Electric motor

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An electric motor uses electrical energy to produce mechanical energy. The reverse process, that of using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo. Traction motors used on locomotives and some electric and hybrid automobiles often perform both tasks if the vehicle is equipped with dynamic brakes. Electric motors are found in household appliances such as fans, refrigerators, washing machines, pool pumps, floor vacuums, and fanforced ovens.The principle of conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in and consisted of a freehanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool of mercury. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine salt water is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors. A later refinement is the Barlows Wheel. These were demonstration devices, unsuited to practical applications due to limited power.The first real electric motor, wich using electromagnets for both stationary and rotating parts was demonstrated by Ányos Jedlik in Hungary, who later developed a motor powerful enough to propel a vehicle.The first commutatortype directcurrent electric motor capable of a practical application was invented by the British scientist William Sturgeon in . Following Sturgeons work, a commutatortype directcurrent electric motor made with the intention of commercial use was built by the American Thomas Davenport and patented in . Although several of these motors were built and used to operate equipment such as a printing press, due to the high cost of primary battery power, the motors were commercially unsuccessful and Davenport went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeons motor, there was no practical commercial market for these motors.The modern DC motor was invented by accident in , when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry.In Nikola Tesla invented the first practicable AC motor and with it the polyphase power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse company.The classic division of electric motors has been that of Direct Current DC types vs Alternating Current AC types. This is more a de facto convention, rather than a rigid distinction. For example, many classic DC motors run happily on AC power, these motors being referred to as universal motors.The classic division of electric motors has been that of Direct Current DC types vs Alternating Current AC types. This is more a de facto convention, rather than a rigid distinction. For example, many classic DC motors run happily on AC power, these motors being referred to as universal motors.The ongoing trend toward electronic control further muddles the distinction, as modern drivers have moved the commutator out of the motor shell. For this new breed of motor, driver circuits are relied upon to generate sinusoidal AC drive currents, or some approximation of. The two best examples are the brushless DC motor and the stepping motor, both being polyphase AC motors requiring external electronic control.Considering all rotating or linear electric motors require synchronism between a moving magnetic field and a moving current sheet for average torque production, there is a clearer distinction between an asynchronous motor and synchronous types. An asynchronous motor requires slip between the moving magnetic field and a winding set to induce current in the winding set by mutual inductance the most ubiquitous example being the common AC induction motor which must slip in order to generate torque.





In the synchronous types, induction or slip is not a requisit for magnetic field or current production eg. permanent magnet motors, synchronous brushless woundrotor doublyfed electric machine.When optimally designed for a given active current i.e., torque current, voltage, polepair number, excitation frequency i.e., synchronous speed, and core flux density, all categories of electric motors or generators will exhibit virtually the same maximum continuous shaft torque i.e., operating torque within a given physical size of electromagnetic core. Some applications require bursts of torque beyond the maximum operating torque, such as short bursts of torque to accelerate an electric vehicle from standstill. Always limited by magnetic core saturation or safe operating temperature rise and voltage, the capacity for torque bursts beyond the maximum operating torque differs significantly between categories of electric motors or generators.Note Capacity for bursts of torque should not be confused with Field Weakening capability inherent in fully electromagnetic electric machines Permanent Magnet PM electric machine are excluded. Field Weakening, which is not readily available with PM electric machines, allows an electric machine to operate beyond the designed frequency of excitation without electrical damage.Electric machines without a transformer circuit topology, such as FieldWound i.e., electromagnet or Permanent Magnet PM Synchronous electric machines cannot realize bursts of torque higher than the maximum designed torque without saturating the magnetic core and rendering any increase in current i.e., torque as useless. Categorization of electric motorsFurthermore, the permanent magnet assembly of PM synchronous electric machines can be irreparably damaged, if bursts of torque exceeding the maximum operating torque rating are attempted.Electric machines with a transformer circuit topology, such Induction i.e., asynchronous electric machines, Induction DoublyFed electric machines, and Induction or Synchronous WoundRotor DoublyFed WRDF electric machines, exhibit very high bursts of torque because the active current i.e., MagnetoMotiveForce or the product of current and windingturns induced on either side of the transformer oppose each other and as a result, the active current contributes nothing to the transformer coupled magnetic core flux density, which would otherwise lead to core saturation.

Electric machines that rely on Induction

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Electric machines that rely on Induction or Asynchronous principles shortcircuit one port of the transformer circuit and as a result, the reactive impedance of the transformer circuit becomes dominant as slip increases, which limits the magnitude of active i.e., real current. Still, bursts of torque that are two to three times higher than the maximum design torque are realizable.The Synchronous WRDF electric machine is the only electric machine with a truly dual ported transformer circuit topology i.e., both ports independently excited with no shortcircuited port. The dual ported transformer circuit topology is known to be unstable and requires a multiphase slipringbrush assembly to propagate limited power to the rotor winding set. If a precision means were available to instantaneously control torque angle and slip for synchronous operation during motoring or generating while simultaneously providing brushless power to the rotor winding set see Brushless woundrotor doublyfed electric machine, the active current of the Synchronous WRDF electric machine would be independent of the reactive impedance of the transformer circuit and bursts of torque significantly higher than the maximum operating torque and far beyond the practical capability of any other type of electric machine would be realizable. Torque bursts greater than eight times operating torque have been calculated.A DC motor is designed to run on DC electric power. Two examples of pure DC designs are Michael Faradays homopolar motor which is uncommon, and the ball bearing motor, which is so far a novelty. By far the most common DC motor types are the brushed and brushless types, which use internal and external commutation respectively to create an oscillating AC current from the DC source so they are not purely DC machines in a strict sense.





