Electrical Machines Multiple Choice Questions on “Elementary Synchronous Machines”.
1. Consider a single phase synchronous machine of cylindrical rotor type. A single phase supply is given to the stator winding and a voltmeter is connected across rotor winding. For what value of θr, the voltmeter reads maximum?
A. 90°
B. 0°
C. 45°
D. 30°
Answer: B
Clarification: When θr=0, the flux linkages with the rotor are maximum, and this is indicated by a maximum voltmeter reading.
2. Consider a single phase synchronous machine of cylindrical rotor type. A single phase supply is given to the stator winding, and a voltmeter is connected across rotor winding. For what value of θr the voltmeter reads a zero?
A. 0°
B. 30°
C. 45°
D. 90°
Answer: D
Clarification: When θr=90°, the voltmeter reading is zero, because the stator winding axis is perpendicular to the rotor winding axis, and the mutual inductance Msr is zero.
3. Which of the following equation best represents the mutual inductance between stator and rotor in a single phase cylindrical rotor synchronous machine? (where θr= space angle between stator and rotor field axis)
A. Msr=0
B. Msr=Mmax
C. Msr=Mmaxcosθr
D. Msr=Mmaxsinθr
Answer: C
Clarification: In a single phase cylindrical rotor synchronous machine, the Msr is maximum, when θr=0, and is zero when θr=90° and cosine function represents it.
4. In a single phase cylindrical rotor synchronous machine, the torque Te tends to ____________ the space angle θr.
A. reduce
B. increase
C. maintain constant
D. all of the mentioned
Answer: A
Clarification: Wfld(is,ir,θr)=1/2 is2Ls+1/2 ir2Lr+ isirMmaxcosθr
Torque, Te=∂Wfld(is,ir,θr)/∂θr= -isirMmaxsinθr
Negative sign indicates that the torque Te tends to reduce angle θr.
5. The following torque expression: Te=-isirMmaxsinθr is applicable in a single phase cylindrical rotor synchronous machine when the rotor is __________
A. revolving
B. stationary
C. revolving or stationary
D. none of the mentioned
Answer: C
Clarification: The torque expression is applicable whether the rotor is revolving or stationary.
6. In a two phase cylindrical rotor synchronous machine, the self inductances Lα, Lβ for phases α,β respectively are __________
A. constant and equal
B. constant and unequal
C. varying and equal
D. varying and unequal
Answer: A
Clarification: Constant and equal, as the reluctance offered to the fluxes produced by phase α and β winding doesn’t vary with rotor movement.
7. In a two phase cylindrical rotor synchronous machine, the mutual inductance Mαβ between α phase and β phase winding is __________
A. Mmax
B. 0
C. Mmin
D. any of the mentioned
Answer: B
Clarification: Since the axes of phase α and phase β windings are normal to each other and air gap is uniform, the mutual inductance Mαβ between α,β winding is zero.
8. In a two phase cylindrical rotor synchronous machine, torque Te is _____________
A. constant at some instant of time
B. changing at every instant of time
C. constant at every instant of time
D. changing at some instant of time