the stator and the rotating part is called the rotor. The coils of wire that are used to produce a magnetic field are called the field and the coils that produce the power are called the armature. The coils of wire that are used to create the field and the armature are sometimes referred to as the “windings”.


A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature.


The exciter field coils are on the stator and its armature is on the rotor. The AC output from the exciter armature is fed through a set of diodes that are also mounted on the rotor to produce a DC voltage. This is fed directly to the field coils of the main alternator, which are also located on the rotor. With this arrangement, brushes and slip rings are not required to feed current to the rotating field coils. This can be contrasted with a simple automotive alternator where brushes and slip rings are used to supply current to the rotating field.

Main Alternator

The non main alternator has a rotating field as described above and a stationary armature (power generation windings). With the armature stationary, the high current output does not have to go through brushes and slip rings. Although the electrical design is not complex, it results in a not so much reliable alternator because the only parts subject to wear are the bearings.

Control System

Varying the amount of current through the stationary exciter field coils controls the strength of the magnetic field in the exciter. This in turn controls the output from the exciter. The exciter output is fed into the rotating field of the main alternator to supply the magnetic field for it. The strength of the magnetic field in the main alternator then controls its output. The result of all this is that a small current, in the field of the exciter indirectly controls the output of the main alternator and none of it has to go through brushes and slip-rings.By varying excitation only reactive power is controlled , system voltage is improved .


AVR is an abbreviation for Automatic Voltage Regulator. An AVR serves the same function as the “voltage regulator” in an automobile or the “regulator” or “controller” in a home power system.

Hybrid automobiles

Hybrid automobiles replace the separate alternator and starter motor with a combined motor/generator that performs both functions, cranking the internal combustion engine when starting, providing additional mechanical power for accelerating, and charging a large storage battery when the vehicle is running at constant speed. These rotating machines have considerably more powerful electronic devices for their control than the simple automotive alternator described above.

Electric motor

Electric motor

Electric motors

machine that converts electrical energy into mechanical energy. When an electric current is passed through a wire loop that is in a magnetic field, the loop will rotate and the rotating motion is transmitted to a shaft, providing useful mechanical work. The traditional electric motor consists of a conducting loop that is mounted on a rotatable shaft. Current fed in by carbon blocks, called brushes, enters the loop through two slip rings. The magnetic field around the loop, supplied by an iron core field magnet, causes the loop to turn when current is flowing through it. In an alternating current (AC) motor, the current flowing in the loop is synchronized to reverse direction at the moment when the plane of the loop is perpendicular to the magnetic field and there is no magnetic force exerted on the loop. Because the momentum of the loop carries it around until the current is again supplied, continuous motion results. In alternating current induction motors the current passing through the loop does not come from an external source but is induced as the loop passes through the magnetic field. In a direct current (DC) motor, a device known as a split ring commutator switches the direction of the current each half rotation to maintain the same direction of motion of the shaft. In any motor the stationary parts constitute the stator, and the assembly carrying the loops is called the rotor, or armature. As it is easy to control the speed of direct-current motors by varying the field or armature voltage, these are used where speed control is necessary. The speed of AC induction motors is set roughly by the motor construction and the frequency of the current; a mechanical transmission must therefore be used to change speed. In addition, each different design fits only one application. However, AC induction motors are cheaper and simpler than DC motors. To obtain greater flexibility, the rotor circuit can be connected to various external control circuits. Most home appliances with small motors have a universal motor that runs on either DC or AC. Where the expense is warranted, the speed of AC motors is controlled by employing special equipment that varies the power-line frequency, which in the United States is 60 hertz (Hz), or 60 cycles per second. Brushless DC motors are constructed in a reverse fashion from the traditional form. The rotor contains a permanent magnet and the stator has the conducting coil of wire. By the elimination of brushes, these motors offer reduced maintainance, no spark hazard, and better speed control. They are widely used in computer disk drives, tape recorders, CD drives, and other electronic devices. Synchronous motors turn at a speed exactly proportional to the frequency. The very largest motors are synchronous motors with DC passing through the rotor.



