Ques 71. In star-delta starting of three-phase induction motor, the starting voltage is reduced to
√3 times of normal voltage
3 times of normal voltage
1/3 times of normal voltage
1/√3 times of normal voltage✔
Star Delta Starter (Y – Δ) is a common type of three-phase (3 phase) induction motor starter generally used in low starting torque motors.
At the time of starting the motor, the rotor is at a standstill and the slip ( between the stator’s magnetic field and rotor) is large which causes the large inrush of armature current (which is 6–7 times the rated value). This large current can damage the stator windings and burn the motor.
To avoid this situation, we use the star-delta starter. At the time of start, motor connections (stator connections) are made in star mode so the impressed voltage is reduced by 1/√3 (Phase voltage = Line voltage/√3) which reduces the starting current. Once the rotor is at 80–90% speed, the centrifugal switch operates changing the connections from star to delta mode (full line voltage is impressed).
Ques 72. An electromagnetic torque is produced as an interaction between flux and current. The angle between flux and current is 45°. If this angle is changed to 30°, flux increases by 100% and current reduce by 25%, then the torque
Increases to 183.7% of the original✔
Reduces to 81.6% of the original
Reduces to 54.4% of the original
Reduces to 66.7% of the original
Electromagnetic torque
T ∝ K ΦIcosθ
Where K is designed constant = (PZ/2πA)
Where
P = number of poles
Z = total number of armature conductor
N = rotation speed of the armature in revolution per minute (r.p.m)
A = number of parallel paths
Now the flux Φ is increased by 100% i.e
Φ2 = Φ1+ 100% Φ1 = 2Φ1
I2 =0.75I1
T1 ∝ Φ1 I1cos45° &
T2 ∝ Φ2 I2cos30° = 2Φ1 x (0.75)I1 x cos30°
Then T2 = 1.838 T1 = 183.7% of T1
Hence the torque increases to 183.7% of the original.
Ques 73. The maximum temperature limit for class F insulation is
130°C
120°C
105°C
155°C✔
The electrical insulation system is sometimes referred to as insulation class or thermal classification. The different classes are defined by NEMA which is given below
Ques 74. Match List-I (Types of motors) with List-II (Characteristics) and select the appropriate response
a → 1, b → 2
a → 2, b → 1✔
a → 1, b → 1
a →2, b → 2
For any dc motor, torque is directly proportional to the flux and armature current. In the case of the series motor, the flux produced is directly proportional to the armature current. Hence torque is directly proportional to the square of armature current.
T ∝ Ia2
So as the current during starting is 1.5 times the rated so starting torque is higher for the series motor.
In dc shunt motor flux produced by field winding is proportional to field current Ish. Here the input voltage is constant so field current and flux is also constant. Therefore, a dc shunt motor is also called a constant flux or constant speed motor.
Ques 75. The motor characteristics best suited for traction purposes are those of
D.C series Motor✔
Synchronous Motor
Induction Motor
D.C shunt Motor
Dc series motors are used in traction applications due to their excellent starting torque properties. Since Torque is directly proportional to the square of the armature current resulting in high starting torque.
Ques 76. If the starting torque of a 3 phase induction motor Tst for DOL starting, that for star-delta starting of the same motor is
Tst/3✔
Tst/√3
√3 Tst
3 Tst
A DOL starter connects the motor terminals directly to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, a high starting current flows through the motor.
Star-delta-starter
In the above figure, the motor is designed for delta running and is started in star.
Zsc = Short circuit phase Impedance
V = Line to line voltage
For DOL starting in delta
Hence with star-delta starting, the starting torque is also reduced to one-third of starting torque as compared to DOL starting.
Ques 77. The iron loss in a 100 KVA transformer is 1 KW and full load copper losses are 2 KW. The maximum efficiency occurs at a load of
100 KVA
70.7 KVA✔
141.4 KVA
50 KVA
Full load KVA = 100 KVA
Iron loss = 1 KW
Copper Loss = 2 KW
The kVA loading corresponding to maximum efficiency
Ques 78. Which one of the following is correct?
The effect of field current on the main flux of a synchronous machine is called armature reaction
The effect of air gap flux on the armature current of a synchronous machine is called armature reaction
The effect of armature current on the main flux of a synchronous machine is called armature reaction✔
The effect of armature current on the air-gap flux of a synchronous machine is called armature reaction
The effect of armature flux on the main field of a synchronous machine is called the armature reaction
Ques 79. A lap wound DC generator having a 250 armature conductor runs at 1200 RPM. If the generator emf is 200V, then the operating flux of the DC generator is
0.08 wb
0.04 wb✔
0.06 wb
0.02 wb
The emf induced in a dc machine is given as
Where
E = PΦZN/60A
Φ = flux per pole in Weber
P = number of poles
Z = total number of armature conductor
N = rotation speed of the armature in revolution per minute (r.p.m)
A = number of parallel paths (Number of parallel paths in lap winding, A = P)
Therefore E = ΦZN/60
200 = 1200 × Φ × 250/60
Φ =0.04wb
Ques 80. The highest speed attainable by D.C shunt motor is
Much higher than the no-load speed
Equal to no-load speed✔
Much lower than the no-load speed
Ideally infinite
In the case of a DC shunt motor, flux is practically constant as the field is parallel to the armature. If the load is reduced then the speed of the motor will remain almost constant because the field current remains almost constant. DC shunt motor has full speed at no load and at full load, speed is slightly less (which can be negligible practically).
From the above discussion, it is clear that the highest speed attainable by the D.C shunt motor is equal to no-load speed
