جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة Al Azhar University Faculty of Engineering نموذج الورقة األولى من المشروع البحثى الجامعة جامعه األزهر الكلية كلية الهندسة ـ بنين ـ القاهرة القسم قسم الهندسه الكهربيه قوي واالت الفرقة الثالثه قوي واالت المادة االت كهربيه الفترة الدراسية دور مايو 2020-2019 أستاذ المادة د.محمد كمال الشاعر النتيجــــــــــــة راسب ناجح الدرجـــــــــــة أرقام حروف الرقم الســـرى اسم الطالب محمود صالح سليم ابراهيم رقم الجلوس 102073 الرقم السرى الدرجــــة أرقام حروف Al Azhar University Faculty of Engineering جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة Starting methods of synchronous motors A synchronous motor is a device which converts the AC into mechanical work at synchronous speed. The starting of the Synchronous Motor does not take place on its own. This means that the Synchronous Motor is not Self. To understand the nature of the starting problem, refer the Following figure. this figure shows a 60- Hz synchronous motor at the moment power is applied to its stator windings. The rotor of the motor is stationary, and therefore the magnetic Al Azhar University Faculty of Engineering جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة Staning problems in a synchronous motor---the torque alternates rapidly in magnitude and direction. so that the net starning torque is zero. The rotor of motor is static so the field at the rotor BR is also static. The stator’s field BS starts to revolve in the stator at synchronous speed. In the above figure, the circuit denoted as a tells about motor at (t=0) seconds when the rotor’s field BR and Bs are in a straight line torque induced equation. tind = KBR x BS So the torque induced in the motor at t=0 according to torque equation will be zero. The circuit denoted as b explains the condition when the time (t = 1/240s). In this minor time interval motor hardly rotates, nonetheless, the direction of stator’s field is towards left side. According to torque equation, the direction of torque on the rotor is anticlockwise. The circuit in above figure denoted as c tells about the condition at (t=1/120s). at this time the rotor’s field and stator’s field are opposite to each other. The circuit denoted d explains the situation at (t=3/240), now both the field of rotor and stator are ninety degrees to each other and torque direction is clockwise. The last circuit that is denoted as e explains the situation at (t= 1/60s) at this time interval both the fields of stator and rotor are in a straight line. You can observe that from the above discussion that after completion of one electric rotation (cycle) first the torque direction was anticlockwise then clockwise and after completion of cycle average torque was 0. Due to these such conditions, motor starts vibration after every electrical rotation (cycle) and gets overheated. So It can be started by the following methods given below. Al Azhar University Faculty of Engineering جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة 1- Starting by Reducing Electrical Freqquency If the rotation speed of field at stator is less so rotor can be very easily interlocked with it. After that speed can again reach its normal operating frequency that is fifty or sixty hertz. Rectifier inverters and cycloconverter can be used for this process first to decreases frequency than increase it. With the invention of this circuitry nowadays it is very easy to vary the frequency from zero to desired value or overrated frequency. If we linked these frequency variable circuits with motor starting circuitry than it is very easy to change frequency at start of motor than increase according to system requirements. Al Azhar University Faculty of Engineering جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة If motor is working at a speed less than the rated its internal generated voltage will also be less than normal value of voltage. With the decrement in internal generated voltage value, there will also decrease in terminal voltage of motor to maintain stator current at a safer level. In any variable frequency starting circuitry voltage will vary directly proportional to the frequency variations. 2- Starting with the help of an External Prime Mover The second technique is to connect external starting motor with a synchronous motor to get its desired rated speed. When the motor gets its required speed then remove the exterior motor connected with the synchronous motor. When an external source is connected with motor removed then speed of rotation of shaft decreases, then the field of rotor BR lags the net field of a machine than generator behavior of motor ends and it starts to operate as a normal motor. After that load can be connected with the synchronous motor for normal operation. This entire process is not as absurd as it looks, as numerous synchronous motors are fragments of motor-generator sets, and the synchronous machine in the motor-generator set can start its operation with the other machine working as the starting motor. Similarly, the starting motor requires to decrease the inertia of the synchronous machine without a load, load connects with motor until it paralleled to the system. As