Brushless DC motors are commonly used where precise speed control is necessary, as in computer disk drives or in video cassette recorders, the spindles within CD, CDROM etc. drives, and mechanisms within office products such as fans, laser printers and photocopiers. They have several advantages over conventional motors Compared to AC fans using shadedpole motors, they are very efficient, running much cooler than the equivalent AC motors. This cool operation leads to muchimproved life of the fans bearings. Without a commutator to wear out, the life of a DC brushless motor can be significantly longer compared to a DC motor using brushes and a commutator. Commutation also tends to cause a great deal of electrical and RF noise without a commutator or brushes, a brushless motor may be used in electrically sensitive devices like audio equipment or computers. The same Hall effect sensors that provide the commutation can also provide a convenient tachometer signal for closedloop control servocontrolled applications. In fans, the tachometer signal can be used to derive a fan OK signal. The motor can be easily synchronized to an internal or external clock, leading to precise speed control. Brushless motors have no chance of sparking, unlike brushed motors, making them better suited to environments with volatile chemicals and fuels. Also, sparking generates ozone which can accumulate in poorly ventilated buildings risking harm to occupants health. Brushless motors are usually used in small equipment such as computers and are generally used to get rid of unwanted heat. They are also very quiet motors which is an advantage if being used in equipment that is affected by vibrations.Modern DC brushless motors range in power from a fraction of a watt to many kilowatts. Larger brushless motors up to about kW rating are used in electric vehicles. They also find significant use in highperformance electric model aircraft.edit Coreless DC motorsNothing in the design of any of the motors described above requires that the iron steel portions of the rotor actually rotate torque is exerted only on the windings of the electromagnets. Taking advantage of this fact is the coreless DC motor, a specialized form of a brush or brushless DC motor. Optimized for rapid acceleration, these motors have a rotor that is constructed without any iron core. The rotor can take the form of a windingfilled cylinder inside the stator magnets, a basket surrounding the stator magnets, or a flat pancake possibly formed on a printed wiring board running between upper and lower stator magnets. The windings are typically stabilized by being impregnated with Electrical epoxy potting systems. Filled epoxies that have moderate mixed viscosity and a long gel time. These systems are highlighted by low shrinkage and low exotherm. Typically UL recognized as a potting compound for use up to C Class H UL File No. E .Because the rotor is much lighter in weight mass than a conventional rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical time constant under ms. This is especially true if the windings use aluminum rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must often be cooled by forced air.These motors were commonly used to drive the capstans of magnetic tape drives and are still widely used in highperformance servocontrolled systems, like radiocontrolled vehiclesaircraft, humanoid robotic systems, industrial automation, medical devices, etc.

Universal motors

A variant of the wound field DC motor is the universal motor. The name derives from the fact that it may use AC or DC supply current, although in practice they are nearly always used with AC supplies. The principle is that in a wound field DC motor the current in both the field and the armature and hence the resultant magnetic fields will alternate reverse polarity at the same time, and hence the mechanical force generated is always in the same direction. In practice, the motor must be specially designed to cope with the AC current impedance must be taken into account, as must the pulsating force, and the resultant motor is generally less efficient than an equivalent pure DC motor.Operating at normal power line frequencies, the maximum output of universal motors is limited and motors exceeding one kilowatt about . horsepower are rare. But universal motors also form the basis of the traditional railway traction motor in electric railways. In this application, to keep their electrical efficiency high, they were operated from very low frequency AC supplies, with and . hertz Hz operation being common. Because they are universal motors, locomotives using this design were also commonly capable of operating from a third rail powered by DC.The advantage of the universal motor is that AC supplies may be used on motors which have the typical characteristics of DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and short life problems caused by the commutator. As a result such motors are usually used in AC devices such as food mixers and power tools which are used only intermittently. Continuous speed control of a universal motor running on AC is easily obtained by use of a thyristor circuit, while stepped speed control can be accomplished using multiple taps on the field coil. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor causing the motor to run on halfwave rectified AC.Universal motors generally run at high speeds, making them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high RPM operation is desirable. They are also commonly used in portable power tools, such as drills, circular and jig saws, where the motors characteristics work well. Many vacuum cleaner and weed trimmer motors exceed , RPM, while Dremel and other similar miniature grinders will often exceed , RPM.Motor damage may occur due to overspeeding running at an RPM in excess of design limits if the unit is operated with no significant load. On larger motors, sudden loss of load is to be avoided, and the possibility of such an occurrence is incorporated into the motors protection and control schemes. In smaller applications, a fan blade attached to the shaft often acts as an artificial load to limit the motor speed to a safe value, as well as a means to circulate cooling airflow over the armature and field windings.With the very low cost of semiconductor rectifiers, some applications that would have previously used a universal motor now use a pure DC motor, sometimes with a permanent magnet field.edit AC motors Main article AC motorIn , Nikola Tesla identified the rotating magnetic field principle, and pioneered the use of a rotary field of force to operate machines. He exploited the principle to design a unique twophase induction motor in . In , Galileo Ferraris independently researched the concept. In , Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.Introduction of Teslas motor from onwards initiated what is sometimes referred to as the Second Industrial Revolution, making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system, also of Teslas invention . Before the invention of the rotating magnetic field, motors operated by continually passing a conductor through a stationary magnetic field as in homopolar motors.

