This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Doubly Excited Magnetic Systems”.

1. Which of the following are examples of doubly-excited magnetic systems?

a) synchronous Machines

b) loudspeakers and Tachometers

c) D.C Shunt Machines

d) any of the mentioned

View Answer

Explanation: All of the above applications require two independent sources of excitation.

2. Most of the electromagnetic energy conversion devices belong to __________

a) singly excited magnetic systems

b) doubly excited magnetic systems

c) multiply excited magnetic systems

d) both doubly excited magnetic systems and multiply excited magnetic systems

View Answer

Explanation: Widely used motor/generators such as synchronous and induction machines belong to doubly excited systems, and many other practical applications require multiply excited magnetic systems.

3. In a doubly excited magnetic system with salient pole type stator and rotor, if the rotor is not allowed to move, then the equation for magnetic field stored energy in establishing the currents from zero to i_{s} and i_{r} is __________

a) W_{fld}=1/2 i_{s}^{2}L_{s}+1/2 i_{r}^{2}L_{r}

b) W_{fld}=1/2 i_{s}^{2}L_{s}+M_{rs} i_{s} i_{r}

c) W_{fld}=1/2 i_{s}^{2}L_{s}+1/2 i_{r}^{2}L_{r}+M_{rs} i_{s} i_{r}

d) W_{fld}=1/2 i_{r}^{2}L_{r}+M_{rs} i_{s} i_{r}

View Answer

Explanation: As the rotor is not allowed to move, dW

_{mech}=0, thus dW

_{elec}=0+dW

_{fld}=i

_{s}dΨ

_{s}+i

_{r}dΨ

_{r}, if in this equation, we introduce the self and mutual inductance terms, (Ψ

_{s}=L

_{s}i

_{s}) and integrate the resulting equation from 0 to i

_{s}, 0 to i

_{r}and 0 to i

_{r}i

_{s}, the respective terms, finally we get W

_{fld}=1/2 i

_{s}

^{2}L

_{s}+1/2 i

_{r}

^{2}L

_{r}+M

_{rs}i

_{s}i

_{r}

4.In a doubly excited magnetic system of salient pole stator and rotor, the magnetic torque (T_{e}) depends on

(i) the instantaneous values of currents i_{s} and i_{r}

(ii) the angular rate of change of inductances

(iii) the differential changes of current di_{s} and di_{r}

(iv) only the instantaneous values of self inductance

Which of the above statements are true?

a) (i),(iii)

b) (i),(ii)

c) (iii),(iv)

d) (i),(iv)

View Answer

Explanation: T

_{e}=1/2i

_{s}

^{2}dL

_{s}/dθ

_{r}+1/2i

_{r}

^{2}dL

_{r}/dθ

_{r}+i

_{s}i

_{r}dM

_{s}

_{r}/dθ

_{r}.

5.In a doubly excited magnetic systems, the magnetic torques and forces act in such a direction as to tend to

a) decrease the field energy at constant currents

b) decrease the field co-energy at constant currents

c) increase the field energy at constant currents

d) none of the mentioned

View Answer

Explanation: T

_{e}= ∂W

_{fld}(i

_{s},i

_{r},θ

_{r})/∂θ

_{r}= ∂W

_{fld}

^{1}(i

_{s},i

_{r},θ

_{r})/∂θ

_{r}

f

_{e}= ∂W

_{fld}(i

_{s},i

_{r},x)/∂x = ∂W

_{fld}

^{1}(i

_{s},i

_{r},x)/∂x

6. In a doubly excited magnetic system of salient pole type stator and rotor, the reluctance torque is present only when _____________

a) both stator and rotor currents are acting

b) stator current is acting alone

c) rotor current is acting alone

d) any of the stator or rotor currents acting alone

View Answer

Explanation: Equation for magnetic torque is T

_{e}=1/2i

_{s}

^{2}dL

_{s}/dθ

_{r}+1/2i

_{r}

^{2}dL

_{r}/dθ

_{r}+i

_{s}i

_{r}dM

_{s}

_{r}/dθ

_{r}, if i

_{r}=0, T

_{e}=1/2i

_{s}

^{2}dL

_{s}/dθ

_{r}and if i

_{s}=0, then T

_{e}= 1/2i

_{r}

^{2}dL

_{r}/dθ

_{r}, and these equations for torque are called reluctance torques.