In electrical engineering, an armature is one of the two principal electrical components of an electro-mechanical machine--a motor or generator. The other is the field winding or field magnets. The role of the "field" component is simply to create a magnetic field (magnetic flux) for the armature to interact with, so this component can comprise either permanent magnets, or electromagnets formed by a conducting coil. The armature, in contrast, must carry current or EMF (usually both), so it is always a conductor or a conductive coil, oriented normal to both the field and to the direction of motion, torque (rotating machine), or force (linear machine). The armature's role is two-fold: (a) to carry current crossing the field, thus creating shaft torque (in a rotating machine) or force (in a linear machine), and (b) to generate an electromotive force ("EMF"). In the armature, an electromotive force ("EMF") is created by the relative motion of the armature and the field. When the machine is acting as a motor, this EMF opposes the armature current, and the armature converts electrical power to mechanical torque (and power, unless the machine is stalled) and transfers it to the load via the shaft. When the machine is acting as a generator, the armature EMF drives the armature current, and shaft mechanical power is converted to electrical power and transferred to the load. (In an induction generator, these distinctions are blurred, since the generated power is drawn from the stator, which would normally be considered the field.)

A growler is used to check the armature for shorts, opens and grounds.


The parts of an alternator or related equipment can be expressed in either mechanical terms or electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term. This may cause confusion when working with compound machines such as brushless alternators, or in conversation among people who are accustomed to work with differently configured machinery.

A DC armature.

Rotor:The rotating part of an alternator, generator, dynamo or motor.
Stator: The stationary part of an alternator, generator, dynamo or motor

Armature: The power-producing component of an alternator, generator, dynamo or motor. The armature can be on either the rotor or the stator.
Field: The magnetic field component of an alternator, generator, dynamo or motor. The field can be on either the rotor or the stator and can be either an electromagnet or a permanent magnet.

In alternating current machines, the armature is usually stationary (the stator). In DC rotating machines other than brushless DC machines, it is usually rotating (the rotor).

The pole piece of a permanent magnet or electromagnet and the moving, iron part of a solenoid, especially if the latter acts as a switch, may also be referred to as armatures

Sunday, September 7, 2008

Water wheel

Water wheel

A water wheel is a hydropower system; a machine for extracting power from the flow of water. Water wheels and hydropower was widely used in the Middle Ages, powering most industry in Europe, along with the windmill. The most common use of the water wheel was to mill flour in gristmills, but other uses included foundry work and machining, and pounding linen for use in paper.

An overshot water wheel standing 42 feet high powers the Old Mill at Berry College in Rome, Georgia

A water wheel consists of a large wooden or metal wheel, with a number of blades or buckets arranged on the outside rim forming the driving surface. Most commonly, the wheel is mounted vertically on a horizontal axle, but the tub or Norse wheel is mounted horizontally on a vertical shaft. Vertical wheels can transmit power either through the axle or via a ring gear and typically drive belts or gears; horizontal wheels usually directly drive their load. A channel created for the water to follow after leaving the wheel is commonly referred to as a "tailrace."

History of Water Wheel Technology


The first use of the water wheel may possibly have occurred in 4th century BC India. According to Terry S. Reynolds, "Joseph Needham noted in 1965 that certain ancient Indian texts from around 350 BC mentioned a cakkavattaka (turning wheel) which commentaries explained as arahatta-ghatĩ-yanta (machine with wheel-pots attached)", on which basis Needham "suggested that the machine in question was a noria and that it was the first water powered prime mover." However Reynolds also writes that "the term used in Indian texts is ambiguous and does not clearly indicate a water-powered device. In fact, as Thorkild Schiøler has noted, it is far more likely that these passages refer to some type of tread- or hand-operated water-lifting device, instead of a water-powered water-lifting wheel."

Irrigation water for crops was provided by using water raising wheels, some driven by the force of the current in the river from which the water was being raised. This kind of water raising device was used in ancient India.

