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
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
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 is and ir is __________
a) Wfld=1/2 is2Ls+1/2 ir2Lr
b) Wfld=1/2 is2Ls+Mrs is ir
c) Wfld=1/2 is2Ls+1/2 ir2Lr+Mrs is ir
d) Wfld=1/2 ir2Lr+Mrs is ir
Explanation: As the rotor is not allowed to move, dWmech=0, thus dWelec=0+dWfld=isdΨs+irdΨr, if in this equation, we introduce the self and mutual inductance terms, (Ψs=Lsis) and integrate the resulting equation from 0 to is, 0 to ir and 0 to iris, the respective terms, finally we get Wfld=1/2 is2Ls+1/2 ir2Lr+Mrs is ir.
4. In a doubly excited magnetic system of salient pole stator and rotor, the magnetic torque (Te) 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)
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
Explanation: Te = ∂Wfld(is,ir,θr)/∂θr = ∂Wfld1(is,ir,θr)/∂θr
fe = ∂Wfld(is,ir,x)/∂x = ∂Wfld1(is,ir,x)/∂x
The positive sign in the formula indicates that force/torque acts in a direction as to tend to increase both field energy and co-energy.
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
Explanation: Equation for magnetic torque is Te=1/2is2dLs/dθr+1/2ir2dLr/dθr+isirdMsr/dθr, if ir=0, Te=1/2is2dLs/dθr and if is=0, then Te = 1/2ir2dLr/dθr, 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?
d) all of the mentioned
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?
a) 1/2is2dLs/dθr and isirdMsr/dθr
b) 1/2is2dLs/dθr and 1/2ir2dLr/dθr
c) 1/2ir2dLr/dθr and isirdMsr/dθr
d) 1/2is2dLs/dθr, 1/2ir2dLr/dθr and isirdMsr/dθr
Explanation: Because the change of reluctance (Rls or Rlr) 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 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)
Explanation: Reluctance torque:
Explanation: 1/2is2dLs/dθr and 1/2ir2dLr/dθr
Positive or negative value of current doesn’t affect the torque direction. The interaction/electromagnetic torque = isirdMsr/dθr, here the direction of is and ir 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
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
Explanation: Electromagnetic torque = isirdMsr/dθr and if either is or ir = 0, then Te=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
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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