Here are 1000 MCQs on Electrical Machines chapterwise.
1. Electromagnetic torque in rotating electrical machinery is present when ________
a) Both stator and rotor windings carry current
b) Air gap is uniform
c) Stator winding alone carries current
d) Rotor winding alone carries current
View Answer
Explanation: Electromagnetic torque = isirdMsr/dθr and if either is or ir = 0, then Te=0.
2. In a rotating electrical machine with 2 poles on the stator and 4 poles on the rotor, spaced equally, the net electromagnetic torque developed is ____________
a) maximum
b) zero or no torque is developed
c) minimum
d) none of the mentioned
View Answer
Explanation: In a machine with 2 stator poles and 4 rotor poles spaced equally, the force of attraction will be cancelled due to the force of repulsion, and hence the net electromagnetic torque will be zero.
3. The interaction torque in a rotating electrical machine depends on which of the following components?
a) sinδ, stator field strength, rotor field strength
b) cosδ, torque angle δ, rotor field strength
c) sinδ, torque angle δ, rotor field strength
d) cosδ, stator field strength, rotor field strength
View Answer
Explanation: Interaction torque Te∝(stator field strength)(rotor field stregth)sinδ.
4. A consideration of the power losses in electrical machines is essential for which of the following reasons?
a) Operating cost
b) Temperature rise
c) Voltage drops
d) All of the mentioned
View Answer
Explanation: A machine with lower efficiency has more losses, and therefore increased operating cost. Also, losses cause heating of the machine and therefore, its temperature rise. Similarly, voltage drop IR is associated with ohmic loss.
5. Which of the following can be obtained by the equivalent circuit of an electrical machine?
a) Complete performance characteristics of the machine
b) Temperature coefficients of the machine components
c) Type of protection to be used in the machine
d) Design parameters of the windings
View Answer
Explanation: Equivalent circuit can help to give the complete performance characteristics of an electrical machine.
6. For an electrical machine with C number of coils and P poles, the distance between the coils connected by an equalizer ring is?
a) C/P
b) C/2
c) 2C/P
d) C/2P
View Answer
Explanation: Backward pitch for an equalizer ring = number of coils/pole pairs = C/P/2 = 2C/P.
7. Which of the following is the primary function of a commutator in a DC machine?
a) To reverse the current direction
b) To maintain constant field flux
c) To regulate the armature speed
d) To reduce the power loss
View Answer
Explanation: The commutator in a DC machine reverses the direction of current in the armature windings, ensuring the current flow remains unidirectional through the external circuit.
8. In an induction motor, which of the following factors determines the speed of the motor?
a) Frequency of the supply
b) Resistance of the rotor
c) Number of poles in the motor
d) Voltage applied to the stator
View Answer
Explanation: The speed of an induction motor is primarily determined by the number of poles in the stator and the supply frequency, known as the synchronous speed.
9. Which of the following is a characteristic of a synchronous motor?
a) It can operate at any speed depending on the load
b) It always operates at synchronous speed
c) It has high starting torque
d) It is self-starting
View Answer
Explanation: A synchronous motor always operates at synchronous speed, which is determined by the frequency of the supply and the number of poles in the motor.
10. Which of the following is the primary advantage of a 3-phase induction motor?
a) Higher starting torque
b) Smooth operation with less vibration
c) High efficiency
d) Simple construction and low cost
View Answer
Explanation: 3-phase induction motors are known for their simple construction, reliability, and cost-effectiveness compared to other motor types.
11. What is the primary function of the stator in a synchronous machine?
a) To generate the magnetic field
b) To provide mechanical power
c) To act as a conductor for the current
d) To control the speed of the rotor
View Answer
Explanation: In a synchronous machine, the stator is responsible for generating the magnetic field required for operation, while the rotor maintains synchrony with this field.
12. Which of the following losses is associated with eddy currents in an electrical machine?
a) Copper loss
b) Hysteresis loss
c) Iron loss
d) Mechanical loss
View Answer
Explanation: Eddy current losses occur in the core material of electrical machines due to the changing magnetic field, contributing to iron loss.
13. Which of the following can be obtained by the equivalent circuit of an electrical machine?
a) Complete performance characteristics of the machine
b) Temperature coefficients of the machine components
c) Type of protection to be used in the machine
d) Design parameters of the windings
View Answer
Explanation: Equivalent circuit can help to give the complete performance characteristics of an electrical machine.
14. In a DC generator, what determines the direction of induced EMF?
a) Direction of the armature current
b) Direction of the magnetic field and motion
c) Speed of rotation of the armature
d) Resistance of the winding
View Answer
Explanation: The direction of the induced EMF in a DC generator is determined by Fleming’s Right-Hand Rule, which depends on the direction of the magnetic field and the motion of the conductor.
15. Why are laminations used in the core of electrical machines?
a) To increase the magnetic flux
b) To reduce eddy current losses
c) To improve mechanical strength
d) To reduce hysteresis losses
View Answer
Explanation: Laminations in the core reduce eddy current losses by increasing the resistance to the flow of eddy currents, thereby improving efficiency.
