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torque a. the rigid supporting structure of an object
Frame b. the action or process of flowing or flowing out
Magnitude c. the speed at which something moves in a particular direction
Velocity d. a narrow opening in a machine or container
Slot e. a force that tends to cause rotation; twisting force to an object
Flux f. the great size or extent of something C. Adjectives:
Allowable a. operated by a machine or machinery
Instant b. directed or proceeding towards the inside
Inward c. forming the outer surface or structure of something
Sinusoidal d. happening or coming immediately
External e. related to a diagram or graph
Mechanical f. making it possible for Exercise 2. Read and translate the following expressions: Electromagnetic phenomena; mechanical torque; the interaction of conductors; mechanical output torque, speed, and position; the simplest type of induction motor; without external connection; cylindrical iron core; three sinusoidal currents; in one cycle of the stator currents; a proportional reduction; the load with no excess torque; the two-pole construction of the figure
Exercise 3. Read the following text and fill in the gaps with the words given below: Electric Motor Electric motor, any of a class of 1 … that convert electrical energy to mechanical energy, usually by employing 2 … phenomena. Most electric motors develop their mechanical torque by the 3 … of conductors carrying current in a direction at right 4 … to a magnetic field. The various types of electric motor differ in the ways in which the conductors and the 5 … are arranged and also in the control that can be exercised over mechanical output torque, 6 …, and position. Most of the major kinds are delineated below.
Words: A. – interaction B. – speed C. – devices D. – angles E. – field F. – electromagnetic
Exercise 4. Read the following text and choose the right answer to the yes or no questions given below: 1. Does the rotor consist of a cylindrical iron core with conductors placed in slots around the surface? A. Yes, he does B. Yes, it is C. Yes, it does D. No, it isn’t
2. The field completes one revolution in one cycle of the stator currents, doesn’t it? A. No, it isn’t B. Yes, it is C. No, it doesn’t D. Yes, it does 3. Does the rotor speed reach a steady value when the torque produced by the rotor currents equals the torque required? A. Yes, it is B. Yes, it does C. Yes, it was D. Yes, it did
4. Must the mechanical output power be provided by an electrical output power? A. Yes, he must B. Yes, it must C. No, it mustn’t D. No, it doesn’t 5. Do a majority of three-phase induction motors operate with their stator windings connected directly to a three-phase electric supply of constant voltage and constant frequency? A. Yes, they do B. Yes, it does C. No, they don’t D. No, it doesn’t
6. Is the supply voltage matched by the time rate of change of the magnetic flux in the stator of the machine in most cases? A. No, they don’t B. Yes, it is C. Yes, it does D. Yes, they do
7. Is for a 60-hertz supply, the field speed 30 revolutions per second? A. No, it doesn’t B. Yes, it is C. No, it isn’t D. Yes, it does
8. Can other synchronous speeds be obtained with a constant frequency supply by building a machine with a larger number of pairs of magnetic poles? A. Yes, they do B. Yes, they must C. Yes, they are D. Yes, they can 9. Are the most common synchronous speeds for 60-hertz motors 1000 and 800 revolutions per minute? A. No, they aren’t B. Yes, they are C. Yes, they do D. No, they don’t
10. Do typical supply voltages range from 230 volts line-to-line for motors of relatively low power to about 15 kilovolts line-to-line? A. No, they don’t B. Yes, they do C. No, they aren’t D. Yes, they are
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. A majority of three-phase induction motors operate with their stator windings connected directly to a three-phase electric supply of constant voltage and constant frequency. Typical supply voltages range from 230 volts line-to-line for motors of relatively low power (e.g., 0.5 to 50 kilowatts) to about 15 kilovolts line-to-line for high-power motors up to about 10 megawatts. Except for a small voltage drop in the resistance of the stator winding, the supply voltage is matched by the time rate of change of the magnetic flux in the stator of the machine. Thus, with a constant-frequency, constant-voltage supply, the magnitude of the rotating magnetic field is held constant, and the torque is roughly proportional to the power component of the supply current. With the induction motor shown in the foregoing figures, the magnetic field rotates through one revolution for each cycle of the supply frequency. For a 60-hertz supply, the field speed is then 60 revolutions per second, or 3,600 per minute. The rotor speed is less than the speed of the field by an amount that is just enough to induce the required voltage in the rotor conductors to produce the rotor current needed for the load torque. At full load, the speed is typically 0.5 to 5 percent lower than the field speed (often called synchronous speed), with the higher percentage applying to smaller motors. This difference in speed is frequently referred to as the slip. Other synchronous speeds can be obtained with a constant frequency supply by building a machine with a larger number of pairs of magnetic poles, as opposed to the two-pole construction of the figure. The possible values of magnetic-field speed in revolutions per minute are 120 f/p, where f is the frequency in hertz (cycles per second) and p is the number of poles (which must be an even number). A given iron frame can be wound for any one of several possible numbers of pole pairs by using coils that span an angle of approximately (360/p)°. The torque available from the machine frame will remain unchanged, since it is proportional to the product of the magnetic field and the allowable coil current. Thus, the power rating for the frame, being the product of torque and speed, will be roughly inversely proportional to the number of pole pairs. The most common synchronous speeds for 60-hertz motors are 1,800 and 1,200 revolutions per minute.
