电工学概论答案

电工学概论答案【篇一：电工学概论习题答案_第二章(下)】?)与u2?173sin(314t?120?)之和u?u1?u2，并写出它们的相量表示式。

?解：u1??30??2?12i)??u2??120???(?12?2)??u?u1?u2??90?因此，u?u1?u2?200sin(314t?90?)2-32. 已知某负载的电流的有效值及初相为2a、45?，电压的有效值及初相为100v、?45?，频率为50hz，写出它们的向量表达式，并判断该负载是什么元件，元件参数为多少？解：i?2?45????45??2??50iu?100??z?因此，该负载是电容，元件阻抗大小为50。

??220?900v i??10?900a，(b) 2-33. 在50hz的单相交流电路中，若(a) u??220?900v i??10?450a，(c) u??220?900v i??10?1200a求三种情况下电u路中的r及x，并写出电路阻抗z的复数式。

解：(a) z?220i10i?22因此，电路中的r?22?，x?0(b) z??因此，电路中的r?，x?(c) z??11i因此，电路中的r?，x?11?2-34. 一个电感线圈接到20v的直流电源时，通过电流为0.5a。

接到50hz、100v的交流电源时通过电流为1.25a，求线圈的r和l。

解：r?200.5?40??1.25?xl?42 又?xl?2?f?l ?l?0.2hz?50?88.6i设u?220?0?，则??i?220?0?50?88.6i?2.2(12?2)?2.2??602-36. 把上述的rl串联电路与一个电容器并联，若并联后的电路总电流为原来rl串联电路的电流，求电容电流及电容量，作出相量图。

解：如图所示?ic?2.2?90?90??xc?1?而xc?2?f?c，故c?12?f?xc?55.2uf?1?c?12?f?c??50?设u?220?0，则?ir?220?050220?0?50i???225225???0??ic????225?90?i?ir?ic=225i45?2-38. 把上述的r与c并联电路与一电感元件串联，接在u?220v，50hz的交流电源上，测得rc并联电路的端电压正好也是220v，求电感元件两端的电压及电感量，作出向量图。

解：z?220225?225i?25?25i由向量图可知，xl?50?，又xl??l?2?f?l，故l?2-39. 有两台单相交流电动机（电感性负载）并联在u?220v的电源上所消耗的功率及功率因数为p1?1kw，cos?1?0.8，p2?0.75kw，cos?2?0.6，求每台电动机的电流，无功功率、视在功率，以及总有功功率，总无功功率，总视在功率，总电流、总功率因数。

?2解：由于p1?ui1cos?1p2?ui2cos8 因此电流 i1?5.68a i2?5.6axl2?f?0.159h?2?1000 var无功功率 q1?ui1sin?1?750varq2?ui2sin视在功率 s1?ui1?1250v?as2?ui2?1250v?a 总有功功率p?p1?p2?1750w 总无功功率 q?q1?q2?1750var 总视在功率s??2474.87v?aa总电流i?s/u?11.25总功率因素 cos??0.7072-40. 在40w日光灯电路中已知电源为220v、50hz灯管端电压为100v，功率消耗为40w，镇流器功率消耗为8w，灯管电流为0.4a，求(1)电路的总有功功率，视在功率和无功功率，功率因数。

(2)镇流器的无功功率，视在功率及端电压。

(3)作出相量图。

解：l电路如上图所示，其中l、r为镇流器电阻电感，r1为日光灯电阻(1) 电路的总有功功率为 p = 40 + 8 = 48w总视在功率为s?ui?220?0.4?88v?a总无功功率为q??73.76var功率因素为cos??p/s?0.54 (2) 镇流器的无功功率qz?q?73.76var视在功率为sz???76.86v?a端电压uz?sz/i?192.15v(3) 向量图uli2-41. 上述电路中欲使功率因数提高到0.9，应并联多大的电容？解：并联电容后无功变化，有功不变变化后的无功功率为q?48?0.9?23.25var 变化量为q变?q?q?50.51var xc?u/q变?958.2c?12?f?xc?3.32uf2解：该负载需要的无功功率及视在功率分别为：p?uicos??90kwq?uisin??90000?0.6?0.8?120kw s??150kv?a若要求该电网的功率因数提高到0.9，则需要并联电容，其大小为c?p?u2(tan?1?tan?2)?900002??50?2202(1.333?0.484)?5.03mf2-43. 解释rlc串联电路中的串联谐振现象，串联谐振有哪几个特征？通过哪些参数调节可以出现串联谐振？解：在rlc串联电路中，当xl?xc时，电路两端的电压与电流同相，即为串联谐振串联谐振有以下特征： (1)电路的阻抗|z|?|z|为最小值。