Around 1150, the astronomer Bhaskara Achārya observed water-raising wheels and imagined such a wheel lifting enough water to replenish the stream driving it, effectively, a perpetual motion machine.

The construction of water works and aspects of water technology in India is described in Arabic and Persian works. During medieval times, the diffusion of Indian and Persian irrigation technologies gave rise to an advanced irrigation system which bought about economic growth and also helped in the growth of material culture.

Greco-Roman Mediterranean

The technology of the water wheel had long been known, but it was not put into widespread use until the Middle Ages when an acute shortage of labor made machines such as the water wheel cost effective. However, the water wheels in ancient Rome and ancient China found many practical uses in powering mills for pounding grain and other substances. The Romans used both fixed and floating water wheels and introduced water power to other countries of the Roman Empire. The Romans were known to use waterwheels extensively in mining projects, with enormous Roman-era waterwheels found in places like modern-day Spain. In the 1st century BC, the Greek epigrammatist Antipater of Thessalonica was the first to make a reference to the waterwheel. He praised it for its use in grinding grain and the reduction of human labor:

Cease from grinding, oh you toilers; women slumber still, Even if the crowing rooster calls the morning star. For Demeter has appointed nymphs to turn your mill, And upon the waterwheel alighting here they are. See how quick they twirl the axle whose revolving rays spin heavy rollers quarried overseas. So again we savor the delights of ancient days, Taught to eat the fruits of Mother Earth in ease.

Ancient China

began to use waterwheels to crush grain in mills and to power the piston-bellows in forging iron ore into cast iron.

Two types of hydraulic-powered chain pumps from the Tiangong Kaiwu of 1637, written by the Ming Dynasty encyclopedist Song Yingxing (1587-1666).

In the text known as the Xin Lun written by Huan Tan about 20 AD (during the usurpation of Wang Mang), it states that the legendary mythological king known as Fu Xi was the one responsible for the pestle and mortar, which evolved into the tilt-hammer and then trip hammer device (see trip hammer). Although the author speaks of the mythological Fu Xi, a passage of his writing gives hint that the waterwheel was in widespread use by the 1st century AD in China (Wade-Giles spelling):

Fu Hsi invented the pestle and mortar, which is so useful, and later on it was cleverly improved in such a way that the whole weight of the body could be used for treading on the tilt-hammer (tui), thus increasing the efficiency ten times. Afterwards the power of animals—donkeys, mules, oxen, and horses—was applied by means of machinery, and water-power too used for pounding, so that the benefit was increased a hundredfold.

In the year 31 AD, the engineer and Prefect of Nanyang, Du Shi (d. 38), applied a complex use of the waterwheel and machinery to power the bellows of the blast furnace to create cast iron. Du Shi is mentioned briefly in the Book of Later Han (Hou Han Shu) as follows (in Wade-Giles spelling):

In the seventh year of the Chien-Wu reign period (31 AD) Tu Shih was posted to be Prefect of Nanyang. He was a generous man and his policies were peaceful; he destroyed evil-doers and established the dignity (of his office). Good at planning, he loved the common people and wished to save their labor. He invented a water-power reciprocator (shui phai) for the casting of (iron) agricultural implements. Those who smelted and cast already had the push-bellows to blow up their charcoal fires, and now they were instructed to use the rushing of the water (chi shui) to operate it...Thus the people got great benefit for little labor. They found the 'water(-powered) bellows' convenient and adopted it widely.

Waterwheels in China found practical uses such as this, as well as extraordinary use. The inventor Zhang Heng (78–139) was the first in history to apply motive power in rotating the astronomical instrument of an armillary sphere, by use of a waterwheel.The mechanical engineer Ma Jun (c. 200–265) from Cao Wei once used a waterwheel to power and operate a large mechanical puppet theater for the Emperor Ming of Wei (r. 226-239).

Islamic period

Muslim engineers during the medieval Islamic period employed water wheels as early as the 7th century, excavation of a canal in the Basra region discovered remains of a water wheel dating from this period. Hama in Syria still preserves one of its large wheels, on the river Orontes, although they are no longer in use. The largest had a diameter of about 20 metres and its rim was divided into 120 compartments.

