Which of the following is NOT a type of DC 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?
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?
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.
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?
The damper winding (squirrel cage) in a synchronous motor provides the initial asynchronous torque for starting until synchronism is achieved.
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A 50 kVA, 2302300 V transformer has a primary resistance of 2Ω. Calculate the equivalent primary resistance referred to secondary side.
R'2 = R1 × (N2/N1)² = 2 × (2300230)² = 2 × 0.01 = 0.02 Ω
In a DC generator, the commutator serves the purpose of:
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:
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:
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?
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:
Ns = 120×450 = 1500 rpm; Nr = Ns(1-s) = 1500×0.95 = 1425 rpm
In a synchronous motor, the pull-out torque depends on:
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:
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?
Both star-delta and autotransformer starting reduce starting current. Star-delta reduces current by 31 of DOL current.
The efficiency of a DC generator is maximum when:
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:
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:
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?
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:
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?
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.
