Two conductor C1 and C2 are laid parallel at center to center distance of 3 m in air, If the lengths of conductors 1 and 2 are 50 m and 10 m respectively, and the currents flowing through them are 750 and 500 A respectively. Use the information to answer QI to Q4
What is the magnetic flux density produced by C1 at the midpoint perpendicular between
a. 0.0001T b. 0.001T c. 10000T d. 1000T
B1 = μo I/2πr = (4π × 10-7 × 750)/(2π × 1.5) = 0.0001 = 1.0 × 10-3 T
What is the magnetic flux density produced by C2 at the midpoint perpendicular between them?
a. 67000T b. 0.00067T c. 0.000067T d. 6700T
B1 = μo I/2πr = (4π × 10-7 × 500)/(2π × 1.5) = 0.000067 = 6.7 × 10-5 T
Calculate force experienced by C1 due to C2?
a.1.2N b. 0.25N c. 0.2N d. 1.25N
F1 = B2 I1 L1
B2 = μo I/2πr = (4π × 10-7 × 500)/(2π × 3) = 3.3 × 10-5 T
Therefore F2 = 3.3 × 10-5 × 750 × 50 = 1.25N
Calculate force experienced by C2 due to C1?
1.2N b. 0.25N c. 0.2N d. 1.25N
F2 = B1 I2 L2
B1 = μo I/2πr = (4π × 10-7 × 750)/(2π × 3) = 5.0 × 10-5 T
Therefore F2 = 5 × 10-5 × 500 × 10 = 0.25N
Permeance is define as?
a. Inverse of reluctivity b. Reciprocal of permeability c. Reciprocal of Conductance d. Same as permeability
Inverse of reluctance.
Time varying magnetic flux can be obtained from?
a. A coil carrying direct current b. Permanent Magnet c. A coil carrying alternating current d. Silicon IV oxide
A coil carrying alternating current
The induced emf in a generator is directly proportional to…
a.Its starting torque b. The sped of its amature c. The reluctant of the core d. Time of operation
The speed of its armature
Hysteresis can be summarily described by a situation where..
a.Current lag flux density b. Current lead flux density c. Flux density lead magnetizing field d. Flux density lag magnetizing field.
flux density lag magnetizing field.
The net effect of fringing is to…
a. Increase the length of the path b. Decrease the area of the path c. Increase the area of the path d. The flux density
Increase the area of the path
Eddy current loses is a primarily function of?
a.The resistance of the core b. The resistance of the coil c. The frequency of the source d. The resistance of the coil
The resistance of the
Core loses can be term as loses due to…?
a.Eddy current and Hysteresis Eddy b.current and copper c. Copper d. Hysteresis
Eddy current and Hysteresis
Using a scale of 1 cm – 100AT/m and 1cm=0.2 T, if the maximum height of hysteresis loop is 14cm. what is the maximum flux density?
a.140T b. 280T c. 2.8T d. 1400T
Bmax = 0.2 × 1.4 = 2.8T
A cast steel magnetic structure made of a bar of cross sectional area of 4 squared centimeter, the length of limbs A, B and C are 25cm, 15cm and 25cm respectively. And the relative permeability of the steel is 600. Neglect flux leakage. Use thisothis question toato Q13 to Q17.
Calculate the magnitude of Flux in the limb C.
a. 3.33mWb b. 2mWb c. 5.55mWb d. 0.4mWb
Bmax = 0.2 × 1.4 = 2.8T
Ra = Rc = l/μA= (25 × 10-2)/(600 × 4π×10-7 × 4 × 10-4 ) = 828931.995 AT/Wb
Rb = l/μA = (15 × 10-2)/(600 × 4π×10-7 × 4 × 10-4 ) = 497359.2 AT/Wb
Øb Rb = Øc Rc
Øc = (Øb Rb )/ Rc = (3.3 × 10-3 × 497359.2)/ 828931.995 = 0.0098 =2.0mWb
What is the value of flux density in limb A?
a. 12.2T b. 5.33mWb c. 5.55mWb d. 0.4mWb
Øa = Øb + Øc = 3.3 + 2 = 5.3mWb
Ba = μH = (μ Øa Ra)/La = (600 × 4π×10-7 × 5.3 × 10-3 × 828931.995)/25 × 10-2
=13.25T
What is the reluctant of the path A is approximately equal to?
a.0.5M AT/wb b. 5MAT/wb c. 500AT/wb d. 0.4MAT/wb
Ra = l/μA= (25 × 10-2)/(600 × 4π×10-7 × 4 × 10-4 ) = 828931.995 AT/Wb
What is the magnetizing force in limb B?
a.11050 AT/m b.1105 AT/m c. 11050N d. 1105N
H = (Øb Rb )/Lb
= (3.3 × 10-3 × 497359.2)/15× 10-2
What is the value of current I?
