1、附录 A： 外文资料 Induction Motor 1 Starting an induction motor High-inertia loads put strain induction motors because they prolong the starting period. The starting current in both the stator and rotor is high during this interval so that overheating becomes a major problem. For motors of several thousand
2、 horsepower, a prolonged starting period may even overload the transmission line feeding the plant where the motor is installed. The line voltage may fall below normal for many seconds, thus affecting other connected loads. To relieve the problem, induction motors are often started on reduced voltag
3、e. This limits the power drawn by the motor, and consequently reduces the brie voltage drop as well as the heating rate of the windings. Reduced voltage lengthens the start-up time, but this s usually not important. However, whether the start-up time is long or short, it is worth remembering the fol
4、lowing rule for a motor that is not loaded mechanical: Rule I - The heat dissipated in the rotor during the starting period (from zero speed to final rated speed) is equal to the final kinetic energy stored in all the revolving parts. This rule holds true, irrespective of the stator voltage or the t
5、orque-speed curve of the motor. Thus, if a motor brings a massive flywheel up to speed, and if the energy stored in the flywheel is then 5000 joules. the rotor will have dissipated 5000 joules in the form of heat. Depending upon the size of the rotor and its cooling system, this energy could easily
6、produce overheating. 2 Plugging an induction motor In some industrial applications, the induction motor and its load have to be brought to a quick stop. This can be done by interchanging two stator leads, so that the revolving field suddenly turns in the opposite direction to the rotor. During this
7、plugging period, the motor acts as a brake. It absorbs kinetic energy from the still-revolving load, causing its speed to hill. The associated mechanical power P, is entirely dissipated as heat in the rotor. Unfortunately, the rotor also continues to receive electromagnetic power P from the stator,
8、which is also dissipated as heat (Fig. 14.10). Consequently, plugging produces 12R losses in the rotor that even exceed those when the rotor is locked. Motors should not he plugged too frequently because high rotor temperatures may melt the rotor bars or overheat the stator winding, in this regard i
9、t is worth remembering the rule following rule for plugging operations for a motor that is not loaded mechanically: Rule 2- The heat dissipated in the rotor during the plugging period (initial rated speed to zero speed) is three times the original kinetic energy of all the revolving parts. Fig. 14.1
10、0 When a 3-phase induction motor is plugged, the rotor losses are very high.3 Braking with direct current An induction motor and its high-inertia load can also be brought to a quick stop by circulating dc current in the stator winding. Any two stator terminals can be connected to the dc source. The
11、direct current produces stationary N, S poles in the stator. The number of poles created is equal to the number of poles which the motor develops normally. Thus, a 3-phase, 4-pole induction motor produces 4 dc poles, no matter how the motor terminals are connected to the dc source. When the rotor sw
12、eeps past the stationary field, an ac voltage is induced iii the rotor bars. The voltage produces an ac current and the resulting rotor losses are dissipated at the expense of the kinetic energy stored in the revolving parts. The motor finally comes to rest when all the kinetic energy has been dissi
13、pated as heat in the rotor. The advantage of dc braking is that it produces far less heat than does plugging. In effect, the energy dissipated in the rotor is only equal to the original kinetic energy stored in the revolving masses, and not three times that energy. The energy dissipated in the rotor
14、 is independent of the magnitude of the dc current. However, a smaller dc current increases the braking time. The dc current can be two or three times the rated current of the motor. Even larger values can be used, provided that the stator dues not become too hot. The braking torque is proportional
15、to the square of the dc braking current. 4 Abnormal conditions Abnormal motor operation may due to internal problems (short-circuit in the stator, overheating of the bearing, etc. ) or to external conditions. External problems may be caused by any of the following: 1. Mechanical overload 2. Supply v
16、oltage changes 3. Single phasing 4. Frequency changes We will examine the nature of these problems in the sections that follow. According to national standards, a motor shall operate satisfactory on any voltage within 10% of the nominal voltage, and for any frequency within 5% of the normal frequenc
17、y. If the voltage and frequency both vary, the sum of the two percentage changes must not exceed 10 percent. Finally, all motors are designed to operate satisfactorily at altitudes up to 1000 m above sea level. At higher altitudes the temperature may exceed the permissible limits due to the poor coo
18、ling afforded by the thinner air. 5 Mechanical overload Although standard induction motors can develop as much as twice their rated power for short periods, they should not be allowed to run continuously beyond their rated capacity. Overloads cause overheating, which deteriorates the insulation and
19、reduces the service life of the motor. In practice the overload causes the thermal overload relays in the starter box to trip. bringing the motor to a stop before its temperature gets too high. Some drip-proof motors are designed to carry a continuous overload of I 5 percent. This overload capacity
20、is shown on the nameplate by the service factor 1.15. The allowable temperature rise is then 10C higher than that permitted for drip-proof motors operating at normal load. During emergencies a drip-proof motor can be made to carry overloads as much as 125 percent, as long as supplementary external ventilation is provided. This is not recommended