Tesla had suggested that the commutators from a machine could be removed and the device could operate on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a perpetual motion machine. Tesla would later attain U.S. Patent ,, , Electric Motor December , which resembles the motor seen in many of Teslas photos. This classic alternating current electromagnetic motor was an induction motor.Michail Osipovich DolivoDobrovolsky later invented a threephase cagerotor in . This type of motor is now used for the vast majority of commercial applications.edit ComponentsA typical AC motor consists of two parts . An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and. An inside rotor attached to the output shaft that is given a torque by the rotating field.edit Torque motorsA torque motor also known as a limited torque motor is a specialized form of induction motor which is capable of operating indefinitely while stalled, that is, with the rotor blocked from turning, without incurring damage. In this mode of operation, the motor will apply a steady torque to the load hence the name.
A common application of a torque motor would be the supply and takeup reel motors in a tape drive. In this application, driven from a low voltage, the characteristics of these motors allow a relativelyconstant light tension to be applied to the tape whether or not the capstan is feeding tape past the tape heads. Driven from a higher voltage, and so delivering a higher torque, the torque motors can also achieve fastforward and rewind operation without requiring any additional mechanics such as gears or clutches. In the computer world, torque motors are used with force feedback steering wheels.Another common application is the control of the throttle of an internal combustion engine in conjunction with an electronic governor. In this usage, the motor works against a return spring to move the throttle in accordance with the output of the governor. The latter monitors engine speed by counting electrical pulses from the ignition system or from a magnetic pickup and, depending on the speed, makes small adjustments to the amount of current applied to the motor. If the engine starts to slow down relative to the desired speed, the current will be increased, the motor will develop more torque, pulling against the return spring and opening the throttle. Should the engine run too fast, the governor will reduce the current being applied to the motor, causing the return spring to pull back and close the throttle.edit Slip ringThe slip ring or wound rotor motor is an induction machine where the rotor comprises a set of coils that are terminated in slip rings to which external impedances can be connected. The stator is the same as is used with a standard squirrel cage motor.By changing the impedance connected to the rotor circuit, the speedcurrent and speedtorque curves can be altered.The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range. By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low current from zero speed to full speed. A secondary use of the slip ring motor is to provide a means of speed control. Because the torque curve of the motor is effectively modified by the resistance connected to the rotor circuit, the speed of the motor can be altered. Increasing the value of resistance on the rotor circuit will move the speed of maximum torque down. If the resistance connected to the rotor is increased beyond the point where the maximum torque occurs at zero speed, the torque will be further reduced.When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation is also very poor.

Stepper motors

Main article Stepper motorClosely related in design to threephase AC synchronous motors are stepper motors, where an internal rotor containing permanent magnets or a large iron core with salient poles is controlled by a set of external magnets that are switched electronically. A stepper motor may also be thought of as a cross between a DC electric motor and a solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the motor may not rotate continuously instead, it steps from one position to the next as field windings are energized and deenergized in sequence. Depending on the sequence, the rotor may turn forwards or backwards.Simple stepper motor drivers entirely energize or entirely deenergize the field windings, leading the rotor to cog to a limited number of positions more sophisticated drivers can proportionally control the power to the field windings, allowing the rotors to position between the cog points and thereby rotate extremely smoothly. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servocontrolled system.Stepper motors can be rotated to a specific angle with ease, and hence stepper motors are used in pregigabyte era computer disk drives, where the precision they offered was adequate for the correct positioning of the readwrite head of a hard disk drive. As drive density increased, the precision limitations of stepper motors made them obsolete for hard drives, thus newer hard disk drives use readwrite head control systems based on voice coils.Stepper motors were upscaled to be used in electric vehicles under the term SRM switched reluctance machine.edit Linear motors Main article Linear motorA linear motor is essentially an electric motor that has been unrolled so that, instead of producing a torque rotation, it produces a linear force along its length by setting up a traveling electromagnetic field.Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev Transrapid train, where the train flies over the ground, and in many rollercoasters where the rapid motion of the motorless railcar is controlled by the rail.edit Doublyfed electric motorDoublyfed electric motors have two independent multiphase windings that actively participate in the energy conversion process with at least one of the winding sets electronically controlled for variable speed operation. Two is the most active multiphase winding sets possible without duplicating singlyfed or doublyfed categories in the same package. As a result, doublyfed electric motors are machines with an effective constant torque speed range that is twice synchronous speed for a given frequency of excitation. This is twice the constant torque speed range as singlyfed electric machines, which have only one active winding set.A doublyfed motor allows for a smaller electronic converter but the cost of the rotor winding and slip rings may offset the saving in the power electronics components. Difficulties with controlling speed near synchronous speed limit applications.edit Singlyfed electric motorSinglyfed electric machines incorporate a single multiphase winding set that is connected to a power supply. Singlyfed electric machines may be either induction or synchronous. The active winding set can be electronically controlled. Induction machines develop starting torque at zero speed and can operate as standalone machines. Synchronous machines must have auxiliary means for startup, such as a starting induction squirrelcage winding or an electronic controller. Singlyfed electric machines have an effective constant torque speed range up to synchronous speed for a given excitation frequency.The induction asynchronous motors i.e., squirrel cage rotor or wound rotor, synchronous motors i.e., fieldexcited, permanent magnet or brushless DC motors, reluctance motors, etc., which are discussed on the this page, are examples of singlyfed motors. By far, singlyfed motors are the predominantly installed type of motors.

