Ac And Dc Machines PdfBy Profecatdul In and pdf 19.01.2021 at 09:38 5 min read
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A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine.
- dc machines construction pdf
- Commutator (electric)
- What is a DC Machine : Construction & Its Working
- Electric motor
dc machines construction pdf
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft.
Electric motors can be powered by direct current DC sources, such as from batteries, or rectifiers , or by alternating current AC sources, such as a power grid, inverters or electrical generators. An electric generator is mechanically identical to an electric motor, but operates with a reversed flow of power, converting mechanical energy into electrical energy.
Electric motors may be classified by considerations such as power source type, internal construction, application and type of motion output. In addition to AC versus DC types, motors may be brushed or brushless , may be of various phase see single-phase , two-phase , or three-phase , and may be either air-cooled or liquid-cooled. General-purpose motors with standard dimensions and characteristics provide convenient mechanical power for industrial use.
The largest electric motors are used for ship propulsion, pipeline compression and pumped-storage applications with ratings reaching megawatts.
Electric motors are found in industrial fans, blowers and pumps, machine tools, household appliances, power tools and disk drives. Small motors may be found in electric watches. In certain applications, such as in regenerative braking with traction motors , electric motors can be used in reverse as generators to recover energy that might otherwise be lost as heat and friction. Electric motors produce linear or rotary force torque intended to propel some external mechanism, such as a fan or an elevator.
An electric motor is generally designed for continuous rotation, or for linear movement over a significant distance compared to its size.
Magnetic solenoids produce significant mechanical force, but over an operating distance comparable to their size. Transducers such as loudspeakers and microphones convert between electrical current and mechanical force to reproduce signals such as speech. When compared with common internal combustion engines ICEs , electric motors are lightweight, physically smaller, provide more power output, are mechanically simpler and cheaper to build, while providing instant and consistent torque at any speed, with more responsiveness, higher overall efficiency, and lower heat generation.
However, electric motors are not as convenient or common as ICEs in mobile applications i. The first electric motors were simple electrostatic devices described in experiments by Scottish monk Andrew Gordon and American experimenter Benjamin Franklin in the s. This law was discovered independently by Charles-Augustin de Coulomb in , who published it so that it is now known with his name.
A free-hanging wire was dipped into a pool of mercury, on which a permanent magnet PM was placed. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a close circular magnetic field around the wire.
Barlow's wheel was an early refinement to this Faraday demonstration, although these and similar homopolar motors remained unsuited to practical application until late in the century. After Jedlik solved the technical problems of continuous rotation with the invention of the commutator , he called his early devices "electromagnetic self-rotors". Although they were used only for teaching, in Jedlik demonstrated the first device to contain the three main components of practical DC motors: the stator , rotor and commutator.
The device employed no permanent magnets, as the magnetic fields of both the stationary and revolving components were produced solely by the currents flowing through their windings. The first commutator DC electric motor capable of turning machinery was invented by British scientist William Sturgeon in The motors ran at up to revolutions per minute, and powered machine tools and a printing press.
Several inventors followed Sturgeon in the development of DC motors, but all encountered the same battery cost issues. As no electricity distribution system was available at the time, no practical commercial market emerged for these motors. It developed remarkable mechanical output power. His motor set a world record, which Jacobi improved four years later in September In , Jedlik built a device using similar principles to those used in his electromagnetic self-rotors that was capable of useful work.
A major turning point came in , when Antonio Pacinotti first described the ring armature although initially conceived in a DC generator, i. A benefit to DC machines came from the discovery of the reversibility of the electric machine, which was announced by Siemens in and observed by Pacinotti in In , Frank Julian Sprague invented the first practical DC motor, a non-sparking device that maintained relatively constant speed under variable loads.
Other Sprague electric inventions about this time greatly improved grid electric distribution prior work done while employed by Thomas Edison , allowed power from electric motors to be returned to the electric grid, provided for electric distribution to trolleys via overhead wires and the trolley pole, and provided control systems for electric operations.