the starting motor is used only to decrease inertia so its power rating should be less than the motor to start. جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة Al Azhar University Faculty of Engineering 3-Motor Starting by Using Amortisseur windings Damper Windings is the most widely used methods to start a synchronous motor. A Damper Winding consists of heavy copper bars inserted in the slots of the pole faces of the rotor as shown in the figure below. These copper bars are short-circuited by end rings at both ends of the rotor. Thus, these shortcircuited Bars form a squirrel cage winding. When a three phase supply is connected to the stator, the synchronous motor with Damper Winding will start. It works as a three -phase induction motor. As soon as the motor approaches the synchronous speed, the DC excitation is applied to the field windings. As a result, the rotor of the motor will pull into step with the stator magnetic field. eind = (v x B) • L where V=velocity of the bar relative to the magnetic field B = magnetic flux density vector L = length of conductor in the magnetic field جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة Al Azhar University Faculty of Engineering Al Azhar University Faculty of Engineering جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة The bars at the top of the rotor are moving to right relative to the magnetic field, so the resulting direction of the induced voltage is out of the page. Similarly, the induced voltage is into the page in the bottom bars . These voltages produce a current fl ow out of the top bars and int o the bottom bars, resulting in a winding magnetic field Bwpointing to the right. By the induced-torque equation 370 ELECTRIC MACHINERY RJNDAMENTALS the resulting torque on the bars (and the rotor) is counterclockwise. Figure 6-1 9b shows the situation at t = 11240 s. Here, the stator magnetic field has rotated 90° while the rotor has barely moved (it simply cannot speed up in so short a time). At this point, the voltage induced in the amortisseur windings is zero, because v is parallel to B. With no induced voltage, there is no current in the windings, and the induced torque is zero. Figure 6-1 9c shows the situation at t = 11120 s. Now the stator magnetic field has rotated 900, and the rotor still has not moved yet. TIle induced voltage [given by Equation ( 1-45)] in the amortisseur windings is out of the page in the bottom bars and into the page in the top bars. The resulting current flow is out of the page in the bottom bars and into the page in the top bars, causing a magnetic field Bwto point to the left. 1lle resulting induced torque, given by T;Dd = k Bw x Bs is counterclockwise. Finally, Figure 6-1 9d shows the situation at time t = 31240 s. Here, as at t = 11240 s, the induced torque is zero. Notice that sometimes the torque is countercl ockwise and sometimes it is essentially zero, but it is always unidirectional. Since there is a net torque in a single direction, the motor's rotor speeds up. (1llis is entirely different from starting a synchronous motor with its normal field current, since in that case torque is first clockwise and then counterclockwise, averaging out to zero. In this case, torque is always in the same direction, so there is a nonzero average torque.) Although the motor's rotor will speed up, it can never quite reach synchronous speed. This is easy to understand. Suppose that a rotor is turning at synchronous speed. Then the speed of the stator magnetic field Bs is the same as the rotor's speed, and there is no relative motion between Bs and the rotor. If there is no relative motion, the induced voltage in the windings wi ll be zero, the resulting current fl ow wi ll be zero, and the winding magnetic field will be zero. Therefore, there will be no torque on the rotor to keep it turning. Even though a rotor cannot speed up all the way to synchronous speed, it can get close. It gets close enough Al Azhar University Faculty of Engineering جامعـــــــــــة األزهــــــــــــــــــــر كليـــــة الهندســــة بنين بالقاهـرة to n'YD< that the regular field current can be turned on, and the rotor will pull into step with the stator magnetic fields. In a real machine, the field windings are not open-circuited during the starting procedure. If the field windings were open-circuited, then very high voltages wou ld be produced in them during starting. If the field winding is short-circuited during starting, no dangerous voltages are produced, and the induced field current actually contributes extra starting torque to the motor. To summarize, if a machine has amortisseur windings, it can be started by the fo llowing procedure: I. Disconnect the field windings from their dc power source and short them out. SYNCHRONOUS MOTORS 371 2. Apply a three-phase voltage to the stator of the motor, and let the rotor accelerate up to near-synchronous speed. The motor should have no load on its shaft , so that its speed can approach n.ync as closely as possible. 3. Connect the dc field circuit to its power source. After this is done, the motor wi ll lock into step at synchronous speed, and loads may then be added to its shaft.