Nanotube nanomotor Main article NanomotorResearchers at University of California, Berkeley, recently developed rotational bearings based upon multiwall carbon nanotubes. By attaching a gold plate with dimensions of the order of nm to the outer shell of a suspended multiwall carbon nanotube like nested carbon cylinders, they are able to electrostatically rotate the outer shell relative to the inner core. These bearings are very robust devices have been oscillated thousands of times with no indication of wear. These nanoelectromechanical systems NEMS are the next step in miniaturization that may find their way into commercial aspects in the future.See also Molecular motorsElectrostatic motordit Materials Further information Materials scienceThere is an impending shortage of many rare raw materials used in the manufacture of hybrid and electric cars Nishiyama Cox . For example, the rare earth element dysprosium is required to fabricate many of the advanced electric motors used in hybrid cars Cox . However, over % of the worlds rare earth elements are mined in China Haxel et al. , and domestic Chinese consumption is expected to consume Chinas entire supply by Cox .citation neededA few nonChinese sources such as Thor Lake and Hoidas Lake in Canada, as well as Mt Weld in Australia are currently under development Lunn . However, it is not known if they will be online in time to supply sufficient production by the time shortage hits.citation needededit Motor standardsIn electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The reverse conversion of electrical energy into mechanical energy is done by a motor, and motors and generators have many similarities. A generator forces electric charges to move through an external electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, the sun or solar energy, compressed air or any other source of mechanical energy.Before the connection between magnetism and electricity was discovered, electrostatic generators were invented that used electrostatic principles. These generated very high voltages and low currents. They operated by using moving electrically charged belts, plates and disks to carry charge to a high potential electrode. The charge was generated using either of two mechanisms Electrostatic induction The triboelectric effect, where the contact between two insulators leaves them charged.Because of their inefficiency and the difficulty of insulating machines producing very high voltages, electrostatic generators had low power ratings and were never used for generation of commerciallysignificant quantities of electric power. The Wimshurst machine and Vande Graaff generator are examples of these machines that have survived.edit Faradays diskFaraday diskFaraday diskIn Michael Faraday discovered the operating principle of electromagnetic generators. The principle, later called Faradays law, is that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. He also built the first electromagnetic generator, called the Faraday disc, a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage, and large amounts of current.This design was inefficient due to selfcancelling counterflows of current in regions not under the influence of the magnetic field. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires, and induces waste heating of the copper disc. Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one currentflow direction.Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher more useful voltages. Since the output voltage is proportional to the number of turns, generators could be easily designed to produce any desired voltage by varying the number of turns. Wire windings became a basic feature of all subsequent generator designs.

Dynamo

Main article DynamoDynamos are no longer used for power generation due to the size and complexity of the commutator needed for high power applications. This large beltdriven highcurrent dynamo produced amperes at volts, or , watts, when spinning at RPM.Dynamos are no longer used for power generation due to the size and complexity of the commutator needed for high power applications. This large beltdriven highcurrent dynamo produced amperes at volts, or , watts, when spinning at RPM.Dynamo Electric Machine End View, Partly Section U.S. Patent , Dynamo Electric Machine End View, Partly Section U.S. Patent , The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into a pulsing direct electric current through the use of a commutator. The first dynamo was built by Hippolyte Pixii in .Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DC electric motor, the AC alternator, the AC synchronous motor, and the rotary converter.A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.Large power generation dynamos are now rarely seen due to the now nearly universal use of alternating current for power distribution and solid state electronic AC to DC power conversion. But before the principles of AC were discovered, very large directcurrent dynamos were the only means of power generation and distribution. Now power generation dynamos are mostly a curiosity.edit Other Rotating Electromagnetic GeneratorsWithout a commutator, the dynamo is an example of an alternator, which is a synchronous singlyfed generator. With an electromechanical commutator, the dynamo is a classical direct current DC generator. The alternator must always operate at a constant speed that is precisely synchronized to the electrical frequency of the power grid for nondestructive operation. The DC generator can operate at any speed within mechanical limits but always outputs a direct current waveform.Other types of generators, such as the asynchronous or induction singlyfed generator, the doublyfed generator, or the brushless woundrotor doublyfed generator, do not incorporate permanent magnets or field windings i.e, electromagnets that establish a constant magnetic field, and as a result, are seeing success in variable speed constant frequency applications, such as wind turbines or other renewable energy technologies.The full output performance of any generator can be optimized with electronic control but only the doublyfed generators or the brushless woundrotor doublyfed generator incorporate electronic control with power ratings that are substantially less than the power output of the generator under control, which by itself offer cost, reliability and efficiency benefits.edit MHD generatorA magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a steam power plant. The first practical design was the AVCO Mk. , developed in . The U.S. government funded substantial development, culminating in a Mw demonstration plant in . In the Soviet Union from until the late s, the MHD plant U was in regular commercial operation on the Moscow power system with a rating of MW, the largest MHD plant rating in the world at that time. MHD generators operated as a topping cycle are currently less efficient than combinedcycle gas turbines.edit TerminologyThe two main parts of a generator or motor can be described in either mechanical or electrical termsMechanical Rotor The rotating part of an alternator, generator, dynamo or motor. Stator The stationary part of an alternator, generator, dynamo or motor.lectrical Armature The powerproducing component of an alternator, generator, dynamo or motor. In a generator, alternator, or dynamo the armature windings generate the electrical current. The armature can be on either the rotor or the stator. Field The magnetic field component of an alternator, generator, dynamo or motor. The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator. For a more technical discussion, refer to the Field coil article.Because power transferred into the field circuit is much less than in the armature circuit, AC generators nearly always have the field winding on the rotor and the stator as the armature winding. Only a small amount of field current must be transferred to the moving rotor, using slip rings. Direct current machines necessarily have the commutator on the rotating shaft, so the armature winding is on the rotor of the machine.