This allowed Sprague to use electric motors to invent the first electric trolley system in —88 in Richmond, Virginia , the electric elevator and control system in , and the electric subway with independently powered centrally-controlled cars.
The latter were first installed in in Chicago by the South Side Elevated Railroad , where it became popularly known as the " L ". Sprague's motor and related inventions led to an explosion of interest and use in electric motors for industry. The development of electric motors of acceptable efficiency was delayed for several decades by failure to recognize the extreme importance of an air gap between the rotor and stator. Efficient designs have a comparatively small air gap. Louis motor, long used in classrooms to illustrate motor principles, is extremely inefficient for the same reason, as well as appearing nothing like a modern motor.
Electric motors revolutionized industry. Industrial processes were no longer limited by power transmission using line shafts, belts, compressed air or hydraulic pressure. Instead, every machine could be equipped with its own power source, providing easy control at the point of use, and improving power transmission efficiency.
Electric motors applied in agriculture eliminated human and animal muscle power from such tasks as handling grain or pumping water. Household uses like in washing machines, dishwashers, fans, air conditioners and refrigerators replacing ice boxes of electric motors reduced heavy labor in the home and made higher standards of convenience, comfort and safety possible.
Today, electric motors consume more than half of the electric energy produced in the US. The first alternating-current commutatorless induction motor was invented by Galileo Ferraris in Ferraris was able to improve his first design by producing more advanced setups in Possible industrial development was envisioned by Nikola Tesla , who invented independently his induction motor in and obtained a patent in May In the same year, Tesla presented his paper A New System for Alternating Current Motors and Transformers to the AIEE that described three patented two-phase four-stator-pole motor types: one with a four-pole rotor forming a non-self-starting reluctance motor , another with a wound rotor forming a self-starting induction motor , and the third a true synchronous motor with separately excited DC supply to rotor winding.
One of the patents Tesla filed in , however, also described a shorted-winding-rotor induction motor. Scott to help Tesla; however, Tesla left for other pursuits in Lamme later developed a rotating bar winding rotor.
Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky invented the three-phase induction motor in , of both types cage-rotor and wound rotor with a starting rheostat, and the three-limb transformer in After an agreement between AEG and Maschinenfabrik Oerlikon, Doliwo-Dobrowolski and Charles Eugene Lancelot Brown developed larger models, namely a hp squirrel cage and a hp wound rotor with a starting rheostat. These were the first three-phase asynchronous motors suitable for practical operation.
At the Frankfurt International Electrotechnical Exhibition, the first long distance three-phase system was successfully presented.
The General Electric Company began developing three-phase induction motors in In an electric motor, the moving part is the rotor, which turns the shaft to deliver the mechanical power. The rotor usually has conductors laid into it that carry currents, which interact with the magnetic field of the stator to generate the forces that turn the shaft. Alternatively, some rotors carry permanent magnets, and the stator holds the conductors.
The rotor is supported by bearings , which allow the rotor to turn on its axis. The bearings are in turn supported by the motor housing. The motor shaft extends through the bearings to the outside of the motor, where the load is applied. Because the forces of the load are exerted beyond the outermost bearing, the load is said to be overhung. The stator is the stationary part of the motor's electromagnetic circuit and usually consists of either windings or permanent magnets.
The stator core is made up of many thin metal sheets, called laminations. Laminations are used to reduce energy losses that would result if a solid core were used. Resin-packed motors, used in washing machines and air conditioners, use the damping properties of resin plastic to reduce noise and vibration.
These motors completely encapsulate the stator in plastic. The distance between the rotor and stator is called the air gap. The air gap has important effects, and is generally as small as possible, as a large gap has a strong negative effect on performance.
It is the main source of the low power factor at which motors operate. The magnetizing current increases with the air gap. For this reason, the air gap should be minimal.
Very small gaps may pose mechanical problems in addition to noise and losses. Windings are wires that are laid in coils , usually wrapped around a laminated soft iron magnetic core so as to form magnetic poles when energized with current. Electric machines come in two basic magnet field pole configurations: salient- and nonsalient-pole configurations.