16. What is the purpose of using a starter in a DC motor?
a) To limit the speed
b) To reduce back EMF
c) To limit the starting current
d) To increase efficiency
View Answer
Explanation: A starter is used in a DC motor to limit the high inrush current during startup, which can damage the windings and other components.
17. Which of the following is true about the slip in an induction motor?
a) It is always zero at synchronous speed
b) It is negative during motoring mode
c) It increases as the load increases
d) It is independent of load
View Answer
Explanation: Slip in an induction motor increases with load because the rotor speed decreases slightly as the load torque increases, causing a greater relative speed between the rotor and the magnetic field.
18. What is the primary cause of hysteresis losses in electrical machines?
a) Variable magnetic field in the core
b) Resistance of the windings
c) Harmonics in the supply
d) Mechanical friction
View Answer
Explanation: Hysteresis losses occur due to the repeated magnetization and demagnetization of the core material as the magnetic field varies, leading to energy loss in the form of heat.
19. Which is the most widely used material in the core of the transformer?
a) cold rolled grain oriented sheet steel
b) cold rolled grain steel
c) soft iron
d) steel
View Answer
Explanation: CRGO has magnetization in the rolling direction and low core losses and very high permeability than present materials.
20. A single phase transformer has emf per turn having 2310/220 V, 50Hz transformer as 13 V. The core area is(in square cm)?
a) 393
b) 277.8
c) 358.92
d) 450.03
View Answer
Explanation: Emf per turn = 1.44*f*flux density * Area*N2
Area = 220/(18*1.44*50*1.4) = 393 sq.cm.
21. Which of the below estimations require results of both open circuit test and short circuit test?
a) Efficiency
b) Equivalent impedance of one side of the winding
c) Voltage regulation for exact circuit
d) All of the mentioned
View Answer
Explanation: All the estimations asked will need results of both the tests.
1000+ Multiple Choice Questions on Electrical Machines
- Magnetic Circuits
- Basic Concepts in Rotating Machines
- Constructional Features of Rotating Electrical Machines
- Principles of Electro mechanical Energy Conversions
- Rating and Loss Dissipation
- Electrical Machine Applications/Armature Reaction and Commutation
- Electromechanical Energy Conversions
- DC Machines
- DC Generators
- DC Motors
- Synchronous Machines
- Synchronous Generators
- Synchronous Motors
- Efficiency and Regulation
- Synchronous Machine Stability
- Induction Machines
- Starting of Induction Motors
- Armature Windings
- Transformers
1. Magnetic Circuits
The section contains multiple choice questions and answers on singly exicted magnetic systems and energy conversion principles.
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2. Basic Concepts in Rotating Machines
The section contains questions and answers on reluctance motor, synchronous motors, singly and doubly excited systems, torque production, rotating machines terms and rotating magnetic field.
3. Constructional Features of Rotating Electrical Machines
The section contains MCQs on constructional features of rotating electrical machines.
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4. Principles of Electro mechanical Energy Conversions
The section contains multiple choice questions and answers on generated emfs.
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5. Rating and Loss Dissipation
The section contains questions and answers on losses and efficiency, machine ratings and loss dissipation.
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6. Electrical Machine Applications/Armature Reaction and Commutation
The section contains MCQs on machine applications and action of commutator.
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7. Electromechanical Energy Conversions
The section contains multiple choice questions and answers on emf generation in armature and torque in dc machines.
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8. DC Machines
The section contains questions and answers on dc machine applications and its circuit models, mmf and flux density waveforms in dc machines, methods of excitation, efficiency and testing of dc machines.
9. DC Generators
The section contains MCQs on rotating amplifiers and operating characteristics of dc generators.
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10. DC Motors
The section contains multiple choice questions and answers on characteristics of dc motors and speed control of dc motors, pmdc and dc motor starting.
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11. Synchronous Machines
The section contains questions and answers on physical concepts, Emf Polygons, power angle and power factor control of synchronous machines, excitation systems for synchronous machines, flux and mmf phasors in synchronous machines,synchronous machine applications and measurement of xd and xq.
12. Synchronous Generators
The section contains MCQs on characteristics of alternators, phasor diagram of a cylindrical rotor alternator and also voltage regulation of an alternator.
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13. Synchronous Motors
The section contains multiple choice questions and answers on synchronous motor phasor diagram, starting of synchronous motors and power factor correction.
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14. Efficiency and Regulation
The section contains questions and answers on power flow through an inductive impedance, losses and efficiency of synchronous machines.
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15. Synchronous Machine Stability
The section contains MCQs on synchronizing power and synchronizing torque, hunting and damper windings and synchronous machine stability.
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16. Induction Machines
The section contains multiple choice questions and answers on principles of operations, rotor frequency, equivalent circuit and its analysis, induction motor phasor diagram and its operating characteristics, induction generator, circle diagrams of induction machines, flux, mmf phase and waves in induction motors.
17. Starting of Induction Motors
The section contains questions and answers on starting of polyphase induction motors.
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19. Transformers
The section contains multiple choice questions and answers on transformer construction and phasor diagram, open circuit and short circuit test, voltage regulation of transformer, transformer losses and its efficiency, autotransformers, single phase transformer, three phase transformer and ideal two winding transformer.
Wish you the best in your endeavor to learn and master Electrical Machines!
