Electrical Machines

Electrical Machines MCQ (Multiple Choice Questions)

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

View Answer

Answer: a
Explanation: Electromagnetic torque = i_si_rdM_s_r/dθ_r and if either i_s or i_r = 0, then T_e=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

Answer: b 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

Answer: a 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

Answer: d 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

Answer: a 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

Answer: c 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

Answer: a 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

Answer: c 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

Answer: b 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

Answer: d 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

Answer: a 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

Answer: c 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

Answer: a 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

Answer: b 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

Answer: b 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

Answer: c 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

Answer: c 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

Answer: a 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

Answer: a 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

Answer: a 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

Answer: d Explanation: All the estimations asked will need results of both the tests.

22. An electro-mechanical energy conversion device is one which converts _______
a) Electrical energy to mechanical energy only
b) Mechanical energy to electrical energy only
c) Electrical to mechanical and mechanical to electrical
d) None of the mentioned

View Answer

Answer: c Explanation: The operating principles of electrical to mechanical and mechanical to electrical conversion devices are similar, hence, the common name electro-mechanical device. However, their structural details differ depending on their function.

23. What is the coupling field used between the electrical and mechanical systems in energy conversion devices?
a) Magnetic field
b) Electric field
c) Magnetic field or Electric field
d) None of the mentioned

View Answer

Answer: c Explanation: Either electric field or magnetic field can be used, however most commonly we use magnetic field because of its greater energy storage capacity.

24. The energy storing capacity of magnetic field is about ________ times greater than that of electric field.
a) 50,000
b) 25,000
c) 10,000
d) 40,000

View Answer

Answer: b Explanation: As the energy storage capacity of the magnetic field is higher, it is most commonly used as coupling medium in electro-mechanical energy conversion devices.

25. The formula for energy stored in the mechanical system of linear motion type is ______
a) 1/2 Jw_r²
b) 1/2 mv²
c) 1/2 mv
d) Jw_r²

View Answer

Answer: b Explanation: Energy stored is kinetic energy, since the system is of linear motion.

26. In an electro-mechanical energy conversion device, which of the following statements are correct regarding the coupling field?

(i) electrical side is associated with emf and current
(ii) electrical side is associated with torque and speed
(iii) mechanical side is associated with emf and current
(iv) mechanical side is associated with torque and speed

a) (i) & (ii)
b) (ii) & (iii)
c) (iii) & (iv)
d) (i) and (iv)

View Answer

Answer: d Explanation: The coupling field will be associated with the electrical quantities on electrical side and vice versa.

27. A coupling magnetic field must react with which of the following statements?

 
(i) electrical system in order to extract energy from mechanical system
(ii) mechanical system in order to extract energy from mechanical system
(iii) electrical system in order to extract energy from electrical system
(iv) mechanical system in order to extract energy from electrical system
(v) electrical or mechanical system for electro-mechanical energy conversion

a) (i), (ii) & (iii)
b) (ii), (iii) & (v)
c) (ii), (iii) & (iv)
d) (ii), (iii) & (v)

View Answer

Answer: b Explanation: To convert electrical to mechanical, the coupling magnetic field must take energy from input, the electrical system and vice versa.

28. The developed electromagnetic force and/or torque in electromechanical energy conversion system, acts in such a direction that tends to ___________
a) increase the stored energy at constant mmf
b) decrease the stored energy at constant mmf
c) decrease the co-energy at constant mmf
d) increase the stored energy at constant flux

View Answer

Answer: b Explanation: fe = -(∂Wfld (φ,x))/∂x = -(∂Wfld (∅,x))/∂x and Te = -(∂Wfld (φ,θ))/∂θ = -(∂Wfld(∅,θ))/∂θ.

29. The developed electromagnetic force and/or torque in electromechanical energy conversion systems, acts in a direction that tends to ___________

(i) increase the co-energy at constant flux
(ii) increase the co-energy at constant mmf
(iii) decrease the stored energy at constant mmf
(iv) decrease the stored energy at constant flux

Which of the above statements are correct?
a) (ii), (iv)
b) (i), (iii)
c) (ii), (iii)
d) (i), (iv)

View Answer

Answer: a Explanation: fe = -(∂Wfld (φ,x))/∂x and fe = (∂Wfld1(F,x))/∂x.

30. A physical system of electromechanical energy conversion, consists of a stationary part creating a magnetic field with electric energy input, and a moving part giving mechanical energy output. If the movable part is kept fixed, the entire electrical energy input will be _______
a) stored in the magnetic field
b) stored in the electric field
c) divided equally between the magnetic and electric fields
d) zero

View Answer

Answer: a Explanation: As the movable part is fixed, Wmech = 0, we know, Welec = Wmech + Wfld, therefore, Welec = Wfld.