I will be very thankful if you answer me some questions about electric machines:

1) All electric machines , transformer, induction machine, DC machine and synchronous machine produce an emf (induced voltage) in their windings, can we measure this voltage at lab? And how? Or we just can calculate it? For example, in transformer the emf at the primary winding doesn’t equal to the applied voltage because of the drop voltage although the winding is connected in parallel with applied voltage, so why?
2) Pole-changing is available only with squirrel-cage type at the stator, why there is no need to change the number of poles at the rotor also?
3) To produce the rotating magnetic field we need, three-phase winding and three-phase source voltage, why it doesn’t happen at the three-phase transformer?
4) Is there any other way to connect the three-phase winding beside the star and delta connections?
5) In induction motors, at normal status the speed of rotating field never reach to the speed of rotor, why? In other word, why we can find the slip only in this kind of machines?
6) For the same value of output power, the single-phase induction motors draw a value of current from the source greater than the three-phase motors, why?
7) Is there any effect for changing of the direction of induction motors rotating ? (for example change of efficiency or power…etc.)
8) DC. Machine is kind of synchronous machine with commutator, the difference is just the placement of armature and field poles, can we consider the dc machine without commutator, poles in stator and armature in rotor can we consider it a synchronous machine?

al salam alaykom wa rahmat allah wa barakatoh
[first)as the impedance of the winding is distribute across the whole winding ,then we will have a voltage drop distributed on the whole winding length,besides there will be magnetic,eddy ciurrent etc losses olso,
accordingly to calculate the induced valtage you require to add the total lossed vectorially to the output voltage
unless you can find a special voltmeter that can be programmed to calculate those losses automatically by special program that simulate the valuse from instrument readindgs.
second)rotor poles in squerral cage induction motor will build up same no of stator poles,so if no of poles in the stator is 4 the rotor induced poles will be the same no of the stator ,this is the reason that we don't take care of rotor poles.
third)to produce rotating feild ,we need multiphase supply with winding the same no of phases but in a condition is that it should be distributed equally on the stator as we said the angle between the windings is the same
in 3 phase system the angle between windings is 120
while in transformer this assembly is not exist.
forth)there are some other connections like zig zag and open delta ,and for each there is special characteristics,better to refer to machine references.
fifth)when the whole stator power transformed to the rotor with out any losses then the slip will be zero and the rotating speed will be same as the rotating feild
this is an ideal situation as we have losses which make the slip,and if we have load the slip will be bigger as it will be an important value in the rotating resistance that explane the rotatong power r2/s(1-s)
and r2 is the the rotor resistance shown from the stator side
sixth)if you make and equation for same power it will be
power in 1phase =same power in 3phase
V I(1 phase) cos x=root 3 V(L-L)I(phase) cos x)
if we suggest the cos x is the same
(then VI single phase=V(L_L) root 3 i(3ph)
note that for the same power the current in 3phse is multiplied by root 3
and don't forgent we have 3 currents like that in the three branches
but olse the voltages are not the same as you see
seventh)changing the direction of rotation will not change any thing in 3phase motor,but iin 1phase motor it dependson motor design and starting method.
eighth)this question i am not sure of the answer but without commutator how can we supply power to the armetur
another thing is that in synchronous machines the stator windings are distributed electricaly with 120 degree shift ,this is not exist in DC machine,.
i hope that i gave some resonable answers,sorry if any mistakes appear and waiting for the corrections if there is any
thanks and regards
hani

شكرا هاني، لاتتصور كم كانت إجابتك لاسئلتي مفيدة جداً ، كما أنها تتسم بالبساطة والوضوح وخالية من أي تعقيد، سؤال آخر إذا لم يكن في ذلك أي إزعاج ، مالذي تعرفه عن ال Reluctance motors هو شبيه جدا بالمحرك ذو القفص السنجابي لكن لا أعرف الفرق بينهما...شكرا مرة ثانية

شكرا لك اخ عادل واشكرك بشده اخ هاني ودائما مميز .... اضافه فيما يخص السؤال الاول يمكن قياس emf عن طريق open circuit وهذه الطريقه تستخدم في المعمل لايجاد قيم الدائرة المكافئة للمولد او المحول مثلا في المولد التزامني لتحديد قيم reactance & resistance نستخدم الخطوات :
1- open circuit
2-short circuit

اما بالنسبة لل reluctance motor للاسف لايوجد لدي اي معلومات عنه او قد يكون هناك تسمية اخرى له.

تحياتي للجميع

السلام عليكم ورحمة الله وبركاته
هذه نبذة بسيطة عن ال reluctance motor او مايسمى بالمحرك المعاوقي هكذا كنا نسميه ايام الدراسة في الجامعة
المحرك يمكن ان يكون حاويا على اقطاب مغناطيسية ملفوفة على الدوار silent pole rotor
بشرط وجود فارق كبير بين Xd and Xq
وهي المعاوقات على مسار القطب وعلى المسار بين القطبين

الفكرة هي انه عند تحليل القدرة power رياضيا يظهر حدين احدهما دالة بال Xd and Xqكما موضح
ُ
total power=[V Ef/Xd sin x + V^2/2Xd (Xd/Xq-1) sin 2x
لاحظ ان الحد الثاني بعد اشارة الجمع يحتوي على Xd and Xq ولا يحتوي على Ef اي انه ليس له علاقة بالمجال المغناطيسي وانما هو داله بالمعاوقات على المحورين d , q
اذا كانت المعاوقات متساوية فان الحد الثاني صفر ويبقى الحد الاول فقط والمحرك يعمل كمحرك متزامن
اما اذا كانت المعاوقات متباينة فان الحد الثاني موجود وله قيمة معينة وهو يمثل ال reluctance torqueويبقى موجود حتى لو كان الحد الاول صفر
وهذا النوع يستعمل في الاحمال القليلة ويكون البدء للمحرك على شكل محرك حثي الى ان يصل الى السرعة المطلوبة حيث يظهر تاثير العزم المعاوقي ويستمر بالدوران بواسطته
هذه المعلومات اخذتها من كتاب ELECTRICAL MACHINES AND THEIR APPLICATIONS
للمؤلفJOHN HINDMARSH
وللمزيد يمكن الرجوع للكتاب
وشكرا