Another wheel that is still in operation is found at Murcia in Spain, La Nora, and although the original wheel has been replaced by a steel one, the Moorish system during al-Andalus is otherwise virtually unchanged.

By the 13th century, what we might call water raising machine technology lifted off with the works of al-Jazari and Taqi al-Din. They both carried out a number of experiments, building fantastic machines, which led to the invention of automated machinery and this has made an enormous impact on civilisation today.

Medieval Europe and Modern

Cistercian monasteries, in particular, made extensive use of water wheels to power watermills of many kinds. An early example of a very large waterwheel is the still extant wheel at the early 13th century Real Monasterio de Nuestra Senora de Rueda, a Cistercian monastery in the Aragon region of Spain. Grist mills (for corn) were undoubtedly the most common, but there were also sawmills, fulling mills and mills to fulfill many other labor-intensive tasks. The water wheel remained competitive with the steam engine well into the Industrial Revolution.

The main difficulty of water wheels was their inseparability from water. This meant that mills often needed to be located far from population centers and away from natural resources. Water mills were still in commercial use well into the twentieth century, however.

Overshot & pitchback waterwheels are suitable where there is a small stream with a height difference of more than 2 meters, often in association with a small reservoir. Breastshot and undershot wheels can be used on rivers or high volume flows with large reservoirs.

The most powerful waterwheel built in the United Kingdom was the 100 hp Quarry Bank Mill Waterwheel near Manchester. A high breastshot design, it was retired in 1904 and replaced with several turbines. It has now been restored and is a museum open to the public.

Modern Hydro-electric dams can be viewed as the descendants of the water wheel as they too take advantage of the movement of water downhill.


A vertically-mounted water wheel that is rotated by water striking paddles or blades at the bottom of the wheel is said to be undershot. This is generally the least efficient, oldest type of wheel (with the exception of the poncelet wheel). It has the advantage of being cheaper and simpler to build, but is less powerful and can only be used where the flow rate is sufficient to provide torque.

Undershot wheels gain no advantage from head. They are most suited to shallow streams in flat country.

Undershot water wheel

Undershot wheels are also well suited to installation on floating platforms. The earliest were probably constructed by the Roman general Belisarius during the siege of Rome in 537. Later they were sometimes mounted immediately downstream from bridges where the flow restriction of arched bridge piers increased the speed of the current.

Breastshot wheel

A vertically-mounted water wheel that is rotated by falling water striking buckets near the center of the wheel's edge, or just above it, is said to be breastshot. Breastshot wheels are the most common type in the United States of America and are said to have powered the American industrial revolution.

The Anderson Mill is undershot, backshot, and overshot using two sources of water. This allows the speed of the wheel to be controlled

Breastshot wheels are less efficient than overshot wheels (see below), more efficient than undershot wheels, and are not backshot (see below). The individual blades of a breastshot wheel are actually buckets, as are those of most overshot wheels, and not simple paddles like those of most undershot wheels (the Poncelet design being a notable exception). A breastshot wheel requires a good trash rake and typically has a masonry "apron" closely conforming to the wheel face, which helps contain the water in the buckets as they progress downwards. Breastshot wheels are preferred for steady, high-volume flows such as are found on the fall line of the North American East Coast.

This is a view inside of the largest water wheel in the UK, situated at the Quarry Bank cotton mill in the UK. It is still working today and powers the looms at Quarry Bank Mill.

Overshot wheel

A vertically-mounted water wheel that is rotated by falling water striking paddles, blades or buckets near the top of the wheel is said to be overshot. In true overshot wheels the water passes over the top of the wheel, but the term is sometimes applied to backshot or pitchback wheels where the water goes down behind the waterwheel.

A typical overshot wheel has the water channeled to the wheel at the top and slightly to one side in the direction of rotation. The water collects in the buckets on that side of the wheel, making it heavier than the other "empty" side. The weight turns the wheel, and the water flows out into the tail-water when the wheel rotates enough to invert the buckets. The overshot design can use all of the water flow for power (unless there is a leak) and does not require rapid flow.

