Electrical Machines Questions and Answers – Doubly Excited Magnetic Systems

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) All of the mentioned
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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
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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_s²L_s+1/2 i_r²L_r
b) W_fld=1/2 i_s²L_s+M_rs i_s i_r
c) W_fld=1/2 i_s²L_s+1/2 i_r²L_r+M_rs i_s i_r
d) W_fld=1/2 i_r²L_r+M_rs i_s i_r
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4. In a doubly excited magnetic system of salient pole stator and rotor, the magnetic torque (T_e) depends on which of the following statements?

(i) the instantaneous values of currents is and ir
(ii) the angular rate of change of inductances
(iii) the differential changes of current dis and dir
(iv) only the instantaneous values of self inductance

a) (i), (iii)
b) (i), (ii)
c) (iii), (iv)
d) (i), (iv)
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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
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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
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7. Which component of torque in the following equation is called the electromagnetic torque of electromagnetic energy conversion device?

Te=1/2is2dLs/dθr+1/2ir2dLr/dθr+isirdMsr/dθr

a) 1/2i_s²dL_s/dθ_r
b) 1/2i_r²dL_r/dθ_r
c) i_si_rdM_sr/dθ_r
d) all of the mentioned
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8. Which components of torque in the following equation are called the reluctance torque terms?

Te=1/2is2dLs/dθr+1/2ir2dLr/dθr+isirdMsr/dθr

a) 1/2i_s²dL_s/dθ_r and i_si_rdM_sr/dθ_r
b) 1/2i_s²dL_s/dθ_r and 1/2i_r²dL_r/dθ_r
c) 1/2i_r²dL_r/dθ_r and i_si_rdM_sr/dθ_r
d) 1/2i_s²dL_s/dθ_r, 1/2i_r²dL_r/dθ_r and i_si_rdM_sr/dθ_r
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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 is and ir
(v) electromagnetic torque doesn't depend on the direction of currents is and ir

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