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?