ExcitationA small early s KVA directdriven power station AC alternator, with a separate beltdriven exciter generator.A small early s KVA directdriven power station AC alternator, with a separate beltdriven exciter generator.Main article Excitation magneticAn electric generator or electric motor that uses field coils rather than permanent magnets will require a current flow to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all. Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger.In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.edit Equivalent circuitEquivalent circuit of generator and load.G = generatorVG=generator opencircuit voltageRG=generator internal resistanceVL=generator onload voltageRL=load resistanceEquivalent circuit of generator and load.G = generatorVG=generator opencircuit voltageRG=generator internal resistanceVL=generator onload voltageRL=load resistanceThe equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generators VG and RG parameters, follow this procedure Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance RGDC. Start the generator. Before connecting the load RL, measure the voltage across the generators terminals. This is the opencircuit voltage VG. Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the onload voltage VL.
Measure the load resistance RL, if you dont already know it. Calculate the generators AC internal resistance RGAC from the following formulaR_{GAC} = {R_L} \left {{{V_G}\over{V_L}}} \rightNote The AC internal resistance of the generatorhen running is generally slightly higher than its DC resistance when idle. The above procedure allows you to measure both values. For rough calculations, you can omit the measurement of RGAC and assume that RGAC and RGDC are equal.Note If the generator is an AC type, use an AC voltmeter for the voltage measurements.The maximum power theorem states that the maximum power can be obtained from the generator by making the resistance of the load equal to that of the generator. This is inefficient since half the power is wasted in the generators internal resistance practical electric power generators operate with load resistance much higher than internal resistance, so the efficiency is greater. Vehiclemounted generatorsEarly motor vehicles until about the s tended to use DC generators with electromechanical regulators. These have now been replaced by alternators with builtin rectifier circuits, which are less costly and lighter for equivalent output. Automotive alternators power the electrical systems on the vehicle and recharge the battery after starting. Rated output will typically be in the range A at V, depending on the designed electrical load within the vehicle. Some cars now have electricallypowered steering assistance and air conditioning, which places a high load on the electrical system. Large commercial vehicles are more likely to use V to give sufficient power at the starter motor to turn over a large diesel engine. Vehicle alternators do not use permanent magnets and are typically only % efficient over a wide speed range. Motorcycle alternators often use permanent magnet stators made with rare earth magnets, since they can be made smaller and lighter than other types. See also hybrid vehicle.Some of the smallest generators commonly found power bicycle lights. These tend to be . ampere, permanentmagnet alternators supplying W at V or V. Being powered by the rider, efficiency is at a premium, so these may incorporate rareearth magnets and are designed and manufactured with great precision. Nevertheless, the maximum efficiency is only around % for the best of these generators % is more typical due to the use of permanent magnets. A battery would be required in order to use a controllable electromagnetic field instead, and this is unacceptable due to its weight and bulk.Sailing yachts may use a water or wind powered generator to tricklecharge the batteries. A small propeller, wind turbine or impeller is connected to a lowpower alternator and rectifier to supply currents of up to A at typical cruising speeds.