7. Which component of torque in the following equation is called the electromagnetic torque of electromagnetic energy conversion device?

T_{e}=1/2i_{s}^{2}dL_{s}/dθ_{r}+1/2i_{r}^{2}dL_{r}/dθ_{r}+i_{s}i_{r}dM_{s}_{r}/dθ_{r}

a) 1/2i_{s}^{2}dL_{s}/dθ_{r}

b) 1/2i_{r}^{2}dL_{r}/dθ_{r}

c) i_{s}i_{r}dM_{s}_{r}/dθ_{r}

d) any of the mentioned

View Answer

Explanation: The torque developed by the interaction of stator and rotor magnetic fields is the electromagnetic torque or interaction torque.

8. Which components of torque in the following equation are called the reluctance torque terms?

T_{e}=1/2i_{s}^{2}dL_{s}/dθ_{r}+1/2i_{r}^{2}dL_{r}/dθ_{r}+i_{s}i_{r}dM_{s}_{r}/dθ_{r}

a) 1/2i_{s}^{2}dL_{s}/dθ_{r} and i_{s}i_{r}dM_{s}_{r}/dθ_{r}

b) 1/2i_{s}^{2}dL_{s}/dθ_{r} and 1/2i_{r}^{2}dL_{r}/dθ_{r}

c) 1/2i_{r}^{2}dL_{r}/dθ_{r} and i_{s}i_{r}dM_{s}_{r}/dθ_{r}

d) 1/2i_{s}^{2}dL_{s}/dθ_{r}, 1/2i_{r}^{2}dL_{r}/dθ_{r} and i_{s}i_{r}dM_{s}_{r}/dθ_{r}

View Answer

Explanation: Because the change of reluctance (Rl

_{s}or Rl

_{r}) are responsible for the production of these torques.

9. Which of the following statements are true about electromagnetic torques and reluctance torques:

(i) electromagnetic torque can exist only if both windings carry current

(ii) reluctance torque depend on the direction of current in stator or rotor windings

(iii) reluctance torque doesn’t depend on the direction of current in stator or rotor windings

(iv) electromagnetic torque depend on the direction of currents i_{s} and i_{r}

(v) electromagnetic torque doesn’t depend on the direction of currents i_{s} and i_{r}

a)(i),(ii),(iii)

b)(ii),(iii),(v)

c)(i),(iii),(iv)

d)(ii),(iii),(iv)

View Answer

Explanation: Reluctance torque:

Explanation: 1/2i

_{s}

^{2}dL

_{s}/dθ

_{r}and 1/2i

_{r}

^{2}dL

_{r}/dθ

_{r}

Positive or negative value of current doesn’t affect the torque direction. The interaction/electromagnetic torque = i

_{s}i

_{r}dM

_{s}

_{r}/dθ

_{r}, here the direction of i

_{s}and i

_{r}changes the torque.

10. Singly and doubly excited magnetic systems applications are respectively:

a) loud speakers and tachometers

b) synchronous motors and moving iron instruments

c) DC shunt machines and solenoids

d) reluctance motors and synchronous motors

View Answer

Explanation: Reluctance motors can work on single excitation, and synchronous motors require double excitation.

11. Electromagnetic torque in rotating electrical machinery is present when ________

a) stator winding alone carries current

b) rotor winding alone carries current

c) air gap is uniform

d) both stator and rotor windings carry current

View Answer

Explanation: Electromagnetic torque = i

_{s}i

_{r}dM

_{s}

_{r}/dθ

_{r}and if either i

_{s}or i

_{r}=0, then T

_{e}=0.

12. All practical electromechanical energy conversion devices make use of the magnetic field rather than the electric field as the coupling medium.This is because

a) electric field systems present insulation difficulties

b) electric field systems have more dielectric loss than the magnetic loss, for the same power rating of the machine

c) in electric field systems, for normal electric field strength, the stored energy density is high

d) in magnetic field systems, for normal magnetic flux density, the stored energy density is high

View Answer

Explanation: As the energy storing capacity of the magnetic field is higher, magnetic field as coupling medium is most common in electromechanical energy conversion devices.

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