- 156A b. 12.156A c. 8.125A d.20.156A
I = (Øa Ra )/N = (5.33 × 10-3 × 828931.995)/500 = 8.786A
Coercive force is the force that?
a.Remove the negative magnetism b.Remove the residual magnetism c. Reduce the residual magnetism d. Reduce the negative magnetism
Remove the negative
What is the hourly loss of energy in Kwh in a specimen of iron of the hysteresis loop of which is equivalent in area to 250 at frequency of 50 Hz, specific gravity of iron is 7.5, weight of specimen is 10 kg?
a.05Kwhr b. 60 Kwhr c. 0.6 Kwhr d. 0.017 Kwhr
s.g = 7.5 = ρobj / ρwater
ρobj = 7.5 × 1000 = 7500 Kg/m3
v = m/ρ = 10/7500 = 0.0013 m3
No of cycles/hour = 250 × 50 = 3000 cycles/hour
Loss/hour =250× 0.0013 × 3000 = 1000/(36× 105) = 27.8 × 10-5 KWh
An hysteresis loop is drawn to the following scale, 1 cm-1000 AT/m; 1 cm = 0.2 T, If the area of the loop is 9 cm squared and the overall height is 14 cm. the hysteresis loss in J/m/cycle is?
a.900J/m3/cycle b.9000900J/m3/cycle c.180900J/m3 d.1800900J/m3/cycle
loss = xy × area =0.2 × 1000× 9 = 1800J/m3/cycle
The property of a material to oppose the establishment of magnetic flux is?
a.Flux resistance b. Flux permeance c. Reluctance d. Suceptibility
Reluctance
What is the magnetic field strength established by a current of 20 A passing through a coil of 100 turns whose length is 1km?
a.5AT/m2 b.0.5m/AT c.2AT/m d.2m/AT
H = NI/L = (20 × 100)/1000 = 2AT/m
What will be the flux density in a magnetic circuit whose area is 200 cm, if a flux of 0.1Wb passes through it?
a.2000Telsa b.5 tesla c. 20 Tesla d. 0.2Tesla
B = Ø/A = 0.1/(200 × 10-4) = 5T
Relative permeability of vacuum is?
a. 4pi x10-7H/m b. 1 c. 1H/m d. 1/4pi
1 no units
The unit of magnetic flux is?
a.Ampere-turn b. tesla c. weber d. coulomb
Weber
Energy loss due to hysteresis is directly proportional to….?
a.Moment of magnetic field b. Area of hysteresis loop c. Volume of hysteresis loop d. Loop current
Area of hysteresis loop
According to Faraday, e.m.f can be produced by all of these except?
a. Rotating coil in a stationary flux b. Moving coil in a vacuum c. Stationary coil in revolving flux d. both coil and flux rotating
Moving a coil in a vacuum.
The direction of magnetic line of force within the magnet itself is?
a.West to East b.North to South c. East to West d. South to North
South to North
Reluctance of a magnetic material depends on the following except?
a.Permeability b.Area c. Length d. Current density
Current density
One of this is the useful effect of hysteresis?
a.Lux creation b.Humming in machine c. Heating d. Memory fabrication
Memory fabrication
Calculate the torque in Newton-meter developed by a 440V, D.C motor having an armature resistance of 0.25 and running at 900r.p.m when a current when a current of 60A?
a.271N-m 325N-m c. 268N-m d. 324N-m
T = EI/[2π(N/60)] = (V-IR)I/[2π(N/60)]
=(440 – 60 × 0.25)60/[2 π(900/60)]
=270.56Nm
Calculate the current drawn by a 110V D.C generator of armature resistance 0.50 and back e.m.f 120V?
a.240A b.20A c.220A d.19.5A
E = V + IR
I =( E –V)/R = (120 -110)/0.5 =20A
The chief disadvantage of a low power factor is that?
a. More power is consumed by the load b. Active power developed by a generator exceeds its rated output capacity c. Current required for a given load power is higher d. Heat generated is more than the desired amount
Current required for a given load power is higher
The frequency of the alternating emf depends on the number of …………and the……… of rotation of the rotor.
a.Poles and flux b. flux and speed c. Speed ad frequency d. pole and speed
Poles and speed
The no-load voltage ratio in a 1-phase. 50Hz core-type transformer is 1200/440. Find the number of tums in each winding if the maximum flux is to be 0.075Wb.
a.72 and 26 turns b. 24 and 74 turns c. 26 an72 turns d. 2.6 and 7.2 turns
E = 4.44fNØm
N1 = 1200/(4.44 × 50 × 0.75) = 72
N2 = 440/(4.44 × 50 × 0.75) = 26
No load test on a transformer is carried out to determine:
a.Magnetizing current and no load loss b. Efficiency of the transformer c. Magnetizing current d. Copper loss
Magnetizing current and no load loss
Which of the following methods is used to prevent the energy lost in the form of eddy current in a transformer?
a.Laminating the core b.Making the coils with wire of low resistance c. Using a sot iron core d. Using special alloys in the core
Laminating the core
The equivalent resistance of the secondary of a transformer having k = 4 and Ra=15 when referred to primary becomes
a. 0.938ohm b. 240ohm c. 1.067ohm d. 6.004ohm
R’2 = R2 /K2 = 15/42 = 0.938Ω
The voltage induced in the three windings of a 3- phase alternator are ….. degrees apart in time phase.
