Which starting method is suitable for a 3-phase induction motor with locked rotor current limit?
Answer: D
Both star-delta and autotransformer starting reduce starting current. Star-delta reduces current by 31 of DOL current.
Q.105Medium
In a DC motor, the starting resistance is required because:
Answer: B
At starting, back EMF is zero. Starting resistance limits initial armature current to safe values: Ia = V/(Ra + Rstart)
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Q.106Medium
Which transformer connection is preferred for step-up operations in power generation?
Answer: C
Star-delta connection reduces neutral point problems on primary (generator side) and utilizes delta on secondary for better voltage utilization.
Q.107Medium
Which type of DC motor provides excellent speed control characteristics and is suitable for cranes and hoists?
Answer: C
Compound DC motors combine advantages of series (high starting torque) and shunt (stable speed) motors, making them ideal for cranes requiring both high starting torque and controlled speed.
Q.108Medium
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.109Medium
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.110Medium
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.111Medium
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.112Medium
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).
Q.113Medium
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.114Medium
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.115Medium
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.116Medium
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.117Medium
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.118Medium
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.119Medium
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.120Medium
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