Enginegenerator

Main article EnginegeneratorAn enginegenerator is the combination of an electrical generator and an engine prime mover mounted together to form a single piece of selfcontained equipment. The engines used are usually piston engines, but gas turbines can also be used. Many different versions are available ranging from very small portable petrol powered sets to large turbine installations.edit Human powered electrical generators Main article Selfpowered equipmentA generator can also be driven by human muscle power for instance, in field radio station equipment.Human powered direct current generators are commercially available, and have been the project of some DIY enthusiasts. Typically operated by means of pedal power, a converted bicycle trainer, or a foot pump, such generators can be practically used to charge batteries, and in some cases are designed with an integral inverter. The average adult could generate about watts on a pedal powered generator. Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio.Traditionally, these are DC serieswound motors, usually running on approximately volts. The availability of highpowered semiconductors such as thyristors and the IGBT has now made practical the use of much simpler, higherreliability AC induction motors known as asynchronous traction motors. Synchronous AC motors are also occasionally used, as in the French TGV.dit Transmission typesBefore the midth century, a single large motor was often used to drive multiple driving wheels through connecting rods that were very similar to those used on steam locomotives. It is now standard practice to provide one traction motor driving each axle through a gear drive.Usually, the traction motor is simply suspended between the truck bogie frame and the driven axle this is referred to as a nosesuspended traction motor. The problem with such an arrangement is that a portion of the motors weight is unsprung, increasing forces on the track. Occasionally, other mounting arrangements are made. In the case of the GG, two truckmounted motors drove each axle through a quill drive. The BiPolar electric locomotives built by General Electric for the Milwaukee Road had gearless motors. The rotating shaft of the motor was also the axle for the wheels. In the case of the TGV power units, a motor mounted to the power unit’s frame drives each axle a tripod drive allows a small amount of flexibility in the drive train allowing the trucks bogies to pivot. By mounting the relatively heavy traction motor directly to the power unit rather than to the truck bogie, better dynamics are obtained allowing muchimproved highspeed operation.edit RatingIn dieselelectric and gas turbineelectric locomotives the horsepower rating of the traction motors is usually % that of the prime mover. This assumes that the electrical generator converts % of the engines output into electrical energy and the traction motors convert % of this electrical energy back into mechanical energy. Calculation % x % = %.edit CoolingBecause of the high power levels involved, traction motors are almost always cooled using forced air.Dynamic braking is the use of the electric traction motors of a railroad vehicle as generators when slowing the vehicle. It is termed Rheostatic if the generated electrical power is dissipated as heat in brake grid resistors and Regenerative if the power is returned to the supply line. Dynamic braking lowers the wear of friction braking components and additionally Regeneration can also lower energy consumption.ontentside Principle of operation Regenerative braking Blended braking Selfload test See also External links Referencesedit Principle of operationDuring braking the motor fields are connected across either the main traction generator Dieselelectric loco or the supply Electric locomotive and the motor armatures are connected across either the brake grids or supply line. The rolling locomotive wheels turn the motor armatures, and if the motor fields are now excited, the motors will act as generators. For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction. Braking effort is proportional to the product of the magnetic strength of the field windings, times that of the armature windings. For permanent magnet motors, dynamic braking is easily achieved by shorting the motor terminals, thus bringing the motor to a fast abrupt stop.

Regenerative braking Main article Regenerative brakingIn electrified systems the similar process of regenerative braking is employed whereby the current produced during braking is fed back into the power supply system for use by other traction units, instead of being wasted as heat. It is normal practice to incorporate both regenerative and rheostatic braking in electrified systems. If the power supply system is not receptive, i.e. incapable of absorbing the current, the system will default to rheostatic mode in order to provide the braking effect.Yard locomotives with onboard energy storage systems which allow the recovery of some of this energy which would otherwise be wasted as heat are now available. The Green Goat model, for example, is being used Canadian Pacific Railway, BNSF Railway, Kansas City Southern Railway and Union Pacific Railroad.edit Blended brakingDynamic braking alone is insufficient to stop a locomotive, as its braking effect rapidly diminishes below about mph kmh Therefore it is always used in conjunction with the regular Air Brake. This combined system is called blended braking. Liion batteries have also been used to store energy for use in bringing trains to a complete halt.Although blended braking combines both dynamic and air braking, the resulting braking force is designed to be the same as what the air brakes on their own provide. This is achieved by maximizing the dynamic brake portion and automatically regulating the air brake portion as the main purpose of dynamic braking is to reduce the amount of air braking required. This conserves air and minimizes the risks of overheated wheels. One locomotive manufacturer, EMD, estimates that dynamic braking provides between % to % of the braking force during blended braking.dit Selfload testIt is possible to use the brake grids as a form of dynamometer or load bank to perform a self load test of locomotive engine HP. With the locomotive stationary, the main generator MG output is connected to the grids instead of the traction motors. The grids are normally large enough to absorb the full engine output power, which is calculated from MG voltage and current output.edit See also

