Free Electrical Machines MCQs with Answers

• A. to reduce the magnetic hum and locking tendency of the rotor
• B. to increase the tensile strength of the rotor bars
• C. to ensure easy fabrication
• D. none of the above

Explanation: Skewing the rotor slots reduces magnetic cogging, locking and objectionable humming by preventing the rotor teeth from aligning…

• A. at the unit value of slip
• B. at the zero value of slip
• C. at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
• D. at the value of the slip which makes the rotor reactance half of the rotor

Explanation: Maximum induction-motor torque occurs when the effective rotor reactance equals rotor resistance, so sX₂ = R₂.

• A. The slip of the motor will be 5%
• B. The rotor will not run
• C. The rotor will run at very high speed
• D. The torque produced will be very large

Explanation: Zero relative speed means zero slip, so no rotor EMF or rotor current is induced and no electromagnetic torque is produced.

• A. external inductance to the rotor
• B. external resistance to the rotor
• C. external capacitance to the rotor
• D. both resistance and inductance to rotor

Explanation: Adding external rotor resistance increases the starting torque of a slip-ring motor by moving the maximum-torque condition toward starting…

• A. 0.01
• B. 0.02
• C. 0.03
• D. 0.04

Explanation: Synchronous speed is Ns = 120f/P = 120(50)/6 = 1000 rpm. Therefore, slip = (1000 − 960)/1000 = 0.04, or 4%.

• A. stator resistance test
• B. no-load test only
• C. short-circuit test only
• D. noue of the above

Explanation: The short-circuit, or blocked-rotor, test supplies known voltage, current and power values used to establish the circle-diagram scale.

• A. open-circuit test only
• B. short-circuit test only
• C. stator resistance test
• D. none of the above

Explanation: The blocked-rotor, or short-circuit, test permits comparison of the applied stator voltage with the induced rotor voltage at standstill…

• A. power factor at starting is high as compared to that while running
• B. power factor at starting is low as compared to that while running
• C. power factor at starting in the same as that while running

Explanation: At starting, rotor frequency is maximum and rotor leakage reactance is significant, so the motor draws a highly inductive current with low…

• A. improve starting torque
• B. reduce copper losses
• C. improve efficiency
• D. improve power factor

Explanation: Deep rotor bars produce a strong skin effect during starting, increasing the effective rotor resistance.

• A. Block rotor test only
• B. No load test only
• C. Block rotor test and no-load test
• D. Block rotor test, no-load test and stator resistance test

Explanation: The no-load test provides the shunt-branch and rotational-loss data, while the blocked-rotor test provides series impedance data.

• A. pulling the motor directly on line without a starter
• B. locking of rotor due to harmonics
• C. starting the motor on load which is more than the rated load
• D. interchanging two supply phases for quick stopping

Explanation: Plugging is dynamic braking by interchanging any two supply phases, which reverses the rotating field while the rotor is still moving.

• A. windage losses
• B. copper losses
• C. transformation ratio
• D. power scale of circle diagram

Explanation: In the blocked-rotor test the motor is stationary, so windage loss is zero and cannot be measured from this test.

• A. twice the full load torque
• B. 1.5 times the full load torque
• C. equal to full load torque

Explanation: A conventional squirrel-cage induction motor is commonly specified as having a starting torque of about 1.5 times its full-load torque in…

• A. double cage rotor
• B. wound rotor
• C. short-circuited rotor
• D. any of the above

Explanation: A slip-ring motor uses a wound rotor whose three-phase winding is connected to slip rings and external resistance.

• A. 730 r.p.m.
• B. 960 r.p.m.
• C. 1440 r.p.m.
• D. 2880 r.p.m.

Explanation: For the same horsepower, the low-speed motor needs more poles and a larger active size to produce the required torque.

• A. stall after sometime
• B. stall immediately
• C. continue to run at lower speed without damage
• D. get heated and subsequently get damaged

Explanation: Induction-motor torque is approximately proportional to the square of applied voltage, so a 30% voltage reduction leaves only about 49% of…

• A. Ball bearings
• B. Cast iron bearings
• C. Bush bearings
• D. None of the above

Explanation: Small induction motors commonly use ball bearings to support the rotor with low friction and maintain alignment.

• A. D.C. series motor
• B. D.C. shunt motor
• C. universal motor
• D. none of the above

Explanation: An induction motor's speed falls only slightly as load increases, closely matching the nearly constant-speed characteristic of a DC shunt…

• A. it will run in reverse direction
• B. it will pick up very high speed and may go out of step
• C. motor takes five to seven times its full load current
• D. starting torque is very high

Explanation: At direct line starting, an induction motor has very low impedance because there is no back emf, so it draws about five to seven times its…

• A. in the opposite direction as the direction of the rotating field
• B. in the same direction as the direction of the field
• C. in any direction depending upon phase squence of supply

Explanation: The rotating stator field induces rotor currents whose interaction produces torque in the same direction as the rotating field.