Exercise 5. Read the text above once again and choose the right answers to the following questions: 1. What does the rotational motion of the magnetic field with respect to the rotor conductors cause? A. It causes a mechanical angular velocity that depends on the frequency of the electric supply. B. It causes a voltage to be induced in each, proportional to the magnitude and the velocity of the field relative to the conductors. C. It causes the stator currents. D. It causes the effect of these currents in producing a magnetic field.
2. What does the rotor consist of? A. It consists of the magnetic field and the allowable coil current. B. It consists of an electrical input power. C. It consists of a cylindrical iron core with conductors placed in slots around the surface. D. It consists of the required voltage in the rotor conductors.
3. How fast is the field speed per minute when it makes 60 revolutions per second? A. It is 1800 revolutions per minute. B. It is 3000 revolutions per minute. C. It is 4000 revolutions per minute. D. It is 3600 revolutions per minute.
4. Where is a magnetic field with an approximately sinusoidal distribution around the air gap with a maximum outward value? A. It is at the bottom. B. It is on the right. C. It is at the top. D. It is on the left.
5. Where is a magnetic field with an approximately sinusoidal distribution around the air gap with maximum inward value? A. It is on the right. B. It is at the bottom. C. It is on the left. D. It is at the bottom.
6. How does the magnetic field rotate in the induction motor? A. It rotates through two revolutions for each cycle of the supply frequency. B. It rotates through one revolution for each cycle of the supply frequency. C. It rotates through one revolution for each two cycles of the supply frequency. D. It rotates through one revolution for each four cycles of the supply frequency.
7. What does one-quarter sinusoid cycle equal to? A. It equals 30°. B. It equals 180°. C. It equals 90°. D. It equals 360°.
8. How do a majority of three-phase induction motors operate? A. They operate with their stator windings connected directly to a three-phase electric supply of constant voltage and constant frequency. B. They operate to produce a rotating magnetic field with a constant magnitude. C. They operate to cause currents to flow in these conductors. D. They operate to accelerate the rotor and to rotate the mechanical load.
9. What is the typical supply voltages range? A. Typical supply voltages range is from 130 volts to about 15 kilovolts. B. Typical supply voltages range is from 230 volts to about 150 kilovolts. C. Typical supply voltages range is from 230 volts to about 15 kilovolts. D. Typical supply voltages range is from 250 volts to about 20 kilovolts.
10. What are the most common synchronous speeds for 60-hertz motors? A. The most common synchronous speeds for 60-hertz motors are 1800 and 2400 revolutions per minute. B. The most common synchronous speeds for 60-hertz motors are 1800 and 1200 revolutions per minute. C. The most common synchronous speeds for 60-hertz motors are 800 and 1000 revolutions per minute. D. The most common synchronous speeds for 60-hertz motors are 800 and 3000 revolutions per minute.
Date: 2015-12-24; view: 1470
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