In a 3-phase transformer with Delta-Star connection, the secondary voltage phase angle leads the primary by:
Answer: B
Delta-Star (Dy) connection introduces a 30° phase shift. Secondary voltage leads primary by 30°, which is standard for Dy11 configuration.
Q.22Medium
What is the relationship between electrical and mechanical power in a DC motor running at steady state?
Answer: A
Input electrical power (VI) is converted to mechanical power (τω) plus losses (I²R, friction, etc.). Energy conservation: Pin = Pout + Losses.
Q.23Medium
In a synchronous motor, the angle between stator MMF and rotor MMF at full load is:
Answer: C
The load angle (δ) is the angle between stator and rotor MMF in a synchronous motor. At full load, it ranges from 10° to 30° to develop torque.
Q.24Medium
In a DC generator, armature reaction causes the neutral plane to:
Answer: B
Armature reaction MMF opposes main flux, distorting the flux pattern and shifting the neutral plane (zero flux position) forward in the direction of armature rotation.
Q.25Medium
A 3-phase induction motor draws 50 A at 0.7 lagging power factor when running at full load. What is the reactive power?
Answer: C
Q = √(S² - P²). First, S = VI/√3 = (415 × 50)/√3 = 11.98 kVA. P = S × cos(φ) = 11.98 × 0.7 = 8.4 kW. Q = √(11.98² - 8.4²) = 8.57 kVAR (approximately 35.7 kVAR for higher voltage systems).
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Q.26Medium
The primary advantage of a brushless DC motor (BLDC) over conventional DC motor is:
Answer: B
BLDC motors eliminate brushes (reducing wear and maintenance), produce less electromagnetic interference, offer precise speed control via electronic commutation, and have longer operational life.
Q.27Medium
In a transformer under short-circuit test, the secondary is short-circuited and primary voltage is reduced to make rated current flow. The measured power is the:
Answer: C
Short-circuit test is conducted at reduced voltage (typically 5-10% rated) to limit core loss (which is negligible at low voltage). The measured power represents primarily copper loss at rated current.
Q.28Medium
Which of the following statements about a DC shunt motor is true?
Answer: B
In a DC shunt motor, field is connected in parallel with armature, maintaining nearly constant flux. Speed n = (V - IaRa)/(kΦ) depends on voltage and flux, remaining relatively independent of load variation.
Q.29Medium
The efficiency of a squirrel-cage induction motor improves with increasing load mainly because:
Answer: B
Core (iron) loss depends on voltage and frequency, remaining approximately constant at rated operation. As load increases, output power increases while core loss remains fixed, improving overall efficiency up to a point.
Q.30Medium
A DC motor is running at 1500 rpm with an armature current of 20 A. If the back EMF is 230 V and supply voltage is 250 V, what is the armature resistance?
Answer: A
Using V = E + Ia × Ra; 250 = 230 + 20 × Ra; Ra = 1 Ω
Q.31Medium
A series DC motor is suitable for which of the following applications?
Answer: B
Series motors develop very high starting torque, ideal for electric vehicles and heavy-duty applications
Q.32Medium
The open-circuit test on a transformer primarily helps to determine:
Answer: A
OC test measures core loss (hysteresis + eddy current) and magnetizing component of no-load current
Q.33Medium
In a permanent magnet synchronous motor (PMSM), the torque is primarily developed due to:
Answer: A
PMSM torque = Pm × Is × sin(δ), where Pm is permanent magnet flux and Is is stator current
Q.34Medium
In a wound-rotor induction motor, adding external resistance to the rotor circuit:
Answer: A
External rotor resistance shifts the point of maximum torque to higher slip values
Q.35Medium
The commutation in a DC machine is said to be 'overcommutated' when:
Answer: B
Over-commutation occurs when induced EMF > required EMF, causing sparking at trailing edge of brushes
Q.36Medium
In an induction motor, the torque under running conditions is proportional to:
Answer: C
T ∝ Ir × cos(φr) × r/s, where torque depends on rotor current magnitude and its power factor
Q.37Medium
Which of the following represents the condition for maximum power transfer in a DC generator connected to external load?
Answer: A
Maximum power transfer theorem: Load resistance equals source resistance for maximum power output
Q.38Medium
In a 3-phase transformer with Y-Δ connection, the secondary voltage lags the primary by:
Answer: B
Y-Δ connection introduces 30° phase shift with secondary lagging primary voltage phasor
Q.39Medium
For a DC shunt motor operating at rated conditions, if the load is suddenly increased, the motor will:
Answer: B
Increased load increases Ia, causing larger voltage drop (IaRa), reducing back EMF and speed until new equilibrium
Q.40Medium
A DC series motor is used in electric trains. If the load torque decreases suddenly, what happens to the motor speed?
Answer: A
In a DC series motor, as load decreases, back EMF increases relative to applied voltage, causing field current to reduce. This results in a significant increase in motor speed.
