Electrical Engineering questions for GATE, PSU recruitment and SSC JE draw from network theory, electrical machines, power systems, control systems, measurements and instrumentation, analog and digital electronics, and electromagnetic fields. Numerical answers include the formula used and the unit at each stage, which is where marks are commonly lost even when the approach is correct.
Which of the following is the primary function of a commutator in a DC machine?
Answer: B
The commutator reverses the direction of current in armature conductors as they move from one pole to another, ensuring unidirectional torque production.
Q.62Easy
The back EMF in a DC motor is given by the expression?
Answer: B
Back EMF in DC motor: Eb = PφZN/120 where P = poles, φ = flux, Z = conductors, N = speed in rpm.
Q.63Easy
In a three-phase induction motor, the rotating magnetic field is produced by:
Answer: B
Three-phase currents 120° apart create a rotating magnetic field that rotates at synchronous speed Ns = 120f/P.
Q.64Easy
The synchronous speed of a 4-pole, 50 Hz induction motor is:
Which of the following is NOT a type of DC machine?
Answer: D
Induction machines are AC machines, not DC machines. DC machines include generators and motors.
Q.69Easy
In a transformer, the core loss is independent of which parameter?
Answer: C
Core loss depends on frequency and flux density but is independent of load current. It remains constant at rated voltage.
Q.70Easy
In a DC generator, the commutator serves the purpose of:
Answer: B
The commutator rectifies the alternating EMF induced in the armature coils to produce unidirectional (DC) output.
Q.71Easy
Which of the following losses in a transformer occurs in both primary and secondary windings?
Answer: C
Copper loss (I²R loss) occurs in both primary and secondary windings. Core losses occur only in the magnetic core.
Q.72Easy
In a DC motor, the back EMF is proportional to which of the following?
Answer: A
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.
Q.73Easy
What is the synchronous speed of a 6-pole, 3-phase induction motor operating at 50 Hz frequency?
Answer: B
Synchronous speed Ns = 120f/P = (120 × 50)/6 = 1000 RPM, where f = frequency and P = number of poles.
Q.74Easy
The slip of an induction motor at rated load is typically in the range of:
Answer: A
For standard induction motors, slip at full load is typically 0.5% to 2%, ensuring minimal speed variation and high efficiency.
Q.75Easy
The copper loss in a transformer is minimum when:
Answer: A
Copper loss PCu = I²R is proportional to current squared. It is minimum when load current is zero (no-load condition).
Q.76Easy
An induction motor is running at a slip of 4%. If the synchronous speed is 1500 RPM, the actual rotor speed is:
Answer: A
Slip s = (Ns - Nr)/Ns = 0.04. Therefore, Nr = Ns(1-s) = 1500(1-0.04) = 1440 RPM.
Q.77Easy
The no-load core loss in a transformer is primarily due to:
Answer: B
No-load losses consist of hysteresis loss (due to core magnetization reversal) and eddy current loss (due to induced currents in core material). Copper loss occurs only under load.
Q.78Easy
A 8-pole, 3-phase induction motor is required to run at approximately 720 RPM at 50 Hz. The approximate slip required is:
Answer: A
Ns = 120f/P = (120 × 50)/8 = 750 RPM. For Nr = 720 RPM, s = (750 - 720)/750 = 75030 = 0.04 or 4%.
Q.79Easy
In a transformer, the hysteresis loss depends on which of the following factors?
Answer: B
Hysteresis loss = η × B^1.6 × f × V, where η is Steinmetz constant, material property is crucial
Q.80Easy
A 3-phase induction motor has 4 poles and operates at 50 Hz. What is the synchronous speed?
