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بسم الله الرحمن الرحيم
بعض الأسئلة التي وردت في مقابلات شخصية لمهندسي الكهرباء
معظم الأسئلة مجمّعة و منقولة بواسطة المهندس الفاضل: محمد سعد الدين (جزاه الله كل خير)
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س: ماهى أهم البيانات على لوحة بيانات الموتور ؟
ج:
Rated vol***e or vol***es
Rated full-load amps for each vol***e level
Frequency
Phase
Rated full-load speed
Insulation class and rated ambient temperature
Rated horsepower
Time rating
Locked-rotor code letter
Manufacturer's name and address
Frame size
Full-load efficiency
Power factor
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س: ماذا يحدث لمحرك حثي ثلاثي الاطوار إذا أنقطع أحد الفيوزات المغذية له اثناء اشتغاله ؟
ج:
يحترق بعد دقائق إذا كان محملا بحمل كامل أو ما يقاربه بسبب ازدياد التيار في ملفات الطورين الآخرين ومن ثم احتراقهما إذا كان المحرك مربوطا بشكل نجمة ، او احتراق ملفات الطور الواقع بين الفيوزين المغذيين المتبقيين إذا كان المحرك مربوطا بشكل مثلثي .
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س:
A 3 PHASE SQUIRREL CAGE INDUCTION MOTOR HAS القفص السنجابى
1.HIGH STARTING TORQUE
2.LOW STARTING TORQUE
LOW STARTING TORQUE
THE RUNNING SPEED OF 3 PHASE INDUCTION MOTOR IS
1.SYNCHRONOUS SPEED
2.LESS THAN SYNCHRONOUS SPEED
3.MORE THAN SYNCHRONOUS SPEED
LESS THAN SYNCHRONOUS SPEED
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MOTOR IS EQUAL TO IF THE ACTUAL SPEED OF INDUCTION
SYNCHRONOUS SPEED
1.DEVALOPE TORQUE
2.NOT DEVELOP TORQUE.
NOT DEVELOP TORQUE
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INDUSTRIAL DEVICES IS
COMMONLY USED A.C. MOTOR FOR
1.SYNCHRONOUS MOTOR
2.COMMUTATOR MOTOR
3. PHASE INDUCTION MOTOR
3. PHASE INDUCTION MOTOR
A 3 PHASE INDUCTION MOTOR IS
1.SELF STARTING
2.NOT SELF STARTING
SELF STARTING
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THE NUMBER OF POLES OF A 3 PHASE INDUCTION MOTOR AT 60 CYCLES RUNNING BELOW 1500 RPM WILL BE
1.4 POLE
2.6 POLE
3.2 POLE
6 POLES
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THE DIRECTION OF ROTATION OF 3 PHASE INDUCTION MOTOR IS
1.SAME AS THAT OF ROTATING MAGNETIC FIELD
2.OPPOSITE TO THE DIRECTION OF ROTATING MAGNETIC FIELD
SAME AS THAT OF ROTATING MAGNETIC FIELD
THE TORQUE IN A 3 PHASE INDUCTION MOTOR IS PROPORTIONAL TO
1.APPLIED VOL***E
2.SQUARE OF THE APPLIED VOL***E .V2
3.SQUARE ROOT OF APPLIED VOL***E
SQUARE OF THE APPLIED VOL***E
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WHAT IS THE DIFFERENCE BETWEEN STARTING OF 3 PHASE AND SINGLE PHASE INDUCTION MOTORS?
3 PHASE MOTOR IS SELF STARTING WHEREAS 1 PHASE MOTOR IS NOT.
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WHAT ARE THE DIFFERENT METHODS OF STARTING OF 3 PHASE INDUCTION MOTORS?
1) DIRECT ON LINE
2) STAR / DELTA
3) REACTOR STARTING
4) AUTO TRANSFORMER
WHAT IS THE EFFECT OF UNDER VOL***E ON 3 PHASE INDUCTION MOTOR ?
THE MOTOR TAKES MORE CURRENT FOR THE SAME LOAD.
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WHAT CHECKS YOU MUST CARRY OUT WHEN A MOTOR TRIPS ON
THERMAL OVERLOAD?
A) CHECK THE HEALTHINESS OF ALL THREE PHASES.
B) ENSURE ALL CONNECTION TIGHTNESS IN THE POWER CIRCUIT RIGHT FROM CONTACTOR UP TO MOTOR TERMINAL.
C) TAKE THE WINDING RESISTANCE MEASUREMENT AND SHOULD BE SAME FOR ALL 3 WINDINGS.
D) ROTATE THE SHAFT AND CHECK FOR JAMMING.IT SHOULD TURN FREELY.
E) CHECK THE CONTACTOR CONTACTS AND MAKE SURE THAT THERE IS NO PITTING ON THEM AND CONTACTS MAKE FIRMLY.
F) START THE MOTOR AND MEASURE THE CURRENTS IN ALL THE THREE PHASES WITH A TONGUE TESTER.IT SHOULD BE WELL WITHIN THE RATING OF THE MOTOR'S RATED CURRENT.IF THE MOTOR STILL TRIPS ON THERMAL OVERLOAD, SURELY THE BI-****LLIC THERMAL OVERLOAD IS TO BE CALIBRATED AND SET UP TO THE FULL LOAD RATING OF THE MOTOR
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IF YOU HAVE A MOTOR OF MORE THAN 100 KW CAPACITY AND YOU ARE ASKED TO COMMISSION IT, WHAT SUGGESTIONS YOU WILL GIVE FOR SELECTING THE SWITCHGEAR AND PROTECTION ?
IT IS NORMAL PRACTICE THAT WHENEVER THE MOTOR CAPACITY INCREASES
BEYOND 90 KW THE USE OF CIRCUIT BREAKER IS MADE INSTEAD OF POWER CONTACTOR DUE THE LIMITATION OF ITS DESIGN. AS REGARDS THE PROTECTION THE USE OF THERMAL BI -****LLIC RELAY IS AVOIDED AND USE OF MOTOR PROTECTION RELAY IS MADE WHICH SHOULD HAVE THE FOLLOWING FEATURES:
A) SHORT CIRCUIT PROTECTION (INSTANTANEOUS) TO BE SET TO 6 TO 8 * IN (IN=RATED CURRENT OF MOTOR).
B) BLOCKED ROTOR PROTECTION (AGAINST STALLING OF MOTOR TO BE SET 2 *IN AND DELAYED BY THE STARTING TIME OF MOTOR).
C) NEGATIVE PHASE SEQUENCE PROTECTION.
D) EARTH FAULT PROTECTION.
E) THERMAL REPLICA
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what are the conventional ways by which the speed of a polyphase induction motor can be changed؟
Answer :
1- For squirrel cage motor
- by using a stator winding which can be connected for a different no of poles .
- by varying the frequency .
2- For wound rotor :
- by cascade connection of two or more motors.
- by inserting vol***e in the rotor circuit .
- by inserting resistance in the rotor circuit
يتبع
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