Description

Moving electric charges an electric current in a magnetic field experience a force that is perpendicular to both their direction of movement and the magnetic field, called the Lorentz force. In the homopolar motor shown on the right, the electric current produced by the battery moves radially through the disk magnet, which has a magnetic field along its longitudinal axis. The resulting Lorentz force in the tangential direction produces a torque in the magnet, which is free to rotate with the attached screw.It is not necessary for the magnet to be electrically conductive, or to move. One can attach the magnet to the battery and allow the wire to rotate freely while closing the electric circuit even at the axis of rotation. Again, where at some point along the electric loop the current in the wire is not parallel to the magnetic field, there occurs a Lorentz force that is perpendicular to bothThis Lorentz force is tangential and produces a torque in the wire, so that the wire rotates.In contrast to other electrical motors, both the orientation and magnitude of the magnetic field and the electric current do not change.Like most electro-mechanical machines a homopolar motor is reversible so that when electrical energy of a suitable kind is put into its terminals, mechanical energy can be obtained from its motion and vice versa, see homopolar generator for details on construction and theory of operation.edit HistoryThe homopolar motor was the first ever device to produce rotation from electromagnetism. It was first built and demonstrated by Michael Faraday in at the Royal Institution in London.edit Sources of confusionPeople are sometimes confused by the fact that there are no changes in the magnetic field or electric current, and no recognizable North-South pole interaction between the magnet and the electric circuit. People often think that field lines cannot be used to understand homopolar machines, or that the field lines rotate -- see Faraday Paradox. Others refer to special relativity to explain the homopolar motor. The homopolar motor also may seem to require a conducting magnet.The homopolar motor can be well explained by the Faraday model of lines of force, with a tangential force hence, a torque resulting where the electric current makes an angle with the magnetic lines of force. The homopolar motor provides a simple demonstration of the Lorentz force.Main article History of the batterylthough an early form of electrochemical battery called the Baghdad Battery may have been used in antiquity, the modern development of batteries started with the Voltaic pile, invented by the Italian physicist Alessandro Volta in .In , Luigi Galvani published a report on animal electricity. He created an electric circuit consisting of two different metals, with one touching a frogs leg and the other touching both the leg and the first metal, thus closing the circuit. In modern terms, the frogs leg served as both the electrolyte and the sensor, and the metals served as electrodes. He noticed that even though the frog was dead, its legs would twitch when he touched them with the metals.Volta realized the frogs moist tissues could be replaced by cardboard soaked in salt water, and the frogs muscular response could be replaced by another form of electrical detection. He already had studied the electrostatic phenomenon of capacitance, which required measurements of electric charge and of electrical potential. Building on this experience Volta was able to detect electric current through his system, now called a voltaic cell, or cell for short. The terminal voltage of a cell that is not discharging is called its electromotive force emf, and has the same unit as electrical potential, named voltage and measured in volts, in honor of Volta. In , Volta invented the battery by placing many voltaic cells in series, literally piling them one above the other. This Voltaic Pile gave a greatly enhanced net emf for the combination, with a voltage of about volts for a -cell pile. In many parts of Europe batteries continue to be called piles.

Volta did not appreciate that the voltage was due to chemical reactions. He thought that his cells were an inexhaustible source of energy, and that the associated chemical effects e.g. corrosion were a mere nuisance, rather than an unavoidable consequence of their operation, as Michael Faraday showed through the principle of ionic mobility in .While early batteries were of great value for experimental purposes, their limitations made them impractical for a large current drain. Later, starting with the Daniell cell in , batteries provided more reliable currents and were adopted by industry for use in stationary devices, particularly in telegraph networks where they were the only practical source of electricity, since electrical distribution networks did not exist then. These wet cells used liquid electrolytes, which were prone to leakage and spillage if not handled correctly. Many used glass jars to hold their components, which made them fragile. These characteristics made wet cells unsuitable for portable appliances. Near the end of the th century, the invention of Dry cell batteries, which replaced liquid electrolyte with a paste, made portable electrical devices practical.Since then, batteries have gained popularity as they became portable and useful for many purposes. According to a estimate, the worldwide battery industry generates US$ billion in sales eachyear, with % annual growth.
dit How batteries work Main article Electrochemical cellA voltaic cell for demonstration purposes. In this example the two half-cells are linked by a salt bridge separator that permits the transfer of ions, but not water molecules.A voltaic cell for demonstration purposes. In this example the two half-cells are linked by a salt bridge separator that permits the transfer of ions, but not water molecules.A battery is a device that converts chemical energy directly to electrical energy. It consists of one or more voltaic cells. Each voltaic cell consists of two half cells connected in series by a conductive electrolyte. One half-cell is the positive electrode the cathode and the other is the negative electrode the anode. In the redox reaction that powers the battery, reduction occurs in the cathode, while oxidation occurs in the anode. The electrodes do not touch each other but are electrically connected by the electrolyte, which can be either solid or liquid. In many cells, the materials are enclosed in a container, and a separator, which is porous to the electrolyte, which prevents the electrodes from coming into contact.Each half cell has an electromotive force or emf, determined by its ability to drive electric current from the interior to the exterior of the cell. The net emf of the battery is the difference between the emfs of its half-cells, as first recognized by Volta. Thus, if the electrodes have emfs \mathcal{E}_ and \mathcal{E}_, then the net emf is \mathcal{E}_{}-\mathcal{E}_{} in other words, the net emf is difference between the reduction potentials of the half-reactions.The electrical potential difference, or \displaystyle{\Delta V_{bat}} across the terminals of a battery is known as terminal voltage and is measured in volts. The terminal voltage of a battery that is neither charging nor discharging is called the open-circuit voltage and equals the emf of the battery. Because of internal resistance, the terminal voltage of a battery that is discharging is smaller in magnitude than the open-circuit voltage and the terminal voltage of a battery that is charging exceeds the open-circuit voltage. An ideal battery has negligible internal resistance, so it would maintain a constant terminal voltage of \mathcal{E} until exhausted, then dropping to zero. If such a battery maintained . volts and stored a charge of one Coulomb than it would perform . Joule of work. In practical batteries, the internal resistance will increase as it is discharged, and the open circuit voltage will also decrease as the cell is discharged. If the voltage and resistance are plotted against time the resulting graphs will typically not be a straight line, and the shape of the curve will vary with the chemistry and internal arrangement employed.The voltage developed across a cells terminals depends on the chemicals used in it and their respective concentrations. For example, alkaline and carbon-zinc cells both measure approximately . volts, due to the energy release of the associated chemical reactions. Because of the high electrochemical potential changes in the reactions of lithium compounds, lithium cells can provide as much as volts or more.