Unlike undershot wheels, overshot wheels gain a double advantage from gravity. Not only is the force of the flowing water partially transferred to the wheel, the weight of the water descending in the wheel's buckets also imparts additional energy. The mechanical power derived from an overshot wheel is determined by the wheel's physical size and the available head, so they are ideally suited to hilly or mountainous country.

Overshot water wheel

Overshot wheels demand exact engineering and significant head, which usually means significant investment in constructing a dam, millpond and waterways. Sometimes the final approach of the water to the wheel is along a lengthy flume or penstock.

Backshot wheel

A backshot wheel (also called pitchback) is a variety of overshot wheel where the water is introduced just behind the summit of the wheel. It combines the advantages from breastshot and overshot systems, since the full amount of the potential energy released by the falling water is harnessed as the water descends the back of the wheel.

A backshot wheel continues to function until the water in the wheel pit rises well above the height of the axle, when any other overshot wheel will be stopped or even destroyed. This makes the technique particularly suitable for streams that experience extreme seasonal variations in flow, and reduces the need for complex sluice and tail race configurations. A backshot wheel may also gain power from the water's current past the bottom of the wheel, and not just the weight of the water falling in the wheel's buckets.

Materials for construction

Although traditionally water wheels have been made mostly from wood, the use of iron or steel in overshot (and pitchback) wheels allows faster rotation (possibly reducing the need for gearing) without extreme reductions in available torque. A wooden wheel with a wooden axle that can easily turn low-speed, high-torque loads such as a run of millstones cannot necessarily sustain high speeds such as are needed for hydroelectric power generation.

Overshot (and particularly backshot) wheels are the most efficient type; a backshot steel wheel can be more efficient than all but the most advanced and well-constructed turbines. Nevertheless, in some situations an overshot wheel is vastly preferable to any turbine.

Tyson turbine

Tyson turbine

The Tyson Turbine is a hydropower system that extracts power from the flow of water. This design doesn't need a casement, as it is inserted directly into flowing water. It consists of a propeller mounted below a raft, driving a power system, typically a generator, on top of the raft by belt or gear. The turbine is towed into the middle of a river or stream, where the flow is the fastest, and tied off to shore. It requires no local engineering, and can easily be moved to other locations.

Kaplan turbine

Kaplan turbine

The Kaplan turbine is a propeller-type water turbine that has adjustable blades. It was developed in 1913 by the Austrian professor Viktor Kaplan.

The Kaplan turbine was an evolution of the Francis turbine. Its invention allowed efficient power production in low head applications that was not possible with Francis turbines.

A Bonneville Dam Kaplan turbine after 61 years of service

Kaplan turbines are now widely used throughout the world in high-flow, low-head power production.


Viktor Kaplan living in Brno, Moravia, now Czech Republic, obtained his first patent for an adjustable blade propeller turbine in 1912. But the development of a commercially successful machine would take another decade. Kaplan struggled with cavitation problems, and in 1922 abandoned his research for health reasons.

Vertical Kaplan Turbine (courtesy Voith-Siemens).

In 1919 Kaplan installed a demonstration unit at Podebrady, Czechoslovakia. In 1922 Voith introduced an 1100 HP (about 800 kW) Kaplan turbine for use mainly on rivers. In 1925 an 8 MW unit went on line at Lilla Edet, Sweden. This marked the commercial success and wide spread acceptance of Kaplan turbines.

Theory of operation

The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. The design combines radial and axial features.

Vertical Kaplan Turbine (courtsey VERBUND-Austrian Hydro Power).

The inlet is a scroll-shaped tube that wraps around the turbine's wicket gate. Water is directed tangentially, through the wicket gate, and spirals on to a propeller shaped runner, causing it to spin.

The outlet is a specially shaped draft tube that helps decelerate the water and recover kinetic energy.