(2) 由于电源电压与电路中的电流同相，因此电路对电源呈电阻性。

(3) 由于xl?xc，有ul?uc。

(4) 串联谐振电路对频率有选择性，可在无线电选择接收信号频率时加以利用。

由谐振条件xl?xc有2?fl?12?fc，因此通过调节l、c、f可出现串联谐振。

2-44. 解释电感线圈与电容并联电路中出现的并联谐振现象？此种谐振现象的最主要特征是什么？其谐振频率与哪些参数有关？解：在电感线圈与电容并联电路中，当电路端电压与电流相位相同，则电路出现并联谐振现象。

并联谐振的主要特征有：(1) 并联谐振时，电路的阻抗z?流i接近最小。

(2) 并联谐振时，由于电源电压与电流相同时(??0)，因此，电路对电源呈电阻性，即|z|相当于一个电阻。

(3) 在谐振时电感线圈和电容器的电流近似相等，而比总电流大许多倍。

(4) 如并联谐振电流改由恒流源供电，当电源为某一频率时电路发生谐振，电路的阻抗为最大，这样起到了选频的作用。

lrc接近最大，在电源电压u一定时，电路中的总电【篇二：英语学习电工学概论习题答案_第四章】cage form and the winding pattern from the structural characteristics of the three-phase asynchronous motor?answer: rotor winding induced electromotive force is generated, currents and electromagnetic torque, the structure type with cage and wound rotor two, squirrel cage rotor ofeach rotor slot insert a piece of copper conducting bar, in an extended at both ends of the core at the notch, with two short circuit copper rings respectively, the all guide bar ends are welded together. if the core is removed, the overall shape ofthe winding is like a cage, so it is called a cage rotor. the rotor winding and the stator winding is similar to that of theinsulated conductor embedded in the rotor slot and connected into a star three-phase symmetric winding, winding threeoutlet terminals are respectively connected to the rotor shaft three slip ring (ring and ring, the ring and the shaft areinsulated from each other), the carbon brush to draw out the current.4-2. how to change the three-phase induction motor to change?answer: any of the two will be connected with three-phase power supply connected three wire switch, three-phase asynchronous motor direction change.4-3. known a three-phase cage induction motor rated power = 3kw, rated speed = 2880r/min. find the number of pole pairs (1);(2) when the rated slip ratio; (3) rated torque.solution: (1) synchronous speed, so the logarithm of the motor pole p is 1;(2)(3) =9.95technical data of 4-4. known y112m-4 type asynchronous motor = 4kw, delta connection method, rated voltage = 380v, = 1440r / min, rated current = 8.8 a, power factor = 0.82,efficiency = 84.5%. find the number of pole pairs (1); (2) input power during normal operation; (3) when the rated slip ratio; (4) rated torque.solution: (1) synchronous speed, so the logarithm of the motor pole p is 2;(2)(3)(4) =26.54-5. known y132m-4 type asynchronous motor rated power7.5kw, rated current = 15.4a, rated speed = 1440r / min, rated voltage = 380v, rated power factor of 0.85 and rated efficiency = 0.87, starting torque and rated torque = 2.2, the starting current and rated current = 7.0, the maximum torque / rated torque = 2.2. tries (1) rated power input; (2) rated torque; (3) rated slip; (4) thestarting current; (5) starting torque; (6) the maximum torque.solution: (1) rated input power(2) =49.74(3)(4) =7=107.8a(5) =2.2=109.43(6) =2.2=109.434-6. in the three-phase asynchronous motor start instant (s = 1), why the rotor current is large, and the power factor of the rotor circuit is small?a: the electric motor is connected to the power supply, the induction electromotive force and the induced current of the rotor circuit are the largest, which is called the starting current or the blocking current. in general, the starting current of the small and medium sized three phase induction motor is 5 to 7 times of the rated current. although the motor starting current is very large, but the power factor of the rotor is very low, in fact, the starting torque of the motor is not large.4-7. if there is a large number of asynchronous motors in the three-phase power grid at the same time, will be the impact of the grid voltage? why?a large number of asynchronous motors start at the same time, the power grid will have a larger voltage drop. because asynchronous motor starting current is 5 ~ 7 times the rated current, if in the three-phase power grid has a large number of asynchronous motor start at the same time, will produce a large starting current, large starting current will in power line produced larger voltage landing, transformer power supplywith other work load.4-8. three-phase induction motor torque characteristics of the shape of the curve by what factors?answer: electromagnetic torque can be expressed aswhen the s is very small, can be neglected, t is proportional to s; when the s is closer to 1, can be omitted, t is inversely proportional to s. so on the torque characteristic curve (t ~ s) as shown in figure.t ~ s curveat that time, said the motor at the rated operating state, at this time the rotor speed for the rated speed, torque for the rated torque, the output of the mechanical power for the rated power. at that time, the motor is in critical condition. at the same time, the electromagnetic torque is the largest value can by calculating t ~ s curve extremum method, to obtain the critical slip. visible and directly proportional to, but has nothing to do with; regardless of is proportional to. only in this way can by changing (wound rotor circuit external rheostat) and reducedto change t ~ s curve shape, as shown in 4.2.3 and 4.2.4.figure t increases the 4.2.3 ~ s curves of 4.2.4 to reduce the t ~ s curve4-9. why should adopt the measure of reducing blood pressure? what is the effect of reducing the starting torque of the motor? under what circumstances can the buck start?answer: reduced voltage starting of the purpose is to reduce the motor starting when starting the effects of electric current on the grid, the method is in starting lower voltage in the stator winding of the motor, to be motor speed is close to the stable, then the voltage returns to normal value. due to the motor torque and voltage is proportional to the square, so reduced voltage starting torque will also decrease accordingly, current step-down start the main method for star - delta (y - delta) change to connect to start and its applicable condition is normal operation of the stator winding is delta connected squirrel cage asynchronous motor.what are some of the main methods of 4-10. reduction?answer: the start method (1) direct starting, (2) y - delta change is connected with a step-down starting, (3) soft starting method. (4) rotor series resistance starting. starting mainly y - delta switching starting and soft start method.why does the series resistance in the rotor circuit of 4-11. wound rotor type induction motor improve the starting performance and speed performance of the induction motor?answer: winding type motor can be used in the rotor circuit in series resistance starting method. this can not only limit the starting current, but also increase the starting torque. so we need to start production of mechanical torque is large, such as cranes, forging machines and other commonly used linear motor drive around. after the motor is started, the starting resistance is removed by the step with the increase of the rotating speed.4-12. three-phase induction motor in the normal operation, if the rotor suddenly stuck, what will be the consequences of the motor?answer: three-phase asynchronous motor in normal operation, if the rotor suddenly stuck (stall), motor current increased immediately for several times of the rated current, if no protective measures promptly cut off the power supply, the motor will be burnt.4-13. three-phase asynchronous motor in normal operation, if suddenly there is a phase power, the motor can run? if the motor is under heavy load, what is the change of the motor current?answer: three-phase asynchronous motor in normal operation, if suddenly there is a phase power, the motor can run. if the motor is overloaded in case of slip increases, the induced electromotive force increases, the motor current increases. long run may burn the motor.4-14. three phase induction motor which are several speed control method? which method is the best performance of the speed control?answer: (1) the number of magnetic poles of the speed change, (2) change of the rotating slip speed, (3) variable frequency speed regulation are the main speed control method of three-phase asynchronous motor. in many speed regulation methods of ac induction motor, the performance of variable frequency speed regulation is the best, which is characterized by high speed range, good stability and high efficiency.4-15. constant supply voltage in the case, if the rating for the error connected y type connection delta connection, what are the consequences? and as the amount of time for a false delta connection type y connection, and what are the consequences? answer: y type equivalent connecting resistance of the same size three resistor delta after connection for direct y connection type size 1/3. therefore, at a supply voltage of constant, if rated delta connected mistakenly connected type y connection, actual larger resistance, smaller current in the