In the salient-pole machine the pole's magnetic field is produced by a winding wound around the pole below the pole face. In the nonsalient-pole , or distributed field, or round-rotor, machine, the winding is distributed in pole face slots.
Some motors have conductors that consist of thicker metal, such as bars or sheets of metal, usually copper , alternatively aluminum. These are usually powered by electromagnetic induction. A commutator is a mechanism used to switch the input of most DC machines and certain AC machines. It consists of slip-ring segments insulated from each other and from the shaft. The motor's armature current is supplied through stationary brushes in contact with the revolving commutator, which causes required current reversal, and applies power to the machine in an optimal manner as the rotor rotates from pole to pole.
In light of improved technologies in the electronic-controller, sensorless-control, induction-motor, and permanent-magnet-motor fields, externally-commutated induction and permanent-magnet motors are displacing electromechanically-commutated motors. A DC motor is usually supplied through a slip ring commutator as described above. AC motors' commutation can be achieved using either a slip ring commutator or external commutation, can be fixed-speed or variable-speed control type, and can be synchronous or asynchronous type.
Universal motors can run on either AC or DC. DC motors can be operated at variable speeds by adjusting the DC voltage applied to the terminals or by using pulse-width modulation PWM. AC motors operated at a fixed speed are generally powered directly from the grid or through motor soft starters. AC motors operated at variable speeds are powered with various power inverter , variable-frequency drive or electronic commutator technologies.
The term electronic commutator is usually associated with self-commutated brushless DC motor and switched reluctance motor applications. Electric motors operate on three different physical principles: magnetism , electrostatics and piezoelectricity.
In magnetic motors, magnetic fields are formed in both the rotor and the stator. The product between these two fields gives rise to a force, and thus a torque on the motor shaft. One, or both, of these fields must be made to change with the rotation of the motor.
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. Electric motors can be powered by direct current DC sources, such as from batteries, or rectifiers , or by alternating current AC sources, such as a power grid, inverters or electrical generators. An electric generator is mechanically identical to an electric motor, but operates with a reversed flow of power, converting mechanical energy into electrical energy. Electric motors may be classified by considerations such as power source type, internal construction, application and type of motion output. In addition to AC versus DC types, motors may be brushed or brushless , may be of various phase see single-phase , two-phase , or three-phase , and may be either air-cooled or liquid-cooled. General-purpose motors with standard dimensions and characteristics provide convenient mechanical power for industrial use.
are called field windings. ❑ The rotor contains the commutator and the conductors across which emf is induced. This part is called armature. DC Machines.
What is a DC Machine : Construction & Its Working
The conversion of this mechanism is known as the commutator, thus these machines are also named as commutating machines. DC machine is most frequently used for a motor. The main benefits of this machine include torque regulation as well as easy speed. The applications of the DC machine is limited to trains, mills, and mines. For example, underground subway cars, as well as trolleys, may utilize DC motors.
In the present day world, electrical energy is generated in bulk in the form of an alternating current. Hence, the use of DC machines, i.
Electric motors play an essential role in almost every industry. Using the right type of motor with high-quality parts and regular servicing keeps your facility running smoothly and prevents damage to the endpoint equipment due to wear or power surges. Gainesville Industrial Electric can help your company select the right industrial electric motors and parts for your applications. Electric motors are machines that convert electrical energy—from either stored power or a direct electrical connection—into mechanical energy through the production of rotational force. The two major types of electric motors are:.
Hence, this classification is valid for both: DC generators and DC motors. This eBook of Electrical Machine-1 is very useful and can help you score really well during your exam. This makes two broad categories of dc machines; i Separately excited and ii Self-excited. Book Summary. There are some applications where large quantities of DC power are required, such as electroplating, electrolysis, etc.
A Primer on Electric Motors
Черт возьми. Я совсем забыл, что электричество вырубилось. Он принялся изучать раздвижную дверь. Прижал ладони к стеклу и попробовал раздвинуть створки. Потные ладони скользили по гладкой поверхности. Он вытер их о брюки и попробовал. На этот раз створки двери чуть-чуть разошлись.