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




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