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2024年3月28日 · electric circuit, path for transmitting electric current. An electric circuit includes a device that gives energy to the charged particles constituting the current, such as a battery or a generator; devices that use current, such as lamps, electric motors, or computers; and the connecting wires or transmission lines. Two of the basic laws that ...
- The Editors of Encyclopaedia Britannica
- Overview
- Induction motors
electric motor, any of a class of devices that convert electrical energy to mechanical energy, usually by employing electromagnetic phenomena.
Most electric motors develop their mechanical torque by the interaction of conductors carrying current in a direction at right angles to a magnetic field. The various types of electric motor differ in the ways in which the conductors and the field are arranged and also in the control that can be exercised over mechanical output torque, speed, and position. Most of the major kinds are delineated below.
The simplest type of induction motor is shown in cross section in the figure. A three-phase set of stator windings is inserted in slots in the stator iron. These windings may be connected either in a wye configuration, normally without external connection to the neutral point, or in a delta configuration. The rotor consists of a cylindrical iron core with conductors placed in slots around the surface. In the most usual form, these rotor conductors are connected together at each end of the rotor by a conducting end ring.
The basis of operation of the induction motor may be developed by first assuming that the stator windings are connected to a three-phase electric supply and that a set of three sinusoidal currents of the form shown in the figure flow in the stator windings. This figure shows the effect of these currents in producing a magnetic field across the air gap of the machine for six instants in a cycle. For simplicity, only the central conductor loop for each phase winding is shown. At the instant t1 in the figure, the current in phase a is maximum positive, while that in phases b and c is half that value negative. The result is a magnetic field with an approximately sinusoidal distribution around the air gap with a maximum outward value at the top and a maximum inward value at the bottom. At time t2 in the figure (i.e., one-sixth of a cycle later), the current in phase c is maximum negative, while that in both phase b and phase a is half value positive. The result, as shown for t2 in the figure, is again a sinusoidally distributed magnetic field but rotated 60° counterclockwise. Examination of the current distribution for t3, t4, t5, and t6 shows that the magnetic field continues to rotate as time progresses. The field completes one revolution in one cycle of the stator currents. Thus, the combined effect of three equal sinusoidal currents, uniformly displaced in time and flowing in three stator windings uniformly displaced in angular position, is to produce a rotating magnetic field with a constant magnitude and a mechanical angular velocity that depends on the frequency of the electric supply.
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The rotational motion of the magnetic field with respect to the rotor conductors causes a voltage to be induced in each, proportional to the magnitude and the velocity of the field relative to the conductors. Since the rotor conductors are short-circuited together at each end, the effect will be to cause currents to flow in these conductors. In the simplest mode of operation, these currents will be about equal to the induced voltage divided by the conductor resistance. The pattern of rotor currents for the instant t1 of the figure is shown in this figure. The currents are seen to be approximately sinusoidally distributed around the rotor periphery and to be located so as to produce a counterclockwise torque on the rotor (i.e., a torque in the same direction as the field rotation). This torque acts to accelerate the rotor and to rotate the mechanical load. As the rotational speed of the rotor increases, its speed relative to that of the rotating field decreases. Thus, the induced voltage is reduced, leading to a proportional reduction in rotor conductor current and in torque. The rotor speed reaches a steady value when the torque produced by the rotor currents equals the torque required at that speed by the load with no excess torque available for accelerating the combined inertia of the load and the motor.
The mechanical output power must be provided by an electrical input power. The original stator currents shown in the figure are just sufficient to produce the rotating magnetic field. To maintain this rotating field in the presence of the rotor currents of the figure, it is necessary that the stator windings carry an additional component of sinusoidal current of such a magnitude and phase as to cancel the effect of the magnetic field that would otherwise be produced by the rotor currents in the figure. The total stator current in each phase winding is then the sum of a sinusoidal component to produce the magnetic field and another sinusoid, leading the first by one-quarter of a cycle, or 90°, to provide the required electrical power. The second, or power, component of the current is in phase with the voltage applied to the stator, while the first, or magnetizing, component lags the applied voltage by a quarter cycle, or 90°. At rated load, this magnetizing component is usually in the range of 0.4 to 0.6 of the magnitude of power component.
- Gordon R. Slemon
2024年3月27日 · His fame was now worldwide. Montesquieu, French political philosopher whose principal work, The Spirit of Laws, was a major contribution to political theory. It inspired the Declaration of the Rights of Man and the Constitution of the United States. Learn more about Montesquieu’s life and work.
2024年4月25日 · Encyclopædia Britannica, Inc. supply and demand, in economics, relationship between the quantity of a commodity that producers wish to sell at various prices and the quantity that consumers wish to buy. It is the main model of price determination used in economic theory. The price of a commodity is determined by the interaction.
ohm, abbreviation Ω, unit of electrical resistance in the metre-kilogram-second system, named in honour of the 19th-century German physicist Georg Simon Ohm.It is equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere (1Ω = 1 V/A); or, the resistance in which one watt of power is dissipated when one ampere flows through it.
5 天前 · electricity, phenomenon associated with stationary or moving electric charges. Electric charge is a fundamental property of matter and is borne by elementary particles. In electricity the particle involved is the electron, which carries a charge designated, by convention, as negative. Thus, the various manifestations of electricity are the ...
2024年4月25日 · static electricity, form of electricity resulting from the imbalance between positive and negative charges within a material that occurs when electrons (the negatively charged particles in an atom) move from one material to another. If the electron-receiving material is either isolated or not an electrical conductor, it tends to hold on to the ...
電動輪椅價格 相關
廣告怎様選擇輪椅 輪椅種類繁多,超過16年的中藥機械製造和康復產品。零售經驗 供應. 選擇適合座椅寬度、深度、高度、靠背及扶手的高度，毎位便用人士身型都不同，最好親身試坐。
低至正價3折，可先租後買。多種品牌ResMed、Philips Dreamstation2、BMC 品質可靠，購買更放心！ 最平$5300 睡眠呼吸機，低至正價3折，可先租後買。2年原廠保養。立減$500 沙田門市，安全可靠
