Which of the following is NOT a type of DC machine?
A DC Generator B DC Motor C Universal Motor D Induction Machine
Induction machines are AC machines, not DC machines. DC machines include generators and motors.
In a transformer, the core loss is independent of which parameter?
A Frequency B Maximum flux density C Load current D Magnetic material used
Core loss depends on frequency and flux density but is independent of load current. It remains constant at rated voltage.
A DC shunt motor shows increased speed. What could be the reason?
A Increased load B Increased shunt field current C Decreased shunt field current D Increased armature resistance
Speed = (V - IaRa)/(kΦ). Decreasing field current reduces flux Φ, increasing motor speed.
An induction motor operating at slip s = 0.04 has rotor frequency of 2 Hz. Find the supply frequency.
A 40 Hz B 50 Hz C 60 Hz D 48 Hz
Rotor frequency = s × f. Therefore, 2 = 0.04 × f, giving f = 50 Hz
Which winding in a synchronous motor provides the initial torque for starting?
A Field winding B Damper winding C Armature winding D Excitation winding
The damper winding (squirrel cage) in a synchronous motor provides the initial asynchronous torque for starting until synchronism is achieved.
A 50 kVA, 230 2300
A 0.02 Ω B 0.2 Ω C 200 Ω D 2000 Ω
R'2 = R1 × (N2/N1)² = 2 × (2300 230
In a DC generator, the commutator serves the purpose of:
A Reducing eddy currents B Converting AC induced EMF to DC C Increasing the magnetic field D Cooling the machine
The commutator rectifies the alternating EMF induced in the armature coils to produce unidirectional (DC) output.
A 3-phase induction motor draws 50 A at 0.8 power factor lagging from 400 V supply. The input power is approximately:
A 13.86 kW B 27.71 kW C 34.64 kW D 20 kW
P = √3 × V × I × cosφ = 1.732 × 400 × 50 × 0.8 = 27,712 W ≈ 27.71 kW
The back EMF of a DC motor is given by:
A Eb = PΦZn/60A B Eb = PΦZn/120A C Eb = VIaRa D Eb = V + IaRa
Back EMF in DC motor Eb = PΦZn/(60A), where P = poles, Φ = flux, Z = conductors, n = rpm, A = parallel paths
Which of the following losses in a transformer occurs in both primary and secondary windings?
A Eddy current loss B Hysteresis loss C Copper loss D Magnetic loss
Copper loss (I²R loss) occurs in both primary and secondary windings. Core losses occur only in the magnetic core.
A 4-pole, 3-phase induction motor operates at a slip of 0.05. The rotor speed is:
A 1500 rpm B 1425 rpm C 1200 rpm D 1350 rpm
Ns = 120×4 50
In a synchronous motor, the pull-out torque depends on:
A Load torque only B Excitation current only C Supply voltage and field excitation D Speed variation only
Pull-out torque τmax = 1.5(PΦIf)/(ωs), depends on pole flux and field current (excitation) and supply voltage.
A transformer is operating at 110% of rated voltage. The core loss will be approximately:
A Same as rated B 10% more than rated C 21% more than rated D 5% less than rated
Core loss Pc ∝ B² ∝ V². At 1.1V, loss = (1.1)² × rated loss = 1.21 × rated ≈ 21% increase
Which starting method is suitable for a 3-phase induction motor with locked rotor current limit?
A Direct online starting B Star-delta starting C Autotransformer starting D Both B and C
Both star-delta and autotransformer starting reduce starting current. Star-delta reduces current by 3 1
The efficiency of a DC generator is maximum when:
A Iron loss equals copper loss B Iron loss equals friction loss C Copper loss equals friction and windage loss D All losses are equal
Maximum efficiency occurs when variable losses (copper) equal constant losses (friction, windage, and core losses).
A 100 kW, 3-phase induction motor has a full-load slip of 0.04. The rotor copper loss at full load is:
A 3.2 kW B 4 kW C 4.8 kW D 6.4 kW
Rotor copper loss = s × Pg, where Pg is air gap power. At full load, assuming 96% efficiency, losses ≈ 4.17 kW ≈ 4 kW
In a DC motor, the starting resistance is required because:
A It increases the starting torque B It protects the motor from excessive starting current C It reduces the core loss D It increases the back EMF
At starting, back EMF is zero. Starting resistance limits initial armature current to safe values: Ia = V/(Ra + Rstart)
Which transformer connection is preferred for step-up operations in power generation?
A Delta-delta B Star-star C Star-delta D Delta-star
Star-delta connection reduces neutral point problems on primary (generator side) and utilizes delta on secondary for better voltage utilization.
The reluctance torque in a synchronous motor is maximum when the load angle is:
A 0° B 45° C 90° D 180°
Reluctance torque τrel ∝ sin(2δ), which is maximum when sin(2δ) = 1, i.e., when δ = 45°
In a DC motor, the back EMF is proportional to which of the following?
A Speed and magnetic flux only B Armature current only C Applied voltage only D Load torque only
Back EMF in DC motor: Eb = PΦN/60, which depends on flux (Φ) and speed (N). It is independent of armature current and load torque.