The turbine does not need to be at the lowest point of water flow, as long as the draft tube remains full of water. A higher turbine location, however, increases the suction that is imparted on the turbine blades by the draft tube. The resulting pressure drop may lead to cavitation.

Variable geometry of the wicket gate and turbine blades allow efficient operation for a range of flow conditions. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications.

Current areas of research include CFD driven efficiency improvements and new designs that raise survival rates of fish passing through.

Horizontal Bulb turbine. (courtsey VERBUND-Austrian Hydro Power).

Since the propeller blades are rotated by high-pressure hydraulic oil, a critical element of Kaplan design is to maintain a positive seal to prevent emission of oil into the waterway. Discharge of oil into rivers is not permitted.


Kaplan turbines are widely used throughout the world for electrical power production. They cover the lowest head hydro sites and are especially suited for high flow conditions.

Inexpensive micro turbines are manufactured for individual power production with as little as two feet of head.

Large Kaplan turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%. They are very expensive to design, manufacture and install, but operate for decades.


The Kaplan turbine is the most widely used of the propeller-type turbines, but several other variations exist:

Propeller turbines have non-adjustable propeller vanes. They are used in where the range of head is not large. Commercial products exist for producing several hundred watts from only a few feet of head. Larger propeller turbines produce more than 100 MW.

Bulb or Tubular turbines are designed into the water delivery tube. A large bulb is centered in the water pipe which holds the generator, wicket gate and runner. Tubular turbines are a fully axial design, whereas Kaplan turbines have a radial wicket gate.

Pit turbines are bulb turbines with a gear box. This allows for a smaller generator and bulb.

Straflo turbines are axial turbines with the generator outside of the water channel, connected to the periphery of the runner.

S- turbines eliminate the need for a bulb housing by placing the generator outside of the water channel. This is accomplished with a jog in the water channel and a shaft connecting the runner and generator.

Tyson turbines are a fixed propeller turbine designed to be immersed in a fast flowing river, either permanently anchored in the river bed, or attached to a boat or barge.

Francis turbine

Francis turbine

The Francis turbine is a type of water turbine that was developed by James B. Francis. It is an inward flow reaction turbine that combines radial and axial flow concepts.

Francis turbine (courtesy Voith-Siemens)

Francis turbines are the most common water turbine in use today. They operate in a head range of ten meters to several hundred meters and are primarily used for electrical power production.


Water wheels have been used historically to power mills of all types, but they are inefficient. 19th century efficiency improvements of water turbines allowed them to compete with steam engines (wherever water was available).

Francis turbine parts

In 1826 Benoit Fourneyron developed a high efficiency (80%) outward flow water turbine. Water was directed tangentially through the turbine runner causing it to spin. Jean-Victor Poncelet designed an inward-flow turbine in about 1820 that used the same principles. S. B. Howd obtained a U.S. patent in 1838 for a similar design.

Francis Runner, Grand Coulee Dam

In 1848 James B. Francis improved on these designs to create a turbine with 90% efficiency. He applied scientific principles and testing methods to produce the most efficient turbine design ever. More importantly, his math and graphical calculation methods improved the state of the art of turbine design and engineering. His analytical methods allowed confident design of high efficiency turbines to exactly match a site's flow conditions.

Theory of operation

The Francis turbine is a reaction turbine, which means that the working fluid changes pressure as it moves through the turbine, giving up its energy. A casement is needed to contain the water flow. The turbine is located between the high pressure water source and the low pressure water exit, usually at the base of a dam.

Francis Turbine and generator

The inlet is spiral shaped. Guide vanes direct the water tangentially to the runner. This radial flow acts on the runner vanes, causing the runner to spin. The guide vanes (or wicket gate) may be adjustable to allow efficient turbine operation for a range of water flow conditions.

Guide vanes at minimum flow setting (cut-away view)

As the water moves through the runner its spinning radius decreases, further acting on the runner. Imagine swinging a ball on a string around in a circle. If the string is pulled short, the ball spins faster. This property, in addition to the water's pressure, helps inward flow turbines harness water energy.

