১.
Leakage flux in electrical machines is
ব্যাখ্যা
Leakage flux does not link the intended winding/armature and does not contribute to energy conversion
৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি (EEE) [ ৮৯২]
মোট প্রশ্ন৩০
সিলেবাস
Exam 3
i) Electrical Machines and Transformers — Introduction: Leakage Flux, Faraday’s Laws of Electromagnetic Induction, Induced EMF, Self and Mutual Inductance, AC Excitation in Magnetic Circuit, Eddy Current Loss
[Source: Class–3 and relevant books]
ii) DC Generator.
[Source: Classes 2–3 and relevant books]
৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি (EEE) [ ৮৯২]
৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি (EEE) [ ৮৯২] · তারিখ অনির্ধারিত · ৩০ প্রশ্ন
২.
In a transformer, leakage flux mainly causes
ব্যাখ্যা
Leakage flux induces leakage reactance which leads voltage drop
৩.
Which design step reduces leakage flux in machines?
ব্যাখ্যা
Smaller air gap improves flux linkage that means less leakage
৪.
Faraday’s Second Law is-
ব্যাখ্যা
E=−NdΦ/dt
৫.
A coil with 200 turns links a flux of 0.02 Wb. If flux collapses to zero in 0.01 s, induced EMF =?
ব্যাখ্যা
E=NΔΦ/Δt=200×(0.02/0.01)=400V
৬.
Direction of induced EMF in electromagnetic circuit is given by-
ব্যাখ্যা
Lenz’s law states that the induced EMF opposes the cause producing it
৭.
Which factor does not affect induced EMF?
ব্যাখ্যা
E = (P * Φ * Z * N) / (60 * A)
E: is the generated EMF in volts.
P: is the number of poles in the generator.
Φ: is the flux per pole in Webers.
Z: is the total number of armature conductors.
N: is the speed of the armature in revolutions per minute (RPM).
A: is the number of parallel paths in the armature winding
৮.
A conductor of length 0.5 m moves with velocity 10 m/s in a 1 T magnetic field. Induced EMF =?
ব্যাখ্যা
E=Blv=1×0.5×10=5V
৯.
If speed of a generator is doubled, induced EMF will be-
ব্যাখ্যা
the induced emf is directly proportional to the speed i.e E∝N
১০.
Two coils have mutual inductance 0.5 H. If current changes at 4 A/s in primary, EMF induced in secondary is-
ব্যাখ্যা
E=Mdi/dt=0.5×4=2V
১১.
When an AC is applied to magnetic circuit, the generated flux is-
ব্যাখ্যা
AC supply produces AC magnetic field
১২.
In AC excitation, core loss consists of
ব্যাখ্যা
Core losses = hysteresis + eddy current losses.
১৩.
AC excitation causes eddy current loss to be proportional to
ব্যাখ্যা
Pe ∝ f2B2t2
B is the maximum flux density
f is the frequency of the magnetic field
t is the thickness of the material
১৪.
Which method reduces eddy current loss
ব্যাখ্যা
Laminations increase resistance to eddy current paths.
১৫.
Eddy currents are-
ব্যাখ্যা
Induced circulating currents in metallic core due to flux change
১৬.
In the EMF equation of a DC generator, P represents-
ব্যাখ্যা
E = PΦZN/60A , where P denotes the number of poles.
১৭.
The main function of the commutator in a DC generator is
ব্যাখ্যা
The commutator rectifies the generated AC into DC
১৮.
Which type of DC generator has the field winding connected in parallel with the armature?
ব্যাখ্যা
In a shunt generator, the field winding is connected parallel to the armature.
১৯.
Residual magnetism in a generator is required for
ব্যাখ্যা
Residual magnetism provides the initial flux needed to start EMF generation
২০.
Armature reaction in a DC generator mainly causes-
ব্যাখ্যা
Armature reaction distorts the main field which affects commutation.
২১.
The no-load characteristic of a DC generator is also called-
ব্যাখ্যা
The open-circuit characteristic (OCC) or No load characterstics is also called the magnetization curve
২২.
Which generator is best for constant voltage applications?
ব্যাখ্যা
Shunt generators maintain nearly constant voltage with varying loads.
২৩.
In a DC generator, if speed is increased by 20% , the generated EMF will-
ব্যাখ্যা
EMF is directly proportional to speed (?∝?). so same amount changes will occur
২৪.
The purpose of interpoles in a DC generator is-
ব্যাখ্যা
interpoles reduce sparking and improve commutation by neutralizing armature reaction
২৫.
The voltage regulation of a DC generator is defined as
ব্যাখ্যা
Voltage regulation is the change from no-load to full-load voltage it is expressed in percentage.
২৬.
The armature winding type suitable for low-voltage, high-current generators is
ব্যাখ্যা
Lap winding has more parallel paths, suitable for high-current applications
২৭.
The load characteristic shows the relation between
ব্যাখ্যা
Load characteristic plots terminal voltage vs load current.
২৮.
The efficiency of a DC generator is maximum when
ব্যাখ্যা
Maximum efficiency occurs when variable copper losses equal constant iron losses
২৯.
A separately excited generator require
ব্যাখ্যা
In separately excited generators, field winding is supplied from an external DC source
৩০.
If the armature resistance of a generator is 0.1 Ω and it supplies a current of 100 A at 200 V terminal voltage, find the induced EMF
ব্যাখ্যা
E=V+IaRa =200+(100×0.1)=210V