Types of batteries

Main article List of battery typesFrom top to bottom Two button cells, a -volt PP battery, an AAA battery, an AA battery, a C battery, a D battery, a large R.From top to bottom Two button cells, a -volt PP battery, an AAA battery, an AA battery, a C battery, a D battery, a large R.There are many kinds of electrochemical cells, including galvanic cells, electrolytic cells, fuel cells, flow cells and voltaic piles. A batterys characteristics may vary due to many factors including internal chemistry, current drain and temperature.However, there are two main types of batteries, each of which has its own advantages and disadvantages. Primary batteries irreversibly within limits of practicality transform chemical energy to electrical energy. When the initial supply of reactants is exhausted, energy cannot be readily restored to the battery by electrical means. Secondary batteries can be recharged that is, they can have their chemical reactions reversed by supplying electrical energy to the cell, restoring their original composition.Historically, some types of primary batteries used, for example, for telegraph circuits, were restored to operation by replacing the components of the battery consumed by the chemical reaction. Secondary batteries are not indefinitely rechargeable due to dissipation of the active materials, loss of electrolyte and internal corrosion.edit Primary batteriesPrimary batteries are ready to produce current as soon as they are assembled. Disposable batteries, also called primary cells, are intended to be used once and discarded. These are most commonly used in portable devices that have low current drain, are only used intermittently, or are used well away from an alternative power source, such as in alarm and communication circuits where other electric power is only intermittently available. Primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms. Battery manufacturers recommend against attempting to recharge primary cells, although some electronics enthusiasts claim it is possible to do so using special types of chargers.Common types of disposable batteries include zinc-carbon batteries and alkaline batteries. Generally, these have higher energy densities than rechargeable batteries, but disposable batteries do not fare well under high-drain applications with loads under O.edit Secondary batteriesSecondary batteries must be charged before use they are usually assembled with active materials in the discharged state. Rechargeable batteries or secondary cells can be recharged by applying electrical current, which reverses the chemical reactions that occur during its use. Devices to supply the appropriate current are called chargers or rechargers.The oldest form of rechargeable battery is the lead-acid battery, a type of wet cell. This battery is notable in that it contains a liquid in an unsealed container, requiring that the battery be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen gas produced by these batteries during overcharging. The lead-acid battery is also very heavy for the amount of electrical energy it can supply. Despite this, its low manufacturing cost and its high surge current levels make its use common where a large capacity over approximately Ah is required or where the weight and ease of handling are not concerns.A common form of the lead-acid battery is the modern car battery, which can generally deliver a peak current of amperes. An improved type of liquid electrolyte battery is the sealed valve regulated lead acid VRLA battery, popular in the automotive industry as a replacement for the lead-acid wet cell. The VRLA battery uses an immobilized sulfuric acid electrolyte, reducing the chance of leakage and extending shelf life. VRLA batteries have the electrolyte immobilized, usually by one of two meansther portable rechargeable batteries include several dry cell types, which are sealed units and are therefore useful in appliances such as mobile phones and laptop computers. Cells of this type in order of increasing power density and cost include nickel-cadmium NiCd, nickel metal hydride NiMH and lithium-ion Li-ion cells. By far, Li-ion has the highest share of the dry cell rechargeable market. Meanwhile, NiMH has replaced NiCd in most applications due to its higher capacity, but NiCd remains in use in power tools, two-way radios, and medical equipment.Recent developments include batteries with embedded functionality such as USBCELL, with a built-in charger and USB connector within the AA format, enabling the battery to be charged by plugging into a USB port without a charger, and low self-discharge LSD mix chemistries such as Hybrio, ReCyko, and Eneloop, where cells are precharged prior to shipping.edit Battery capacity and discharging device to check battery voltage.
A device to check battery voltage.The more electrolyte and electrode material there is in the cell, the greater the capacity of the cell. Thus a small cell has less capacity than a larger cell, given the same chemistry e.g. alkaline cells, though they develop the same open-circuit voltage.Because of the chemical reactions within the cells, the capacity of a battery depends on the discharge conditions such as the magnitude of the current, the duration of the current, the allowable terminal voltage of the battery, temperature and other factors.The available capacity of a battery depends upon the rate at which it is discharged. If a battery is discharged at a relatively high rate, the available capacity will be lower than expected.The battery capacity that battery manufacturers print on a battery is the product of hours multiplied by the maximum constant current that a new battery can supply for hours at F° C°, down to a predetermined terminal voltage per cell.A battery rated at A·h will deliver A over a hour period at room temperature. However, if it is instead discharged at A, it will run out of charge before the hours as theoretically expected.The symbol for a battery in a circuit diagram.The symbol for a battery in a circuit diagram.For this reason, a battery capacity rating is always related to an expected discharge duration.