Guide vanes at full flow setting (cut-away view)

At the exit, water acts on cup shaped runner features, leaving with no swirl and very little kinetic or potential energy. The turbine's exit tube is shaped to help decelerate the water flow and recover the pressure.


Large Francis turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%.

Francis Inlet Scroll, Grand Coulee Dam

Francis type units cover a wide head range, from 20 meters to 700 meters and their output varies from a few kilowatt to 1000 megawatt. Their size varies from a few hundred millimeters to about 10 meters.

Small swiss-made Francis turbine

In addition to electrical production, they may also be used for pumped storage; where a reservoir is filled by the turbine (acting as a pump) during low power demand, and then reversed and used to generate power during peak demand.

Francis turbines may be designed for a wide range of heads and flows. This, along with their high efficiency, has made them the most widely used turbine in the world.

Banki turbine

Banki turbine

A Crossflow turbine, Banki-Michell turbine, or Ossberger turbine is a water turbine developed by the Australian Anthony Michell, the Hungarian Donát Bánki and the German Fritz Ossberger. Ossberger manufactured this turbine as a standard product. Ossberger's first patent was granted in 1922. Today, the company founded by Ossberger is the leading manufacturer of this type of turbine.

Unlike most water turbines with an axial or a radial flow, in a crossflow turbine the water passes through the turbine type transversely, across the turbine. Like a waterwheel, the water is admitted at its edge. After passing the runner, it leaves on the opposite side. Going through the runner twice provides additional efficiency. When the water leaves the runner, it also helps clean the runner of small debris and pollution. The cross-flow turbine is a low-speed machine.

Banki turbine. Image credit; European Communities, Layman's Guidebook (on how to develop a small hydro site)

Although the illustration shows one nozzle for simplicity, most practical crossflow turbines have two, arranged so that the water flows do not interfere.

Crossflow turbines are often constructed as two turbines of different capacity that share the same shaft. The turbine wheels are the same diameter, but different lengths to handle different volumes at the same pressure. The subdivided wheels are usually built with volumes in ratios of 1:2. The subdivided regulating unit (the guide vane system in the turbine's upstream section) provides flexible operation, with ⅓, ⅔ or 100% output, depending on the flow. Low operating costs are obtained with the turbine's relatively simple construction.

Details of design

The turbine consists of a cylindrical water wheel or runner with a horizontal shaft, composed of numerous blades (up to 37), arranged radially and tangentially. The blades' edges are sharpened reduce resistance to the flow of water. A blade is made in a part-circular cross-section (pipe cut over its whole length). The ends of the blades are welded to disks to form a cage like a hamster cage; instead of the bars, the turbine has trough-shaped steel blades.

The water flows first from the outside of the turbine to its inside. The regulating unit, shaped like a vane or tongue, varies the cross-section of the flow. The water jet is directed towards the cylindrical runner by a fixed nozzle. The water enters the runner at an angle of about 45 degrees, transmitting some of the water's kinetic energy to the active cylindrical blades.

Ossberger turbine section

The regulating device controls the flow based on the power needed, and the available water. The ratio is that (0–100%) of the water is admitted to 0-100%×30/4 blades. Water admission is to the two nozzles is throttled by two shaped guide vanes. These divide and direct the flow so that the water enters the runner smoothly for any width of opening. The guide vanes should seal to the edges of the turbine casing so that when the water is low, they can shut off the water supply. The guide vanes therefore act as the valves between the penstock and turbine. Both guide vanes can be set by control levers, to which an automatic or manual control may be connected.

The turbine geometry (nozzle-runner-shaft) assures that the water jet is effective. The water acts on the runner twice, but most of the power is transferred on the first pass, when the water enters the runner. Only ⅓ of the power is transferred to the runner when the water is leaving the turbine.

Ossberger turbine runner

The water flows through the blade channels in two directions: outside to inside, and inside to outside. Most turbines are run with two jets, arranged so two water jets in the runner will not affect each other. It is, however, essential that the turbine, head and turbine speed are harmonised.

The cross-flow turbine is of the impulse type, so the pressure remains constant at the runner.

