A DC shunt motor is running at no load. If the field winding is suddenly short-circuited, the motor will:
A Stop immediately B Speed up dangerously C Run at half speed D Continue at same speed
Flux reduces drastically when field is short-circuited. As back EMF reduces, armature current increases causing motor speed to increase dangerously.
The efficiency of a transformer is maximum when:
A Core loss = Copper loss B Core loss = 2 × Copper loss C Copper loss = 0 D Core loss = 0
Transformer efficiency is maximum when core loss (Pi) equals copper loss (Pcu), i.e., Pi = Pcu.
The starting current of a 3-phase induction motor is typically how many times the full-load current?
A 2-3 times B 4-7 times C 10-12 times D 15-20 times
Induction motors draw 4-7 times full-load current during starting because rotor speed is zero and rotor resistance appears as reactance.
Which method is NOT used for starting a three-phase induction motor?
A Direct on line starting B Star-delta starting C Autotransformer starting D Frequency variation starting
Standard starting methods are DOL, star-delta, autotransformer, and rotor resistance. Frequency variation is used for speed control, not starting.
A DC series motor is suitable for:
A Constant speed operation B Applications requiring high starting torque C Precision positioning D Light load conditions
DC series motors have high starting torque (proportional to load current) and are used in electric vehicles, cranes, and hoists.
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The power factor of an induction motor at full load is typically:
A 0.7-0.8 lagging B 0.9-0.95 leading C 0.85-0.95 lagging D Unity (1.0)
Induction motors operate at lagging power factor (0.85-0.95) due to reactive magnetizing current required for flux production.
What is the relationship between slip and rotor frequency in an induction motor?
A fr = s × fs B fr = s × (1-s) × fs C fr = (1-s) × fs D fr = fs/s
Rotor frequency fr = slip × stator frequency = s × fs, where fs = 50 Hz for 50 Hz supply.
In a synchronous motor, the damper winding is used to:
A Reduce noise B Provide starting torque (asynchronous torque) C Reduce harmonics D Improve power factor
Damper windings provide asynchronous starting torque to accelerate the rotor near synchronous speed before synchronous operation begins.
A 100 kVA transformer has 1000 primary turns and 100 secondary turns. The turns ratio and voltage ratio are:
A k=0.1, voltage ratio=10:1 B k=10, voltage ratio=10:1 C k=0.1, voltage ratio=1:10 D k=100, voltage ratio=100:1
Turns ratio k = N2/N1 = 1000 100 1 10
A DC shunt motor shows increased speed. What could be the reason?
A Increased load B Increased shunt field current C Decreased shunt field current D Increased armature resistance
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.
A 40 Hz B 50 Hz C 60 Hz D 48 Hz
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?
A Field winding B Damper winding C Armature winding D Excitation winding
The damper winding (squirrel cage) in a synchronous motor provides the initial asynchronous torque for starting until synchronism is achieved.
A 50 kVA, 230 2300
A 0.02 Ω B 0.2 Ω C 200 Ω D 2000 Ω
R'2 = R1 × (N2/N1)² = 2 × (2300 230
A 3-phase induction motor draws 50 A at 0.8 power factor lagging from 400 V supply. The input power is approximately:
A 13.86 kW B 27.71 kW C 34.64 kW D 20 kW
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:
A Eb = PΦZn/60A B Eb = PΦZn/120A C Eb = VIaRa D Eb = V + IaRa
Back EMF in DC motor Eb = PΦZn/(60A), where P = poles, Φ = flux, Z = conductors, n = rpm, A = parallel paths
A 4-pole, 3-phase induction motor operates at a slip of 0.05. The rotor speed is:
A 1500 rpm B 1425 rpm C 1200 rpm D 1350 rpm
Ns = 120×4 50
Which starting method is suitable for a 3-phase induction motor with locked rotor current limit?
A Direct online starting B Star-delta starting C Autotransformer starting D Both B and C
Both star-delta and autotransformer starting reduce starting current. Star-delta reduces current by 3 1
In a DC motor, the starting resistance is required because:
A It increases the starting torque B It protects the motor from excessive starting current C It reduces the core loss D It increases the back EMF
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?
A Delta-delta B Star-star C Star-delta D Delta-star
Star-delta connection reduces neutral point problems on primary (generator side) and utilizes delta on secondary for better voltage utilization.
Which type of DC motor provides excellent speed control characteristics and is suitable for cranes and hoists?
A Series motor B Shunt motor C Compound motor D Separately excited motor
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.