- 120o b.60o c. 90o d.240o
120°
An electric motor in which both the rotor and stator fields rotate with the same speed is called a/an……motor?
a. d.c b. Synchronous c. excited d. universal
Synchronous
The sole purpose of a commutator in a D.C generator is to?
a.Increase output voltage b.Reduce sparking at brushes c. Convert the induce a.c into d.c d. provide smother
Convert the induced A.C into D.C
The direction of induced e.m.f can be found with the help of?
a.Fleming’s right-hand rule b. Len’s law c. Right flat hand rule d. All of the above
Fleming’s right-hand, Flat right hand rule and Lenz’s law
One of the characteristics of a single phase motor is that it
a. It is self starting b.Can rotate in one direction only c. Is not self starting d. requires only one winding
Is not self-starting
The following methods are true for speed control of a D.C motor except?
i. Flux control Amature resistance control iii. Applid voltage control iv. Back e.m.f control v. Torque control
a. I and V b. IV and V c. II and IV d. III and IV
Torque Control and back e.m.f control
A wave winding must go atleast …………. Around the armature before it closes back where it started.
a.Four times b.Twice c. Thrice d. once
Twice
A 4-pole, lap connected d,c motor has 600 conductors and draws an armature current of 10A. If the flux per pole is 0.02Wb. Calculate the armature torque developed.
a. 18.3Nm b. 19.1Nm c. 20.0Nm d. 17.3Nm
T = EI/[2πN/60] = [ZØN/60 × (p/a) × I]/[2π(N/60)]
=(0.02 × 600 × 10) / 2π
= 19.1Nm
The no-load current of a transformer is 5.0A at a 0.3 power factor, when supplied at 230V, 50Hz
The number of tums on the primary winding is 200. Use the information to answer questions 47 and 48.
Calculate the maximum value of flux in the core
a. 5.18mWb b. 5.18Wb c. 4.18Wb d. 4.18mWb
E = 4.44fNØm
Øm = E/4.44fN = 230/(4.44 × 50 × 200) = 5.18mWb
Find the magnetizing current
a. 3.77A b. 4.77A c.5.77A d. 1.50A
Iμ = 5 × 0.3 = 1.5A
Which of the following is true about ideal transformer:
a.Winding have resistance c. Pressure of magnetic linkage Core has infinite permeability d. Negligible copper loss
Negligible copper losses
Which of the following is a basic element of a Transformer?
a.Iron loss b. Eddy current c. Copper losses d. Core
Core
A conductor of length 2m moves at an angle of 30° to a uniform magnetic field of flux density 1.5Wb/m2, with a velocity of 50m/s. Find the emf induced in it?
a. 2.6v b. 1.5v c. 75v d. 129.9v
E = Blv sinθ = 1.5 × 2 × 50 × sin 30 =75V
The following AC motors are true for the single phase motors except?
i. Induction motor Compound motors iii. Shunt series motors iv. AC series motor
a. I and II II and IV c. II and III d. I and IV
Compound and shunt series motors.
The output current corresponding to maximum efficiency for a transformer haying core loss of 100W and equivalent resistance referred to secondary of 0.25 is?
a. 1.25A 5A c. 20A d. 16A
I2R = P
I = √(P/R) = √(100/0.25) =20A
Who stated the law that the direction of the induced emf is always such as to oppose the change producing it?
a.Faraday b.Maxwell c. Lenz d. Fleming
Lenz
Efficiency can be computed by determining…….From no-load or open-circuit test and…….from the short-circuit test.
a.Core loss and copper loss b.Eddy current and iron loss c. Copper loss and hysteresis loss d. Core loss and iron loss
Core loss and copper
The main purpose of performing open circuit test on a transformer is to measure its:
a.Copper loss b. Core loss c. Total loss d. Insulation resistance
Core loss
The magnitude of the induced emf is directly proportional to
a.No of turns in a circuit b.Rate of change of mmf c. Rate of change of magnetic flux linkage d. Flux density
Rate of change of the magnetic flux
In a 25KVA, 2000/200V, single phase transformer, the iron and copper losses are: 250W and 250W respectively. Calculate the efficiency of the transformer?
a. 95% b. 94% c. 100% d. 98%
Efficiency = 1 – (iron loss + copper loss)/input power
1- (250 +250)/25 × 103 =98%
The physical basis of a transformer is mutual induction between two circuits linked by a…
a. Different magnetic flux b.Mutual magnetic flux c. Magnetic flux d. Common magnetic flux
Mutual magnetic flux
A transformer transforms:
a. Voltage Frequency c. Power d. Voltage and current
Voltage and current
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