To improve its behaviour under a partial load, a cross-flow turbine is usually built with two chambers. Each chamber has its own runner, but the runners share the same axle. The chambers are subdivided at Q×⅔ and Q×⅓. The smaller chamber is used with small flows, the larger one with medium flows, and both chambers are used with large flows as follows: Q×⅓ + Q×⅔ = Q.


The peak efficiency of a crossflow turbine is somewhat less than a Kaplan, Francis or Pelton turbine. However, the crossflow turbine has a flat efficiency curve under varying load. With a split runner and turbine chamber, the turbine maintains its efficiency while the flow and load vary from 1/6 to the maximum.

Since it has a low price, and good regulation, crossflow turbines are mostly used in mini and micro hydropower units less than two thousand kW and with heads less than 200 m.

Particularly with small run-of-the-river plants, the flat efficiency curve yields better annual performance than other turbine systems, as small rivers' water is usually lower in some months. The efficiency of a turbine determine whether electricity is produced during the periods when rivers have low heads. If the turbines used have high peak efficiencies, but behave poorly at partial load, less annual performance is obtained than with turbines that have a flat efficiency curve.

Due to its excellent behaviour with partial loads, the crossflow turbine is well-suited to unattended electricity production. Its simple construction makes it easier to maintain than other turbine types; only two bearings must be maintained, and there are only three rotating elements. The mechanical system is simple, so repairs can be performed by local mechanics.

Another advantage is that it can often clean itself. As the water leaves the runner, leaves, grass etc. will not remain in the runner, preventing losses. So although the turbine's efficiency is somewhat lower, it is more reliable than other types. No runner cleaning is normally necessary, e.g. by flow inversion or variations of the speed. Other turbine types are clogged easily, and consequently face power losses despite higher nominal efficiencies.

Pelton wheel

Pelton wheel

A Pelton wheel, also called a Pelton turbine, is one of the most efficient types of water turbines. It was invented by Lester Allan Pelton (1829-1908) in the 1870s, and is an impulse machine, meaning that it uses Newton's second law to extract energy from a jet of fluid.

Pelton wheel from Walchensee, Germany hydro power station


The pelton wheel turbine is a tangential flow impulse turbine, water flows along the tangent to the path of the runner. Nozzles direct forceful streams of water against a series of spoon-shaped buckets mounted around the edge of a wheel. Each bucket reverses the flow of water, leaving it with diminished energy. The resulting impulse spins the turbine. The buckets are mounted in pairs, to keep the forces on the wheel balanced, as well as to ensure smooth, efficient momentum transfer of the fluid jet to the wheel. The Pelton wheel is most efficient in high head applications.

Figure from Pelton's original patent (October 1880)

Since water does not easily compress, almost all of the available energy is extracted in the first stage of the turbine. Therefore, Pelton wheels have only one wheel, unlike turbines that operate with compressible fluids.


Peltons are the turbine of choice for high head, low flow sites. However, Pelton wheels are made in all sizes. There are multi-ton Pelton wheels mounted on vertical oil pad bearings in the generator houses of hydroelectric plants. The largest units can be up to 200 megawatts. The smallest Pelton wheels, only a few inches across, are used with household plumbing fixtures to tap power from mountain streams with a few gallons per minute of flow, but these small units must have thirty metres or more of head. Depending on water flow and design, Pelton wheels can operate with heads as small as 15 metres and as high as 1,800 metres.

Plan view of a Pelton turbine installation (courtesy Voith Siemens Hydro Power Generation).

In general, as the height of fall increases, less volume of water can generate a bit more power. Energy can be expressed as W = Fs (where W is the work measured in joules, F is the force and s is the displacement measured in metres). In the instance of fluid, flow power is expressed as P=kρV/t (where k is a constant, ρ is the pressure, V is the volume and t is the time). The power, P, increases in direct proportionality to the flow rate and grows with f(Pressure^3/2.) Thus in the case of Pelton Wheel designs, it is usually better to seek a large pressure using a large head rather than to go for a fast flow rate.