circuit, resulting in lack of power. if the amount?a delta connection error word type fast connection, the actual resistance is small, the current in the circuit becomes larger, it is possible to burn out resistance.4-16. thermal relay in the three-phase asynchronous motor control circuit to play what role? can it play the role of short circuit protection?answer: thermal relay is used for ac asynchronous motor overload protection, it uses the thermal effect of current and action. now the production of heat relay has three groups of heating elements and has a phase failure protection function, namely in the current serious imbalance or even disconnectionof a phase, the mechanical structure can also make a trip ofthe gusset plate will move off contact disconnect and protects the motor because of the phase interruption caused by phase current overload and off the ring.4-17. test to be able to directly control the control circuit of a three-phase cage asynchronous motor at two locations.answer:small capacity cage type motor direct starting control circuit, which used the air switch q, accontactor km, button sb, thermal relay fr and fuse fu and other control electrical appliances. work first air q switch closed,into the power, when the press the start button (dynamic close contact points), the coil of the ac contactor km energized, moving core is attracted, the three main contacts (contact closed. the speed of the motor through electric starting. when the release button contact restored to original broken position, but due to and parallel ac contactor km of auxiliary contacts (contact) and main contact is closed at the same time, so contactor coil circuit is switched on, and make contacts of the contactor is maintained in a closed position. the auxiliary contact is called the self locking contact, which has the function of self locking for the motor to run for a long time. if the stop button is stopped, the circuit of the contact coil is cut off, the moving iron core and the contact point are restored to the open position, and the motor is stopped.the control circuit has the functions of short circuit protection, overload protection, pressure loss and under voltage protection, besides the starting and stopping control functions of the motor.4-18. test to be able to make a point of control, but also can be used as a direct starting control (two switch conversion) control circuit.answer:4-19. test of the positive inversion control circuit in the contact of the contacts of the contactor what role?answer: if the two ac contactor at the same time, their main contacts are closed, will cause short circuit. in order to ensure the two contacts by not working at the same time, in thecontrol circuit, connected in series with the other contactor move off of auxiliary contact points and namely positiverotation contactor of a dynamic fault of auxiliary contact points on connected in reverse rotation contactor coil circuit, and the overturning contactor of a dynamic breaking auxiliary point connected in positive rotation contactor coil circuit. these two are called interlocking contacts. in this way, when you press a positive starting button, positive rotation contactor coils, and the main contact, motor forward. at the same time, the interlocking contacts are disconnected from the coil circuit of the reverse contactor. therefore, even if the reverse starting button is mistaken, the contactor can not be moved. at thesame time the two contactor only allows one to work in a controlled role called interlocking or interlocking.4-20. programmable controller has what function? what is the advantage of the traditional relay contactor control method when it is used in the motor control?answer: the relay - contactor control is the traditionalindustrial control mode, it relay contacts and coils accordingto the control logic relation of certain connected control circuit, control contactor on-off, followed by the motor or solenoid device drive mechanism motion. however, because the relay itself accounts for a certain size and power consumption, often a failure, coupled with the fixed wiring, so that change control logic is difficult. so the application of complex control system reliability and flexibility are relatively poor.【篇三：电工学概论习题答案第四章】txt>4-1. 怎样从三相异步电动机的结构特征来区别笼型和绕线型？答：转子绕组的作用是产生感应电动势、流过电流和产生电磁转矩，其结构型式有笼型和绕线型两种，笼型转子的每个转子槽中插入一根铜导条，在伸出铁心两端的槽口处，用两个短路铜环分别把所有导条的两端都焊接起来。