Electrical Machines - Synchronous Machines

Synchronous Machines - GATE previous year questions......

1. The torque angle of a synchronous machine operating from a constant voltage bus, is usually defined as the space angle between [GATE- 1992]
a. Rotor mmf wave and stator mmf wave
b. Rotor mmf wave and resultant flux density wave
c.  Stator mmf wave and resultant flux density wave
d. Stator mmf wave and resultant wave

2. A synchronous motor operates at 0.8 p.f lagging. If the field current of the motor is continuously increased [GATE-1993]
a. the power factor decreases upto a certain value of field current and there after it increases
b. the armature current increases upto a certain value of field current and thereafter it decreases
c. the power factor increases upto a certain value of field current and there after it decreases
d. the armature current decreases upto a certain value of field current and thereafter it increases

3. A synchronous motor on load draws a current at a leading power factor angle 'x'.If the internal power factor angle – which is the phase angle between the excitation e.m.f. and the current in the time phasor diagram is 'y', then the air gap excitation m.m.f lags the armature m.m.f by [GATE- 1995]
a. A
b. Pi/2 + A
c. Pi/2 - A
d. Pi/2+x
( where A is )

4. A cylindrical rotor synchronous motor is switched on the supply with its field windings shorted on themselves. It will [GATE- 1996]
a. It will not start
b. Start but not run at synchronous speed
c. Start as induction motor and run as synchronous motor
d. Start and run as synchronous motor

5. The armature of a single phase alternator is completely wound with T single turn coils distributed uniformly. The induced voltage in each turn is 2V (rms). The emf of the whole winding is [GATE- 1998]
a. 2 T volts
b. 1.11 T volts
c. 1.414 T volts
d. 1.273 T volts

6. A three phase alternator is wound with a 60 degree phase-spread armature windings and develops 300 kVA. If the armature is reconnected utilizing all the coils for single phase operation with a phase spread of 180 degrees, the new rating of the machine is
[GATE- 1999]
a. 100 kva             b. 200 kva
c. 250 kva              d. 300 kva

7. It is desirable to eliminate harmonic voltage from the phase voltage of an alternator. The coils should be short-pitched by an electrical angle of [GATE-2000]
a. 30 degrees        b. 36 degrees
c. 72 degrees        d. 18 degrees

8. X,X',X"are steady state d-axis synchronous reactance, transient d-axis reactance and sub-transient d-axis reactance of a synchronous machine respectively. Which of the following statements is true? [GATE-2001]
a.  X>X'>X"
b. X">X'>X
c.  X'>X">X
d. X>X">X

9. The flux per pole in a synchronous motor with the field circuit ON and the stator disconnected from the supply is found to be 25 mWb. When the stator is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20 mWb while the motor is running on no load. Assuming no load losses to be zero, the no load current down by the motor from the supply [GATE-2002]
a. Lags the supply voltage
b. Leads the supply voltage
c. In phase with the supply voltage
d.is Zeo

10. A 400 V, 50 kVA, 0.8 pf leading ∆-connected ,50 Hz synchronous machine has asynchronous reactance of 2 Ω and negligible armature resistance . The friction and windage losses are 2 kW and the core loss is 0.8 kW. The shaft is supplying9 kW load at a power factor of 0.8 leading . The line current drawn is  [GATE-2004]
a. 12.29A                b.16.24 A
c. 21.29 A                d. 36.88A

11.  A 3-phase ,400 V, 5 kW, star connected synchronous motor having an internal reactance of 10 Ω is operating at 50 % load, unity power factor . Now, the excitation is increased by 1%. What will be the new load in percent ,if the power factor is to be kept same ? Neglect all losses and consider linear magnetic circuit [GATE-2006]
a. 67.9 %                  b. 56.9 %
c. 51 %                      d. 50 %

12. A 100 kVA, 415(line) ,star-connected synchronous machine generates rated open circuit voltage of 415 V at a field current of 15 A. The short circuit armature current at a field current of 10A is equal to the rated armature current . The per unit saturated synchronous reactance is [GATE-2007]
a. 1.731 b. 1.5   c. 0.666  d. 0.577

13. A synchronous motor is connected to an infinite bus at 1.0 pu voltage and draws 0.6 pu current at unity power factor . Its synchronous reactance is 1.0 pu and resistance is negligible
The excitation voltage (E) and load angle (δ) will respectively be  [GATE-2008]
a. 0.8 pu and 36.86 degrees lead
b. 0.8 pu and 36.86 degrees lag
c. 1.76 pu and 30.96 degrees lag
d. 1.76 pu and 30.96 degrees lead

14. A 500MW, 21kV,, 50Hz, 3-phase, 2-pole synchronous generator having a rated p.f=0.9, has a moment of inertia of
27.5* 10^3 kg-m^2 . The inertia constant (H) will be  [GATE-2009]
a. 2.44 s.                    b. 2.71 s
c. 4.88 s.                     d. 5.42 s

15. The direct axis and quadrature axis reactance’s of a salient pole alternator are 1.2p.u and 1.0p.u respectively. The armature resistance is negligible. If this alternator is delivering rated kVA at upf and at rated voltage then its power angle is [GATE-2011]
a. 30 degrees
b. 45 degrees
c. 60 degrees
d. 90 degrees